Evolution: Its Science and Doctrine 9781487571863

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EVOLUTION: ITS SCIENCE AND DOCTRINE L'EVOLUTION: LA SCIENCE ET LA DOCTRINE

ROYAL SOCIETY OF CANADA "STUDIA V ARIA" SERIES

1. Studia Varia: Literary and Scientific Papers-Etudes litteraires et scientifiques (1956). Edited by E.G. D. MURRAY

2. Our Debt to the Future: Symposium Presented on the Seventy-fifth Anniversary, 1957-Presence de demain: Colloque presente au Soixante-quinzieme Anniversaire, 1957. Edited by E. G. D. MURRAY

3. The Canadian Northwest: Its Potentialities; Symposium Presented to the Royal Society of Canada in 1958-L'Avenir du Nord-Ouest Canadien; Colloque presente a la Societe Rayale du Canada en 1958. Edited by FRANK H. UNDERHILL 4. Evolution: Its Science and Doctrine; Symposium Presented to the Royal Society of Canada in 1959-L'Evolution: La Science et la Doctrine; Colloque presente a la Societe Royale du Canada en 1959. Edited by THOMAS W. M. CAMERON

Evolution: Its Science and Doctrine Symposium presented to the

ROY AL SOCIETY OF CANADA in 1959

L'Evolution: La Science et la Doctrine ,

Colloque presente ,

a la

SOCIETE ROYALE DU CANADA en 1959 EDITED BY

THOMAS W. M. CAMERON, F.R.S.C. PUBLISHED FOR THE SOCIETY BY UNIVERSITY OF TORONTO PRESS

1960

Copyright, Canada, 1960 University of Toronto Press Printed in Canada London: Oxford University Press Reprinted in 2018 ISBN 978-1-4875-7218-1 (paper)

FINANCIAL ASSISTANCE FROM THE CANADA COUNCIL TOWARDS THE PUBLICATION OF THE STUDIA VARIA SERIES IS GRATEFULLY ACKNOWLEDGED BY THE ROYAL SOCIETY OF CANADA. THE FACT THAT A GRANT HAS BEEN MADE DOES NOT IMPLY, HOWEVER, THAT THE CANADA COUNCIL ENDORSES OR IS RESPONSIBLE FOR THE STATEMENTS OR VIEWS EXPRESSED IN THE PARTICULAR VOLUMES

PREFACE

EARLY IN 1859 Charles Darwin submitted for publication a manuscript entitled "An abstract of an essay on the origin of species and varieties through Natural Selection." In November of that same year, it was published under the title of On the Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. Its publication touched off a violent controversy which, when the tumult had died down, left the theory of biological evolution firmly established. Evolution was not a new concept. In one way or another, it can be seen in the thoughts of mankind since the dawn of history. In more modem times it had been expounded by Darwin's own grandfather, by Count Lamarck, and, in a modified manner, by numerous geologists, including Charles Lyell. But the idea of descent with modification was still too much in conflict with the idea of special creation to be generally accepted. In 1858, however, Wallace and Darwin simultaneously showed that the Malthusian thesis On Population could apply equally well to plants and animals as well as to man and so enable the fittest in any species to survive and reproduce and so gradually change the character of the original species. Their papers were published in the pages of the Linnean Society of London but received little attention. In the following year, however, Darwin published his carefully documented volume in which, by citing the effects of selection by breeders of variations in cultivated plants and domestic animals so that the breeds could be changed in character and new varieties produced, he demonstrated a method by which in non-cultivated plants and wild animals, descent with modification could take place and new species could eventually be produced. This he called "Natural Selection," and in Natural Selection he provided a possible mechanism for evolution which could be understood by the layman. Natural Selection was a statement of fact, but Darwin himself did not believe that it was the only mechanism by which evolution could come about (he did not in fact use the word evolution until much later) and he had no real explanation as to how the selected variations were produced-only that the environment selected them and so permitted the fitter individuals to survive and so change the species.

vi

PREFACE

The success of the book was instantaneous, partly because the educated public was ready to listen to a reasonable explanation and partly because Thomas Huxley and Herbert Spencer were available and anxious to fight for the theory in the face of all opposition. The opposition from those who believed in single creations and catastrophism was considerable but merely served to fan the flame of public interest. Hence the concept of descent with modification spread and widened and the doctrine of Evolution did more to revitalize human thinking during the past century than any other force. It is for this reason that the Royal Society of Canada commemorated the centenary of the publication of The Origin of Species by organizing a symposium on Evolution at its annual meeting in June 1959. A great many discoveries have been made in the past hundred years bearing on the history of living creatures, on the methods by which variation takes place, on the mode and tempo of human thought, and even on the origin and development of inorganic creation. This volume of "Studia Varia" consists of papers on these subjects, by Canadians, presented at the symposium. They are not necessarily printed in the same order as read but are grouped more or less by subject-geological, biological, philosophical, sociological, and cosmological. THOMAS

W. M.

CAMERON

A NOTE ON GEOLOGICAL TIME

EVOLUTION IS THE HISTORY of living things throughout geological time. Geological time is measured in millions of years and fossils are recognizable during the past 500 of them. This long period is broken into several major divisions called eras: Palaeozoic ( or Primary) in which ancient animals and archaic plants flourished; Mesozoic ( or Secondary) which saw the rise of the vertebrates and the development of the reptiles; and Cenozoic (or Tertiary) which is the age in which we now live. These eras are divided further into periods, named, quite arbitrarily if somewhat illogically, after the places where sediments of that age were found or from some characteristic of the sediment itself. Cambrian (from Cymry, meaning Welshman) is so called because the earliest rocks from this formation were found in Wales and Cumberland. Similarly Silurian or Ordovician ( from the Roman names of tribes in southeast and in north Wales respectively) refer to the localities where these formations were first studied. Permian is named after Perm in eastern Russia, Devonian after Devon in England, and Jurassic from the Jura mountains. However, Carboniferous refers to the presence of coal, and Cretaceous to that of chalk in these two formations. The Cenozoic era, which occupies the last sixty million years or so, is further divided into subdivisions first established by Charles Lyell for successions of snails in various strata and since universally adopted and called epochs. The Pleistocene (meaning most recent) contained 95 per cent of species now living, and in descending order came the Pliocene (more recent), Miocene (less recent), Oligocene (few recent), Eocene (dawn recent) and Paleocene (old dawn recent) . These subdivisions are ( with the exception of the Pleistocene) sometimes called the T ertiary period while the Pleistocene is called the Quaternary. The sequence of life in these divisions can be traced with some accuracy, but the duration of each is only approximate. The diagram on the following page attempts to show this pictorially.

GEOLOGICAL ERA

PERIOD

EPOCH

TIME ABOUT 540 MILLION YEARS AGO

LOWER

CAMBRIAN MIDDLE

0

0 N

0

UPPER

:

LOWER

~

MIDDLE

~

...J

cc ORDOVICIAN L&J

...J

UPPER SILURIAN

ABOUT 330 MILLION YEARS AGO

DEVONIAN OR OL D RED SANDSTONE LOWER OR N MISSISSIPPIAN ~ CARBONIFEROUS each other are not clear, but they do not seem derivable one from another.

THE GEOLOGICAL RECORD OF EVOLUTION

9

In the ictidosaurs the mammalian stage is almost reached ( 1 ) . The reptilian jaw articulation of articular against quadrate is retained in vestigial form but side by side with it is the mammalian articulation of dentary against squamosal. The ictidosaurs were perfectly intermediate between reptiles and mammals, but it is very unlikely that they were the ancestors of all mammals. In the Jurassic period we find true mammals well established and divisible into five major groups on the basis of tooth structure. These groups are the Allotheria ( multituberculates), the Triconodonta, the Symmetrodonta, the Docodonta, and the Pantotheria. It has long been held (8, pp. 142-4) that the Allotheria were independently derived from a separate reptilian stock. The Tritylodonta, a group which has at last become well known by reason of recent discoveries, seems to be more or less intermediate between ictidosaurs and allotheres. Recent discoveries in the basal Jurassic of England have disclosed that the reptilian jaw articulation, at least in the vestigial form, was still present in the earliest docodonts and symmetrodonts ( 3). The living monotremes are so different from any other known mammals, living or fossils, that one is forced to accord them a very early origin, possibly at the reptilian level. The Triconodonta may be derived from the late Triassic Protodonta, a poorly known reptilian group of uncertain affinities. Thus, the evidence is strong that the passage from reptile to mammal was made independently in the ancestors of the allotheres, triconodonts, monotremes, and docodonts, with the symmetrodonts and pantothers possibly having a common origin. There is evidence among the earliest mammals of at least five separate crossings of the line between reptiles and mammals. Polyphyletic origin of birds is not supported by fossil evidence. Even if the two known Upper Jurassic birds represent separate genera, Archaeopteryx and Archaeornis, they are clearly close relatives and derivable from a common reptilian ancestor, but the many bird-like structures in the archosaurian reptiles, particularly the two orders of dinosaurs, show that there were several potential phyla leading towards the birds, whether or not only one crossed the arbitrary boundary. There are those who postulate a separate origin of penguins from that of all other birds, but this view is not supported by the anatomy, which indicates that penguins are descendants of birds in which some power of flight had been developed. Polyphyletic evolution is most impressive when seen in the development of one class of vertebrates from another, but fossil evidence indicates that it is common in the evolution of lower categories. At these

10

LORIS S. RUSSELL

levels it is usually described as parallel evolution. It is particularly well documented in the fossil record of mammals. On purely morphological grounds the dasyurids of Australia and the borhyaenids of South America should be in the same family, yet geological and geographical evidence forces one to believe that these two groups arose independently from opossum-like ancestors. Among the so-called Insectivora the zalamdodont condition seems to have been achieved independently in at least three lines. In fact the whole order Insectivora is generally acknowledged to be a polyphyletic one. The rodents display many examples of parallelism but probably the most striking one is the independent development of porcupines in Africa and South America. Many of these examples are used, not as arguments for parallelism, but for former continental connections. Careful study, as in the case of the porcupines, reveals fundamental morphological differences between the two branches. Polyphyletic development of the horse family in Miocene and Pliocene time is well established in western North America. Even Homo sapiens may have been polyphyletic in origin. Some anthropologists hold that the beginnings of the physical characteristics distinguishing the present-day races of mankind can be recognized in fossil hominids that are regarded as belonging to separate species or even genera. The palaeontological record as we know it today includes many examples of polyphyletic or parallel evolution in which the development of similar morphological features has been achieved in two or more independent lines. This was probably the rule rather than the exception. Given a similar genetic assemblage, upon which similar selective factors were operating, it seems probable that similar mutations would arise independently and for similar reasons would be preserved. Is it presumptuous to suggest that perhaps the genetic mechanism of inheritance includes genes that control the kinds of mutations that form, and that such factors are inherited like any others? This is beyond the power of the geological record to establish, but the fossil succession does reveal patterns the genetic basis of which must be postulated, and subsequently tested by the evidence of experimental genetics. REFERENCES 1. CROMPTON, A. W. (1958). The cranial morphology of a new genus and

2.

species of ictidosauran. Proc. Zool. Soc. London, 130, pt. 2: 183-216. ERIC (1952). On the fish-like tail in the ichthyostegid stegocephalians with descriptions of a new stegocephalian and a new crossopterygian from the Upper Devonian of Greenland. Meddelelser om Gr~nland, 114, no. 12.

JARVIK,

THE GEOLOGICAL RECORD OF EVOLUTION

11

3. KERMACK, K. A., and FRANCES MussETT (1958). The jaw articulation in Mesozoic mammals. XVth Internat. Zool. Congress, Section V, paper 8. 4. OLSON, E. C. (1944) . Origin of mammals based upon cranial morphology of the therapsid suborders. Geol. Soc. Amer., Spec. Paper no.

ss.

S. - - - ( 1947). The family Diadectidae and its bearing on the classification of reptiles. Fieldiana: Geology, 11, no. 1. 6. PEABODY, F. E. (1952). Petrolacosaurus kansensis Lane, a Pennsylvanian reptile from Kansas. University of Kansas, Paleont. Contrib., art. 1: 1-41. 7. SXVE-S0DERBERGH, G. (1932) . Preliminary note on Devonian stegocephalians from East Greenland. Meddelelser om Grj,'lnland, 94, no. 7. 8. SIMPSoN,'G. G. (1929) . American Mesozoic mammalia. Mem. Peabody Mus. Yale University, 3, pt. 1.

THE LOWER CAMBRIAN FAUNA Vladimir J. Okulitch,

F. R. s. c.

THE EARLY CAMBRIAN EPOCH contains the earliest well-known fauna and flora. Lower Cambrian fossils have been collected and described from every continent and it is possible now to form a fairly clear concept of their characteristics. It is true that some indications of life have been found in rocks older than the Lower Cambrian, but on closer examination many such "fossils" have turned out to be inorganic or the reported occurrences were shown to be of later age. At present our knowledge of Precambrian life is limited to some algae, worm burrows, and questionable imprints of one or two brachiopods, sponge spicules, and even more questionable coelenterata. This, plus some indirect evidence afforded by the presence of carbonaceous material, suggests that life was present in the Precambrian, but how extensive and how advanced it was remains unknown. It is, therefore, obvious that any consideration of the path of organic evolution has to start with an evaluation of the Lower Cambrian fauna, as the earliest known.* A generation ago it was optimistically predicted that further work would bridge the gap between the known Cambrian and unknown Precambrian faunas. However, this further work has tended to strengthen rather than to erase the demarcation between the Cambrian and the Precambrian. Apparently, all over the world the fossiliferous Lower Cambrian strata are underlain by essentially completely unfossiliferous layers of rock, which, except for the absence of fossils, are not different lithologically from the Palaeozoic rocks. Much has been written on the possible reasons for the absence or near absence of Precambrian fossils. This is not the place to repeat these speculations, and the writer has no new thoughts to offer. A very good summary was given by Professor Percy E. Raymond in his Presidential Address to the Paleontological Society in 1935. Practically all the hypotheses agree that the absence of *The writer must acknowledge the assistance offered by his graduate students to assemble information on the Lower Cambrian life. He is particularly indebted to Mr. Ray Yole, Mr. Darcy Scott, Mr. Parker Calkin, and Mr. Michael Williams who undertook the laborious task of searching through numerous publications to ensure that no significant information was overlooked.

THE LOWER CAMBRIAN FAUNA

13

skeletal tissue, rather than the absence of life, explains the lack of fossils in the Precambrian rocks. A rather common fallacy repeated by textbooks and lecturers alike is a statement that all the major animal phyla are represented in the Lower Cambrian. This is possibly true if the entire Cambrian period is included and if the Graptolites are regarded as members of the Hemichordata. However, if only the Lower Cambrian is considered the statement is far from true; in fact the early Cambrian fauna is extremely poor in the numbers of both genera and higher taxa. It is also noteworthy that the Lower Cambrian is largely unfossiliferous, the fossils occurring only locally. Such isolated fossiliferous localities may be rich in the number of individuals, but each locality is almost always dominated by a limited number of genera and species. This seems to indicate the possibility that early Cambrian life was concentrated in a relatively few favourable localities, and that each such locality supported a particular fauna appropriate to its environment. The spottiness is both horizontal (geographical) and vertical (stratigraphical). We can therefore visualize the Lower Cambrian life as one in the early stages of colonization of the sea bottom. Small isolated colonies were established here and there and only much later were these outposts of life knit together as populations grew. In general the rocks become more fossiliferous as one moves upward stratigraphically, and this probably reflects a rapid increase in the density of the population. It should also be kept in mind that the Lower Cambrian may have lasted about 30,000,000 years, and that a great many ecological and faunal changes have taken place during this time. No attempt has been made in this presentation to deal exhaustively with genera and species found in the Lower Cambrian. Rather this is an attempt to present an over-all picture, without going into details, of the life of the epoch. LOWER CAMBRIAN FAUNA

Only three phyla are represented by a reasonably large number of genera and individuals. In order of abundance of individuals they are Arthropoda, Archaeocyatha, and Brachiopoda. Protozoa, Annelida, Mollusca, Coelenterata, Echinoderma, and Algae occur in relatively small numbers. It is interesting to note that in 1890 Walcott listed 59 genera and 141 species as occurring in the Lower Cambrian of North America. In 1947 Raymond indicated that the total number of known Lower Cambrian species was 455 (19). My own estimate would be close to 900, the main

14

VLADIMIR J. OKULITCH

increase being in the known number of species of Archaeocyatha. In a rough estimate the species would be distributed as follows: Protozoa Porifera Archaeocyatha Coelenterata Echinoderma Vermes Brachiopoda Mollusca Arthropoda

1 per cent 1 30 1 1 3

21

10

32

PLANTS

As far as we are aware, no terrestrial plants existed during the Lower Cambrian; apparently the continents were bare of all forms of life. It should be noted that this dearth of plant cover, with a concomitant absence of soils, must have permitted a much more rapid erosion of the continents. Although algae were present in the seas, only those forms which could secrete calcium carbonate, or which had enough body to leave an impression on the sea muds are recorded. The lack of detail in such fossils has led to the lumping of various forms together under such designations as "fucoids" and "girvanella." PROTOZOA

The presence of Protozoan fossils is somewhat doubtful. Foraminifera described from Russia and Newfoundland have not been recognized as such by Dr. Cushman. The specimens which were referred to the foraminifera are all arenaceous, simple or non-coiled genera. The Radiolaria reported from Thuringia, Brittany, and Australia, are either uncertain or the age of the rocks in which they have been found is under question. 1 One might expect that Protista would be numerous in the oldest fossiliferous beds. However, their small size and fragility of their tests apparently ruled against them. PORIFERA

The sponges are represented in the Lower Cambrian by disjointed siliceous spicules. These include the Silicispongia Leptomitus and Protospongia occurring in the Lower Cambrian of eastern North America 1 A. S. CampbeJI in Moore, Treatise on Invertebrate Paleontology, D. Protista D 19.

THE LOWER CAMBRIAN FAUNA

15

( 34). According to Shimer and Shrock ( 24), Choia and a calcareous sponge Camarocladia range throughout the Cambrian. ARCHAEOCYATHA

The Archaeocyatha are exclusively Cambrian marine organisms with world-wide distribution. They lived in large numbers on calcareous bottoms, forming "gardens" or carpets of sessile benthos, but lacked the ability to build topographically prominent bioherms. Where present, the number both of individuals and of species is extremely high. Vologdin ( 30) estimates the total known number of species as approaching 600. Even after allowing for probable synonymy and the general variability of individuals, the number of distinct species is exceptionally high. However, the geographical extent of each "colony" is limited and the colonies are spaced far apart. The fact that similar species are known from localities ranging from Australia to Labrador suggests a wide dispersal by ocean currents of at least the larva. In North America and Australia the Archaeocyatha became extinct at the end of the Lower Cambrian. In Russia they may have continued till the end of the Middle Cambrian. No discernible descendants have been recognized in later times. The phylum is, therefore, a short lived one. A rather well-marked evolutionary development is discernible during the Lower Cambrian. The earliest representatives have simpler and coarser skeletons while the latest exhibit exquisitely detailed and highly porous skeletons. All the skeletons are composed of calcium carbonate. Thus, as pointed out by Zhuravleva (35; 36) and Okulitch and Greggs ( 12), the early Lower Cambrian is characterized by Archaeocyathus, Protopharetra, and similar genera, the middle horizons by Ethmophyllids, whereas the highest zones contain coscinocyathids and Pycnoidocyathus. COELENTERATA

Supposed coelenterata are rare and poorly preserved in the Lower Cambrian. Camptostroma and Brooksella are regarded by some as early Cambrian scyphozoans. Although it is difficult to imagine preservation of a jelly-fish in Cambrian sandstone, the specimens mentioned above, as well as some reported from Australia, strongly resemble such an animal. It is probable, therefore, that soft-bodied coelenterata lived in the Lower Cambrian seas. Occasionally imprints of their bodies were

16

VLADIMIR J. OKULITCH

preserved in bottom muds. On the other hand, there is no authentic record of corals from Lower Cambrian strata. VERMES

Worm-like creatures were presumably present in early Cambrian times. The evidence exists in the form of burrows, trails, castings, and a few agglutinated tubes. Undoubted annelids were present in the faunas of the Middle Cambrian Burgess shale. Vertical burrows are commonly referred to Scolithus; it has been suggested that these may have been made by phoronids but some were possibly the work of annelids. Worm castings or coprolites have been reported from a number of localities. These consist of cylindrical pellets and rope-like structures. In some cases, trails have been given generic names such as Cruziana, Planolites, and Climactichnites. It is quite possible that some of these structures are in reality algal in origin. BRACHIOPODA

The brachiopods of the Lower Cambrian are generally small, thinshelled and primitive. Both the Articulata and the Inarticulata are present. The inarticulate forms have their shells made mainly of chitinophosphatic material. In contrast, the Articulata are generally calcareous. The class Inarticulata comprises examples of two orders, the Atremata and Neotremata, represented by Lingulella, Acrotreta, Micromitra, and Obolella. The Articulata are represented by Rustella, Kutorgina, and Nisusia. Rustella and Nisusia have simple, sub-equivalved shapes, with imperfectly developed articulation. Kutorgina has a definite interarea and possibly is an ancestral orthid. These representative forms have a world-wide distribution, and occur at various stratigraphic levels of the Lower Cambrian. They are found in arenaceous, argillaceous, and calcareous rocks, and were apparently able to live under a variety of ecological conditions. No obvious evolutionary changes are discernible within the Lower Cambrian; indeed, real evolutionary progress did not begin until Ordovician times. MOLLUSCA

The Mollusca are represented in the Lower Cambrian by several classes. None of these, with the possible exception of the gastropods and some of uncertain affiliation, are found in large numbers.

THE LOWER CAMBRIAN FAUNA

17

Class Pelecypoda Undoubted pelecypods are rare in the Lower Cambrian. Walcott (34) lists Fordilla and Modioloides from eastern North America; both are small and lack detail. Poulsen (15) described better specimens of Fordilla from East Greenland. While it is possible that these objects are carapaces of Crustacea, both Walcott and Poulsen prefer listing them with the pelecypods. Class Gastropoda Primitive gastropods with low, cap, or cup-like shells are found throughout Lower Cambrian rocks. Most typical of these are the genera Scenella, Helcionella, and Discinella. Their shells have an oval outline and are low cups, with the apex subcentral or anterior. Knight (5) places these in the Monoplacophora. The genus Oelandia, while resembling H elcionella, has an extended or up-tilted margin similar to some Bellerophontids. Its apex is not coiled. Knight (5) places this genus in the Coreospiridae; it may be ancestral to the later Bellerophontacea. The genus Pelagiella resembles gastropods externally. However, Knight ( 5) and Wenz ( 31 ) express doubts as to its systematic position. It is here listed with incertae sedis. These shells are symmetrically coiled, and if they are gastropods they lived at the same time as the much more primitive Monoplacophorans. Class Cephalopoda Authorities differ on whether cephalopods are present in the Lower Cambrian. The difference of opinion hinges on whether Salterella and V olborthella are in fact cephalopods. Both Salterella and V olborthella are small, conical, septate shells. They are circular in section and possess a central axial tube or siphuncle. They have been classed as cephalopods and as a separate and extinct phylum. The lack of bilateral symmetry seems to remove them from the Mollusca. The controversy has been well summarized by Rousseau H. Flower ( 3). Until it is resolved it appears most suitable to regard the genera mentioned above as incertae sedis. I ncertae Sedis The fairly common Lower Cambrian genera: Hyolithes, Hyolithellus, and Helenia have been placed with Pteropods, benthonic annelids, "worms," and even with the conularids. In general these are conical shells of circular, elliptical, or subtriangular cross-section. An operculum

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VLADIMIR J. OKULITCH

is present in some. No agreement exists as to their relationship to other known organisms. ECHINODERMA

Very few fossils assignable to this phylum have been found in the Lower Cambrian. These few fossils consist of isolated plates, and have been named Eocystites and Stromatocystites. Presumably they were primitive cystoids. A single genus Macrocystella is regarded by some as a primitive crinoid. Edrioasteroids are represented by a few specimens. PHYLUM ARTHROPODA

The Arthropods, represented largely by trilobites, were the dominant form of life during the Lower Cambrian. They appeared suddenly and in such variety and high degree of organization and specialization that it is impossible to avoid the conclusion that they had a long history preceding the Cambrian. The trilobites will be dealt with first. Class Trilobita The five generally recognized orders of the trilobites are represented in the Lower Cambrian by four orders. They comprise the Agnostida, small blind trilobites, with two thoracic segments and similar cephalon and pygidium; Eodiscida with dissimilar cephalon and pygidium, small number of thoracic segments, and either with or without eyes; Olenellida, which differ from other trilobites by their lack of facial sutures, small pygidium, and large number of thoracic segments; and the Opisthoparia, the largest and longest-lived of trilobite groups, differentiated by their facial sutures, holochroal eyes, and a great variety of cephalic, thoracic, and pygidial structures. The Olenellida range throughout the Lower Cambrian; the Opisthoparia appear in the second half of the Lower Cambrian. Being mobile in both their larval and their adult stages, trilobites gained a very wide geographical distribution. In a significant recent paper, Christina Lochman-Balk and James L. Wilson (10) thus describe the Lower Cambrian biostratigraphy: The Olenellida were world-wide in distribution and were apparently already an old stock before they acquired a preservable exoskeleton. Olenellus, Fremontia, Wanneria and Paedumias had acquired a preference for the intermediate and shelf biofacies around Laurentia, and Callavia, Holmia, Kjerulfia and Olenelloides for the extracratonic-open ocean environment. Nevadia appears early in the Lower Cambrian in the Cordilleran region where it is associated with Olenellus and Paedumias in westernmost miogeosynclinal sites. Nevadella appears in the later half of the epoch in the same sites and with the same associations. Nevadia and Nevadella have also

THE LOWER CAMBRIAN FAUNA

19

been recorded from the Acado-Baltic province in the late Lower Cambrian beds associated with Cal/avia. Elliptocephala was indigenous to a boundary biofacies adjoining the inner edge of the extracratonic euxinic realm in eastern North America, and since it is not known north of Massachusetts it is suspected of having southern origins. Although no sharp fauna} boundary is present between the Lower and Middle Cambrian epochs, all the Olenellida disappeared at this time while the opisthoparian genera underwent a rapid expansion. The interesting Australian and south Asian genus Redlichia was thought to be restricted to the Lower Cambrian, but recent studies by Opik (13) indicate that it crosses the boundary into the Middle Cambrian. The Olenellida are probably the most primitive of trilobites, because of the large number of thoracic segments, poorly differentiated pygidia, and lack of facial sutures. These would seem to indicate relative proximity to the ancestral annelid stock. It is rather difficult to decide whether they became completely extinct by the end of Lower Cambrian or gave rise to such forms as Paradoxides and Redlichia. It is at least open to speculation based on study of larval stages, that olenellids may have given rise to the earliest arachnoids and eventually to Limulus. Arthropods of somewhat uncertain systematic position which occur in the Lower Cambrian are: Protocaris, Tuzoia, Bradoria, Indiana, Dyelymella, Hipponicharion, Beyrichona, Hymenocaris, and Anamalocaris. Tuzoia and Anamalocaris are fairly common in the Lower Cambrian of the Canadian Rocky Mountains. Tuzoia, Hymenocaris, and Anamalocaris are classed with the Hymenocarina of the Homopoda. This class includes a diversified group of extinct shrimp-like animals. Some of the others are placed with the Archaeostraca. This is a poorly known group. Nothing, for instance, is known about their appendages and segmentation. Some of the Lower Cambrian fossils defy all attempts at classification. Such, for instance, is Stenotheca. It consists of a small, oval, low conical shell, which may be crustacean or molluscan in origin. Another is Girvanella. Its minute tubular structure is usually considered to indicate an algal origin. Oldhamia is a small branching structure and possibly also represents an alga. In this general survey of the Lower Cambrian fauna it becomes obvious that several taxa are unrepresented. These include Ctenophora, most worm phyla, Bryozoa, a number of molluscan classes, a great many divisions of the Arthropods, the Hemichordata, and the Chordata. Other phyla such as Protozoa, Coelenterata, and Echinoderma are represented by a very few and usually doubtful species.

20

VLADIMIR J. OKULITCH

Anyone attempting the study of Lower Cambrian fossils is impressed by the difficulty of placing many of the early animals into the existing pigeon-holes of the systematist. Some examples of this have been given for the Mollusca. Undoubtedly many fossils represent groups which are quite unrelated to those now in existence. The largest group of such organisms is the Archaeocyatha, which cannot be placed either with the Porifera or with the Coelenterata, and must be grouped into a distinct phylum. Doubtless there are others. It is, of course, not at all strange that in the remote past groups existed which have left no descendants. These represent the short dead branches on the tree of life. REFERENCES 1. CLARK, T . H . (1925). On the nature of Salterella. Trans. Roy. Soc., 19 : 1-12. 2. COOPER, G. ARTHUR, et al. (1952) . Cambrian stratigraphy and paleontology near Caborca, northwestern Sonora, Mexico. Smithsonian Misc. Coll., 119 : 1-184. 3. FLOWER, R. H. ( 1954). Cambrian cephalopods. New Mexico Institute of Mining and Technology, Socorro, N . Mexico, Bull. 40: 1-51. 4. GRABAU, A. W. (1936) . Palaeozoic formations in the light of pulsation theory, vol. I, Lower and Middle Cambrian. The National University of Peking, Univ. Press. 5. KNIGHT, J. BROOKS (1952). Primitive fossil gastropods and their bearing on gastropod classification. Smithsonian Misc. Coll., 117: 1-56. 6. KOBAYASHI, T. ( 1937) . On Salterella canulata and its allies. Jap. Jour. Geol. and Geog., 14 : 173-83. 7. - - - (1943) . Outline of the Cambrian faunas of Siberia. Proc. Imperial Acad., Tokyo, 19 : 205-14. 8. - - - (1958). On some Cambrian gastropods from Korea. Jap. Jour. Geol. and Geog., 29 : 111-18. 9. LocHMAN, CHRISTINA (1956). Stratigraphy, paleontology and paleogeography of the Elliptocephala asaphoides strata in Cambridge and Hoosick quadrangles, New York. Geol. Soc. Amer. Bull., 67 : 1331-96. 10. LocHMAN-BALK, CHRISTINA and JAMES LEE WILSON (1958). Cambrian biostratigraphy in North America. four. Paleont., 32 : 312-50. 11. OKULITCH, V. J. (1955) . Archaeocyatha: Treatise on invertebrate paleontology, Part E: E 1-E 20. 12. OKULITCH, V. J. and R. G. GREGGS (1958) . Archaeocyathid localities in Washington, British Columbia, and the Yukon Territory. Jour. Paleont., 32: 617-23. 13. OPIK, A. A. ( 19 50) . The Cambrian trilobite Redlichia : Organization and generic concept. Commonwealth of Australia, Department of National Development, Bureau of Mineral Resources, Geology and Geophysics, Bull. 42 : 1-40. 14. OPIK, A. A. et al. (1957). The Cambrian geology of Australia. Commonwealth of Australia, Department of National Development, Bureau of Mineral Resources, Geology and Geophysics, Bull. 49: 1-284.

THE LOWER CAMBRIAN FAUNA

21

15. POULSEN, Ora. (1932). The Lower Cambrian faunas of east Greenland. Meddelelser om Gr~nland, 87, no. 6: 1-66. 16. - - - (1958). Contribution to the palaeontology of the Lower Cambrian Wulff River formation. Meddelelser om Gr~nland, 162, no 2: 1-24. 17. RAsETTI, FRANCO (1943). Lower Cambrian trilobites from the conglomerates of Quebec. lour. Paleont., 22: 1-24. 18. - - - (1955). Lower Cambrian ptychopariid trilobites from the conglomerates of Quebec. Smithsonian Misc. Coll., 128: 1-35. 19. RAYMOND, P. E. (1947). Prehistoric life, chaps. 111 and IV, pp.19-39. Harvard University Press. 20. RICHTER, RUDOLF and EMMA RICHTER (1948). Zur Frage des UnterKambriums in Nordost-Spanien. Senckenbergiana, 29: 23-39. 21. RODGERS, JoHN (ed.) ( 1956). El Sistema Cambrico, su paleogeografia yel problema de su base. Symp. XX Internat. Geol. Congress, Mexico, pts. I and II. 22. SHAW, ALAN B. (1954). Lower and Middle Cambrian faunal succession in northwestern Vermont. Geol. Soc. Amer. Bull., 66: 1033-46. 23. - - - (1955). Paleontology of northwestern Vermont. V, The Lower Cambrian fauna. Jour. Paleont., 29: 775-805. 24. SHIMER, H. W. and R. R. SHROCK (1943). Index fossils of North America. Wiley. 25. SHINDEWOLF, 0 . H . ( 1956). Ueber Prakambrische Fossilien. Geotektonisches Symposium zu Ehren von Hans Stille, pp. 455-80. 26. SHROCK, R. R. and W. H. TWENHOFEL ( 1953). Principles of invertebrate paleontology. McGraw-Hill. 27. SPRIGG, R. C. (1947). Early Cambrian (?) jellyfishes from the Flinders Ranges, South Australia. Trans. Roy. Soc., S. Australia, 71, pt. 2: 212-24. 28. TASCH, PAUL (1952). Adaptive trend in eyeline development in the Olenellidae. Jour. Paleont., 26: 484-8. 29. TERMIER, GENEVIEVE and HENRI TERMIER (1950) . Invertebres de l'ere primaire. 30. VOL0GDIN, A., E. LERMONTOVA, B. YAVORSKY and M. TANISCHEVSKY (1940). Atlas of the leading forms of the fossil faunas of the U.S.S.R., vol. I, Cambrian, 1-193. 31. WENZ, W. (1938). Gastropoda. Handb. Palaozool., brsg. O.H. Shindewolf, 6, Lief 1, VIII, 1-240. 32. - - - (1940). Ursprung und friihe stammes-geschichte der gastropoden. Arch. fiir Molluskunde, 72: 1-10. 33. WHITEHOUSE, F. W. (1939). The Cambrian faunas of north-eastern Australia, pt. 3, Polymezid trilobites. Queensland Museum, Mem., 11: 179-282. 34. WALCOTT, C. D. (1890). The fauna of the Lower Cambrian or Olenellus zone. U.S.G.S. Tenth Annual Report for 1888-9, pt. 1: 511-760. 35. ZHURAVLEVA, I. I. (1951). On the age of Archaeocyatha horizons of Siberia. Contr. Acad. Sci. U.S.S.R., 80: 97-100. 36. ZHURAVLEVA, I. I. and K. K. ZELENOV ( 1955). On the bioherms of the multi-colored suite of the Lena River. Contr. Acad. Sci. U.S.S.R., 56: 57-77.

EVOLUTION OF PALAEOZOIC LIFE: ORDOVICIAN TO PERMIAN

M. Y. William~, F.R.s.c. INTRODUCTION THE PALAEOZOIC ERA, or the Era of Ancient Life, is dated from 520 to 185 millions of years ago, covering a lapse of time of some 335 millions of years. This great time interval has been divided (in America) into seven sub-equal periods by great changes in the earth which have left their record, fragmental but significant, in sedimentary formations distributed over the continents. The length of each period was more than ample for forms of life to adapt themselves through evolutionary changes to the great variety of conditions arising from earth movements and their consequences-the building of mountains and the changes in land and sea relations. During the era, changes in environment were adequate for great advances in life, from the simpler marine algae and invertebrates of the early Cambrian to ancient conifers and mammal-like reptiles of the close of the Permian. Starting at the close of the Cambrian period, some 80 millions of years in length, this paper considers changes in life as recorded by fossils for the succeeding six periods : Ordovician, eighty millions of years in length; Silurian, forty million years; Devonian, fifty-five million years; Mississippian, thirty million years; Pennsylvanian, twenty-five million years; and Permian, twenty-five million years.1 The fossil record is limited in many particulars. Preservation of dead animals and plants is an accidental process depending upon their escape from scavengers and natural decay, accidental burial, presence of hard parts, and the chemical interaction between the enclosing sediments and the dead object; in the case of teeming marine life, hard insoluble parts and accumulating sediments assure abundant fossil records. Near shore or littoral remains are generally comminuted by animals, waves, and currents. Land forms, exposed to predation, exidation, erosion, and

lThe Mississippian and Pennsylvanian are termed lower and upper Carboniferous in Europe.

EVOLUTION OF PALAEOZOIC LIFE

23

generally lacking burial, are rarely preserved-perchance in swamps, in lake or river deposits, or in the estuaries of rivers. Of the fossil record but a very small part has been discovered or will ever be examined. The outcrops of sedimentary rocks make up only a very small fraction of the thousands of feet of formations occupying the great basins of the continents. Again, the sea contains the only complete record of sedimentation, a record accessible only in a very minor way. In spite of the incompleteness and imperfection of the fossil record, it is the only direct testimony of the form and environment of the life of the past, and in reality there is far more information available than can be used. William (Strata) Smith, writing between 1816 and 1820, noted that "each stratum contained organized fossils peculiar to itself," and, as an English engineer engaged in building canals, he made use of the fossils which he found in the rocks to identify the beds in different localities. Thus guide fossils were recognized. Further study has shown the appearance of simpler forms before more complex, illustrated in the case of phyla, classes, orders, and particularly by genera and species. Response to environment is marked: sandstones, shales, limestones, dolomites, salt, freshwater, and continental formations have remains of animals and plants of distinctive character. The succession of strata in the geological column provides a relative time scale for contained fossil remains by which correlation of beds from place to place is carried out. Reference to age in years is gradually being approximated by physical-chemical means. As guide fossils have been given particular study they are used extensively in this article for their evolutionary significance ( 7). FOSSIL PLANTS

Although the preservation of marine plants is rarely good, fossil "algae" were apparently common in the Precambrian rocks, some having been described and named, for example, Newlandia concentrica Walcott from the Beltian Series of Montana. Various Ordovician forms are recognized and by lower Silurian time fucoids, as they are known, had acquired distinctive characters. The deltaic deposits of Devonian time contain early records of land plants which had gradually become established in the newly formed uplands of the Appalachian region and elsewhere. Development of the land flora kept pace with the spread of land and swamp conditions during the Pennsylvanian period. The increasing aridity and lowering of

24

M. Y. WILLIAMS

temperature during the later Permian checked the growth and expansion of plants as it did of life in general. Nonvascular Plants ( All quotations in this and the following sections are from Arnold.) Bacteria or similar objects, for instance, Micrococcus and Bacillus, are reported by C. E. Bertrand and Bernard Renault in rocks from Devonian to Jurassic age in Europe and Russia. These and other finds appear to verify the presence of bacteria in very ancient times but as they are recognized solely on their resemblance to modem bacteria the proof of their evolution is nil. Fungi: "Throughout the Devonian and Carboniferous there is ample evidence of fungus activity within plant tissues although the actual remains of the organisms are seldom present." "The fossil record has thrown no light on the problem of the evolution or origin of fungi." Algae: "Although there is substantial evidence that algae have been in existence since pre-Cambrian times, the fossil record shows us rather little concerning the evolution of this group of thallophytes through the vast stretches of the Paleozoic, Mesozoic, and Cenozoic eras." "This comparative stability is the result of the aquatic environment in which abrupt changes were lacking." Bryophyta: Fossil remains are rare. Early Vascular Plants Psilophytales, "the simplest and possibly the oldest of the known vascular plant groups," is based on Psilophyton princeps described from the Gaspe sandstone (Middle Devonian Erian) by Sir William Dawson in 1858. Other ancient vascular plants include Schizopodium, Hamilton, New York; Barinophyton, Middle and Upper Devonian of eastern Canada, western New York, Maine, Germany, South Wales, and Norway; Aneurophyton, Middle Devonian of Germany and New York; Eospermatopteris, stumps, etc., in Middle Devonian of Gilboa, New York. Ancient Lycopods According to Arnold ancient lycopods "developed rapidly during the Devonian and early Carboniferous, and reached their developmental climax during the middle of the latter period. They suffered a sharp decline during the Permian, and appear in greatly diminished numbers in the rocks of the Mesozoic."

EVOLUTION OF PALAEOZOIC LIFE

25

Ancient Scouring Rushes and their Relatives Arnold says that "Their earliest representatives appeared during the Devonian, the group became a conspicuous element of the flora of the Carboniferous, and their subsequent decline has reduced them . . . in the Recent flora [to] 1 genus and some 25 species." The Ancient Ferns "The oldest ferns (middle Devonian) are only partly distinct from their psilophytic forebears, but in the late Devonian we find them . . . as prominent elements of the flora. Ferns increased markedly in species and in numbers of individuals during the early Carboniferous and they had become a diversified plant group before the close of the Paleozoic coal age. Some of the Paleozoic ferns appear to be related to modem families." The Pteridosperms (Seed Ferns) "It is probable that the pteridosperms originated at some time during the Devonian period from fernlike stock which had not advanced far beyond the psilophytic stage." "The only evidence we have of the existence of pteridosperms in the Devonian consists of a few petrified stems. No organs positively identified as seeds have been found below the Lower Carboniferous. In the Pocono and Price formations, which lie at the base of the Mississippian and overlie the Devonian in the eastern part of North America, we find . . . cupulate seeds of the Lagenostoma or Calymmatotheca type." "The pteridosperms suffered a great decline during the Permian. They were abundant at the beginning, but with the onset of widespread aridity which was followed in some parts of the earth by extensive glaciation, many of the genera became totally extinct. The few that survived to the end of the period did so in greatly modified form." The Cordaitales "The Cordaitales is a group of Paleozoic gymnosperms which, together with the seed-ferns, constituted the bulk of the seed plants of the Carboniferous coal forests." "Their origin is obscure, and the oldest known members reveal such an advanced state of development that it is certain that they were preceded by a long evolutionary stage which is entirely unknown." The Ancient Conifers "The geological history of the conifers begins with the Upper Carboni-

26

M. Y. WILLIAMS

ferous (Pennsylvanian) epoch, and the order probably reached its developmental climax in the late Jurassic or early Cretaceous." THE ANIMAL KINGDOM Animals represented by distinctive Palaeozoic fossils are considered below under their biological classification. PHYLUM I: PROTOZOA CLASS: SARCODINA Order: Foraminifera These single-celled organisms are common as fossils from Cambrian to Recent, ranging in size from microscopic to several inches in diameter in the Tertiary. Some of the most primitive tests are composed of chitin, others of sand grains, sponge spicules, or mica flakes cemented by chitinous, siliceous, or calcareous secretions. Most shells are composed of calcium carbonate. Foraminifera have changed so continuously as to become excellent guide fossils. The Fusulinidae, shaped superficially like grains of wheat, are common and form reliable guides for Pennsylvanian and Permian formations. They died out with the Permian. PHYLUM II : PORIFERA (Sponges) Classes Silicispongiae and Calcispongiae are represented from Cambrian to Recent time. Of the Silicispongiae, 8 genera are guides to the Cambrian; 4 to the Ordovician; 2 to the Silurian; 1 to the Devonian; 1 to the Mississippian; and 1 to the Pennsylvanian. Of the Calcispongiae 4 genera extend from the Cambrian to the Pennsylvanian. Sponges are rarely well preserved and their identification generally depends upon a microscopic study of their spicules. A number of forms in the Ordovician, Silurian, Devonian, and Permian are, however, distinctive, for example, Receptaculites (Ordovician), Astraeospongia (Silurian), and H ydnoceras (Devonian). PHYLUM III: COELENTERATA CLASS 1 : HYDROZOA Of this class there is practically no Palaeozoic record. The graptolites are considered below. CLASS 2: STROMATOPOROIDEA These great reef-builders lived a life comparable to the compound corals of their time. Their skeleton consisted of laminae and pillars of

EVOLUTION OF PALAEOZOIC LIFE

27

calcareous material perforated by canals. Generally massive or discoid, they attained a maximum diameter of about two feet. One aberrant genus of Ordovician time, Beatricea, was nearly cylindrical with a vesicular axis, and attained a length of more than twelve feet with a diameter of more than twelve inches. Confined to the Palaeozoic, guide genera have been recognized as follows : Ord. 6 Ord.-Dev. 1 Sil.-Dev. 3 Dev.

CLASS

3:

1

GRAPTOZOA

The grnptolites, having many characters common with the hydroids, were confined to the Palaeozoic era and were especially characteristic of the muddy seaways of Ordovician time. The distribution of the two orders is outlined below ( reference is to guide fossils) . Order 1: Dendroidea (U. Camb.-L. Miss.) Genera : U. Camb. I U. Camb.-U. Sil. 2 U. Camb.-L. Miss. I Ord.-Sil. I Ord.-Dev. 1 Sil. 2 Dictyonema-6 species U. Camb. I ; L. Ord. 2; m. Sil. I ; L. Dev. 1; m. Dev. I. Order 2: Graptoloidea (Ord.-Sil.) Genera: Ord. 41 Ord.-Sil. 1 Sil. 2 Didymograptus-7 species L. Ord. 5; m. Ord. 2. Climacograptus-6 species M. Ord. 4; U. Ord. 2; Diplograptus-7 species I Ord. Canada, United States, Europe, and Australia. 3 M. Ord.-1 common to Europe and North America. I L. Ord. North America and Europe 2 U. Ord. The Graptoloidea demonstrate the world distribution of planktonic life during the early Palaeozoic most conclusively.

28

M. Y. WILLIAMS

4: SCYPHOZOA The family Conularidae is Palaeozoic in age and includes four genera characterized by pyramidal shells ( 3 genera), tubular shells (1 genus) Conularia is represented by 6 species: 1 M. Ord.; 1 Sil.; 1 L. Dev.; 1 M. Dev.; 1 Miss.; and 1 Penn. CLASS

Two similar genera occur sparingly in middle Ordovician. CLASS

5:

ANTHOZOA

1: TETRACOROLLA (Ordovician to Permian). These cupcorals, generally horn-shaped, are strictly Palaeozoic. Represented sparingly in the Upper Ordovician, they are common but limited in genera and species in the Silurian, reach their maximum development in diversification and numbers in the Devonian, and die out with the Permian. Seventeen genera are restricted to the Devonian, and fifteen more are recognized for the period, of which ten extend upward from older formations and five extend upward into the Mississippian. The genus Cystiphyllum has five species : one Middle Silurian and four Middle Devonian of North America and the Eifel of Europe. SUBCLASS 2 : scu1zocoRALLA ( Ordovician to Mesozoic) . These honeycomb corals are best known from the genus Favosites, with nine well-known species ranging as follows : Silurian 2; Lower Devonian 1; Middle Devonian 6. As pointed out by Lambe, Silurian species of Favosites have spines on their outer wall which are replaced by squamulae in Devonian species.2 The genus Paleofavosites differs from Favosites in having mural pores in or near the angles of the corallites. The species aspera d'Orbigny (prolificus Whiteaves) extends from the base of the Richmondian into basal Lockport ( or from Upper Ordovician into Middle Silurian) on Anticosti Island (in Manitoba and Wyoming about the same) . On Manitoulin Island it is common and appears to be confined to the basal Silurian (Manitoulin) dolomite. SUBCLASS 3: ALCYONARIA (Ordovician to Recent). 2 Palaeozoic genera: Ord. Sil.-Dev. 1 Appendix to Alcyonaria Genera : Ord.-Sil. 1 Sil.-Dev. 5 Sil.-Penn. 1 Sil.-Perm. 6 Dev. 2 Penn. 1 Dev.-Penn. 3 2L. M. Lambe, Contributions to Canadian Paleontology (1899) , IV, Part I. SUBCLASS

EVOLUTION OF PALAEOZOIC LIFE

29

Of the above, Coenites has six recognized species: two Silurian, three Devonian, and one Permian. Family Halysitidae (chain corals) The chain corals have attracted wide attention because of their attractive form, good preservation, and common occurrence in two widespread reef horizons-in the Ordovician Richmond and the Silurian Niagaran. Mistaken identity of genera and species without regard to evolutionary tendencies has caused much confusion in determining the age of the formations, especially in northern Canada. Buehler (2) has divided the family into two genera: Catenipora, mostly Ordovician (ten species versus six Silurian) and Halysites, confined to the Silurian ( fifty-five species). Hamada ( 4) has indicated the "Evolutional trends in the Halysitidae" as follows: to

Ordovician 1. Without mesocorallites

2. Small corallites 3. Rectangular autocorallites 4. Horizontal tabulae

5. Without septal spinules 6. Small lacunae

Gotlandian With mesocorallites Large corallites Rounded autocorallites Strongly convex tabulae; dissepiment-like incomplete tabulae in mesocorallites With septal spinules; rudimentary septa! spinules Large irregular, narrow or meandering lacunae of coralla

Size of corallites varies with age and more particularly with environment, the optimum being in mid-Silurian Niagaran, or Gotlandian time of magnesium-rich seas, widespread reef growth, and large, thick-shelled molluscs and brachiopods. Among the tabulate corals Syringopora is conspicuous with species distributed as follows: Niag.-Sil. Mid-Dev. Miss.-Penn. Penn.

2 3 1 1

SUBCLASS 4: HEXACORALLA. This dominant subclass in modern seas appeared in the Triassic, replacing the ancient forms which did not survive the Appalachian Revolution. The Hexacoralla are not recognized in the Palaeozoic formations.

30

M. Y. WILLIAMS

PHYLUM IV: ECHINODERMA SUBPHYLUM 1: PELMAT0Z0A CLASS CLASS CLASS CLASS

1: 2: 3: 4:

CYST0IDEA EDRIOASTER0IDEA BLAST0IDEA CRIN0IDEA

The first three classes of crinoid-like marine forms are confined to the Palaeozoic rocks, which include "encrinal" limestones composed mainly of fragments of tests and columns with some well-preserved calyces of these prolific creatures. The cystids and edriosters climaxed in the Ordovician, the blastoids in the Devonian, Mississippian, and Pennsylvanian. The class Crinoidea clearly had common ancestors with the other Pelmatozoa and their comminuted remains are commonly indistinguishable. Three subclasses are confined to the Palaeozoic, as are all but the higher orders, reaching their maximum variety and numbers in the Ordovician. Well represented in the Silurian and Devonian, another climax occurred in the lime-depositing seas of the Mississippian and Pennsylvanian. Only the suborder Articulata includes post-Palaeozoic Crinoids, with five families extending from the Triassic to the Recent. SUBPHYLUM 2: ELEUTHER0Z0A CLASS 1 : STELLER0IDEA This class includes the Asteroidea ( true starfishes) which were common in the Palaeozoic but generally poorly preserved, the Auluroidea which were confined to the Ordovician, and the Ophiuroidea or brittle stars which are known from the Mississippian and are common today. CLASS 2: ECHIN0IDEA ( sea urchins) One order ranged from the Silurian through the Permian, one from Mississippian to Recent, and one from Jurassic to Recent. CLASS 3: H0L0THUR0IDEA (sea cucumbers) These range from Cambrian to Recent. PHYLUM V: ANNELIDA The phylum Annelida ( segmented worms) has left clearly recognized fossil remains. "Teeth" or silico-chitinous jaws are known from the Ordo-

EVOLUTION OF PALAEOZOIC LIFE

31

vician to the Eocene; tubes from Ordovician to Recent time; and burrows from Cambrian to Devonian. CON0D0NTS ( an unclassified group of organisms) Small ''jaws" composed of calcium phosphate much like apatite, are associated with unctuous shales and clays of Palaeozoic age. The oldest are simple conelike forms, but they develop into highly complicated shapes and are excellent horizon markers. Conodonts are generally assumed to represent the jaw armour of an extinct order of primitive fishes. PHYLUM VI : BRY0Z0A (Polyzoa) CLASS

1:

ENTOPROCTA

This class has no known fossils. CLASS

2:

ECTOPROCTA

This class has freshwater and marine forms; the latter have a good record and many index fossils. Bryozoa were common among other reef-building organisms of the Palaeozoic seas. Of five orders, two are entirely Palaeozoic; two are Ordovician to Recent, and one is Mesozoic to Recent. PHYLUM

VII:

BRACHIOPODA

Brachiopods were plentiful in all normal Palaeozoic seaways and are found in all kinds of sediments except conglomerates, saline deposits and associated shales, some mudstones, and red beds. Their hard parts-shells, spiralia, etc.-are generally well preserved and their vast numbers and ever-changing species and genera make brachiopods par excellence the best known guide fossils of Palaeozoic time. After an explosive development of advanced genera and species in Pennsylvanian and Permian time, brachiopods all but disappeared with the close of the Palaeozoic, leaving only a few forms to continue down to the present. Brachiopods first appear in Cambrian formations, the class Inarticulata or hingeless forms being most common. Their phosphatic shells opened by sliding sideways over each other. It is notable that the family Lingulidae has survived to the present with almost no change. The modern genus Hyattidia, differs from Lingula of the early Palaeozoic in shell markings which could scarcely survive on fossils from Ordovician time. Articulata or hinged brachiopods appear in later Cambrian time and subsequent variations are in hinge structure, the foramen through which

32

M. Y. WILLIAMS

the pedicle passes, and the support for the brachia. These supports developed from simple crura to most elaborate calcareous spiralia in the Spiriferacea. Recent classification places emphasis upon the character of the carbonate shells of the Articulata, whether lmpunctate, Pseudopunctate, or Punctate. Only eight out of twenty-one superfamilies had representatives in the Mesozoic and only six have living species ( 9). Speciation is exemplified in the numerous varieties found together as in the case of Mucrospirifer pennatus of Hamilton (Upper Devonian) beds at Thedford, Ontario, and elsewhere. Specimens from the same bed and location, with approximately the same height (.75 inches) may have a length of hinge line varying from 1.1 to 2 inches. The wider hinges are drawn out to fine points. Brachiopods reached maximum proportions in Silurian time, some of the pentameroids being three or more inches in height. PHYLUM

CLASS

VIII:

1:

MOLLUSCA

PELECYPODA-LAMELLIBRANCHIA

Pelecypods are aquatic animals living freely in marine, brackish, and fresh water, for the most part at moderate depth. Appearing in the late Cambrian, these animals have changed mostly in hinge structure, siphonal development, and shell shape; some have become attached to the substratum, some have become burrowers, some borers in rock or wood, and some free swimmers. Pelecypod shells consist generally of calcium carbonate, in whole or in part in the form of aragonite which does not fossilize as well as does calcite, and therefore many shells are preserved as moulds or casts and their detailed character is lost. For this reason, perhaps more than their little marked evolutionary changes, pelecypods are rarely good guide fossils, especially in the Palaeozoic. They are much more important in later sediments. New families appeared as follows (9): Camb. Ord. Sil.

Dev.

Carb.

Perm.

Trias.

Jur. Cret.

1 10 10 9 ( some fresh and brackish water) 3

1 14 15 18

Eocene Miocene-Pliocene Pleistocene-Recent

15

2 3

33

EVOLUTION OF PALAEOZOIC LIFE CLASS

2:

GASTROPODA

Gastropods or snails are more highly developed than pelecypods in the following characters: they have a well-developed head commonly provided with stalked eyes and sensory organs, and a mouth with a radula set with teeth and in some forms with horny jaws; a body with a one- or two-chambered heart; a digestive tract with liver, kidneys, and numerous glands; gills or lungs are common; the foot is developed for locomotion by crawling by rhythmic waves, in some cases for jumping, and in still others for swimming. Reproduction is by eggs or in some cases the young are born alive. The shell is single, composed of calcite or aragonite, with a chitinous epidermis in many forms. Lower Cambrian gastropods generally had low-spired or cap-shaped shells. Many families were present in Ordovician time but generally with smooth, flat-coiled or low helical shells. Pulmonate gastropods are found in the Devonian, land snails in the Carboniferous, and freshwater forms from Jurassic time onward. As in the case of pelecypods, Palaeozoic gastropods are generally represented only as moulds or casts and so prove poor guide fossils. Shimer and Shrock recognize two Palaeozoic subclasses. ( 1) Protogastropoda Order: Cynostraca Genera: Camb. Camb.--Ord. Ord. Ord.-Sil. ( 2) Eugastropoda Order 1 : Archaeogastropoda Genera: U. Camb. 1 Camb.--Ord. 1 Ord. 28 Ord.-Sil. 13 Ord.-Dev. 3 Ord.-Miss. 1 Ord.-Penn. 1 Ord.-Trias. 2 Sil. 3 Sil.-Dev. 7 Sil.-Perm. 5 Order 2: Mesogastropoda Genera: Ord.-Perm. Dev. Miss.-Penn. Penn.-Perm.

5 3

1 1

Dev. Dev.-Penn. Dev.-Perm. Miss. Miss.-Penn. Miss.-Perm. Miss.-Trias. Penn. Penn.-Perm. Perm.

1 1 1 1

8 3 3

1

7

10 1 6 6

1

34

M. Y. WILLIAMS

Order 3: Neogastropoda Genera: Ord.-Sil. Dev.-Perm. Miss. Miss.-Penn. Miss.-Perm. Penn. Penn.-Perm. Other Gastropoda are Mesozoic to Recent. CLASS

3:

I

1

1 1 1 1 I

SCAPHOPODA

These molluscs, sometimes known as "elephant tusk shells," show little change through the ages and are probably as common in the present near shore environment as at any time in their history. Genera: Dentalium; Ord.-Recent (The subgenus P/agiog/ypta is Carb.-Trias.) Cadulus: Cret.-Recent Mollusca: Jncertae Sedis Hyolithes: Camb.-Perm. Tentaculites: Ord.-Dev. Styliolina: Dev. CLASS

4:

AMPHINEURA-CHITONS

Order I: Polyplacophora (common chitons) (L. Ord.-Recent) 1 Genus Chiton; Species: Ord. Penn. ?-Recent I Tert.-Recent 2 Pleistocene-Recent 3 Order 2: Aplacophora (Recent) CLASS

5:

CEPHALOPODA

This, the highest class of molluscs, is represented from the Cambrian to the present by free-swimming predaceous marine animals, typically housed in chambered shells, only the outer chamber being occupied. Excellent vision is possessed by modern forms, the paired eyes being in some respects superior to those of the vertebrates. Of three recognized subclasses, only the Nautiloidea and Ammonoidea are known from Palaeozoic rocks. SUBCLASS 1: NAUTILOIDEA (Cambrian to Recent). The animals of this class lived in shells with successive living chambers separated from the abandoned chamber by a simple concave septum drawn backwards into a siphuncular tube and joining the wall of the shell in a smooth simple curve or suture line. Evolution was marked by lengthening of the

EVOLUTION OF PALAEOZOIC LIFE

35

shell cone, changes in shell aperture, modifications of the siphuncle sheath, and curvature of the shell. Order 1: Protochoanites, Camb., e.g. Valborthella, L. and M. Camb. Salterella, Camb.-Ord. Order 2: Holochoanites, Ord.-Sil. Breviconic-moderately curved 15 Genera: Ord. 1 Ord.-Sil. Order 3: Mixochoanites U. Ord.-Sil. Genera: Ord. 2 1 Ord.-Sil. Order 4: Orthochoanites Ord.-Recent Penn. 5 Genera: Ord. 26 2 Miss. Ord.-Sil. 8 Miss.-Penn. 2 Ord.-Dev. 4 Miss.-Perm. 3 Ord.-Miss. 1 Penn. Sil. 11 5 1) (Trias.-Tert. Sil.-Dev. 4 ( Cret.-Tert. Dev. 11 4) Dev.-Penn. 4 SUBCLASS 2: AMMONOIDEA (Upper Silurian to Cretaceous) . The ammonites are characterized by complex and generally angulated and intricate suture lines. Recognized as goniatites in the Upper Silurian, representatives of this subclass had developed highly complex but relatively smooth suture lines during Carboniferous time. The characteristic ammonite suture line, however, began with the Jurassic. Along with complexity of suture line went as a rule involute coiling of the shell in one plane. Order 1: lntrasiphonata (Devonian). The siphuncle is dorsal.

Genus-Platyclymenia-Devonian.

Order 2: Extrasiphonata (Upper Silurian-Cretaceous). The siphuncle is ventral. This order includes the goniatites and ammonites. Genera: U. Sil. Dev. Dev.-Penn. Miss. Miss.-Penn. Miss.-Perm. Penn. Penn.-Perm. Perm. Perm.-Trias.

rare

6 2 7 2 1

14 5 15 1

Typical ammonites are confined to the Mesozoic.

36

M. Y.

WILLIAMS

SUBCLASS 3: COLEOIDEA (Dibranchia) Order 1 : Belemnoidea Order 2: Sepioidea All are Mesozoic and later. PHYLUM IX: ARTHROPODA CLASS 1: CRUSTACEA SUBCLASS 1: AGN0STIA (Lower Cambrian to Ordovician). Thirteen genera are listed from Cambrian, none from Ordovician. SUBCLASS 2: TRIL0BITA (Lower Cambrian to Permian). Of some sixty-eight genera of trilobites listed for the Cambrian only four occur in the Ordovician ( 7) . Post-Cambrian trilobites: 3 Of seventy-nine genera listed as guide fossils. thirty-five are confined to the Ordovician, thirteen to the Ordovician and Silurian, and thirteen are exclusively Devonian. Six genera are represented in the Mississippian, two in the Pennsylvanian, and three in the Permian ( of one family). Trilobites thus culminated early in the Palaeozoic and were exterminated by the changes which marked its close. Their evolution was marked by maximum size ( twenty-seven inches in length) in the Ordovician. The rise of cephalopods and of fishes in the Devonian probably marked the doom of the trilobites. SUBCLASS 3: H0M0P0DA. Order Pseudonotostraca is represented by the genus Protocaris in the Lower and Middle Cambrian. Order Hymenocarina is represented by Hymenocaris in the Cambrian of North America and of Europe. SUBCLASS 4: XEN0P0DA. This subclass is represented by Sidneyia in the middle Cambrian. SUBCLASS 5: ARCHAE0STRACA (Cambrian to Mississippian). One order in Cambrian. One order extended from Ordovician to Mississippian ( only five prominent genera). One order with two genera in the Devonian. BTwenhofel and Shrock (9) report by the close of the Cambrian period 100 genera and nearly 1,000 species of trilobites. Many genera were extinct before the opening of the Ordovician but new forms appeared and by the close of the Ordovician there were 1,200 species in 125 genera; a decline set in and only 600 species belonging to 40 genera are known from Silurian strata. Only 40 genera and 200 species are known from the Devonian; and from the Upper Devonian, family after family died out, only a single family with few representatives remaining in the Permian, and all died out at its close. No new families appear after the Ordovician.

EVOLUTION OF PALAEOZOIC LIFE

37

6: BRANCHIOPODA (Lower Cambrian to Recent). genus-Lower Cambrian genus-Devonian (Schizodiscus) genus-Devonian to Recent (Estheria) genus-Pennsylvanian to Permian (Leaia)

SUBCLASS

One One One One

SUBCLASS 7: OSTRACODA ( Ordovician to Recent). These excellent guide fossils have the following Palaeozoic range of genera.

Ord. Ord.-Sil. Ord.-Dev. Ord.-Carb. Ord.-Recent Sil. Sil.-Dev. Sil.-Carb. Dev. SUBCLASS

8:

11 3 10 7 4 5 4 5 7

Dev.-Carb. 4 (Dev.-Jur.) 1 (Dev.-Tert.) 1 Miss. 14 Miss.-Penn. 5 Miss.-Perm. 5 Penn. 3 Penn.-Perm. 1 (Mesozoic and later 21)

CIRRIPEDIA (BARNACLES)

Genera:

(Cambrian to Recent) 2 1 2 2 1 1

Camb.-Dev. Ord.-Dev. Dev. Cret. Cret.-Recent Eocene-Recent

SUBCLASS 9: MALACOSTRACA (Cambrian to Recent) Division: Syncarida ( Carboniferous to Recent) Genus-Palaeocaris (Pennsylvanian) Division: Peracarida ( Carboniferous to Recent)

Genera: Dev.-Miss. Penn.

1

Division: Eucarida Order: Decapoda (Cretaceous to Recent) Genera: Jura.-Recent Cret. Cret.-Oligo. Cret.-Recent Tert. Eo.-Recent Mio.-Recent

2

1

2 1 1 1 2 1

2: MYRIAPODA This class is composed of centipedes and millipedes (Devonian to Recent). There are three genera in the Pennsylvanian of America. The myriapods were among the first recorded air-breathers. CLASS

38

M. Y. WILLIAMS CLASS 3: INSECTA

SUBCLASS: APTERYG0TA. These are primitively wingless, and are not definitely known before the Cretaceous but are probably ancestral to the subclass Pterygota known from Upper Carboniferous. Examples of the latter are Palaeodictyoptera (Mississippian to Permian) and Blattaria (cockroaches) which constitute 60 per cent of the insects of the Upper Carboniferous period, 30 per cent of the insects of the Permian, and 10 per cent of the insects of later periods ( 7) . Twenhofel and Shrock list eleven other orders from the Carboniferous, all extinct excepting Blattaria, the modern cockroaches. The Protodonta ( dragonfly forms), ranging from Pennsylvanian into Triassic time, reached a wingspread of over two feet. The archaic forms became more modern in Permian time, Raymond reporting representatives of eleven existing orders. The Carboniferous insects record the first mastery of the air. CLASS 4: ARACHNIDA SUBCLASS 1: MER0ST0MATA (Cambrian to Recent) Order ( 1) : Aglaspida 1 Genera: Camb. M. Camb. 1 U. Camb. 3 Dev.-Recent Order (2): Xiphosura U. Carb. Euproops Dev.-Perm. Paleolimulus Meso.-Recent Limulus Camb.-Perm. Order ( 3) : Eurypterida 2 Genera: Ord.-Sil. Ord.-Perm. 1 Sil.-Dev. 1

SUBCLASS 2: EMB0L0BRANCHIATA (Silurian to Recent) Penn. 4 genera PHYLUM X: CHORDATA SUBPHYLUM: VERTEBRATA SUPERCLASS: PISCES CLASS 1: AGNATHA (jawless vertebrates) This primitive class of vertebrates is possibly represented by Palaeospondylus found in the lower Old Red sandstone of Caithness, Scotland. The specimens, generally less than an inch in diameter, represent the whole skeleton.

EVOLUTION OF PALAEOZOIC LIFE

39

The Ostracoderms belong in this class and may be represented by scales and plates found in Ordovician sediments in Colorado, Wyoming, and Michigan (3). Upper Silurian ostracoderms are represented by distinctive remains. These jawless bony-plated fish reached their maximum diversity and size (twelve inches) in the Devonian (Norway, Spitzbergen, Greenland, and the British Isles). The ostracoderms flourished during the Devonian period and disappeared at its close. 2: PLACODERMI (with primitive jaws) Placoderms developed to 30 feet in length in Devonian time and all were extinct before Mississippian time except the Acanthodii which disappeared in Permian time. Some remains are found in marine deposits but the placoderms are thought to have lived mainly in rivers and estuaries. The placoderms are the oldest known jawed vertebrates, the jaws having developed, it is thought, from the third pair of gill arches. "The placoderms were evolutionary experiments that failed" ( 3). Examples: Acanthodians or "spiny sharks" (Silurian to Permian). Arthrodires ( armoured fishes) dominate in late Devonian time with a length up to thirty feet (Silurian to Devonian). Antiarchs, for example, Bothriolepis (Devonian of Gaspe), Macropetalichthys (Devonian), Stegoselachians ( Devonian of Europe). CLASS

3: CHONDRICHTHYES (sharks) Genera: Cladoselache in Upper Devonian Cleveland shales south of Lake Erie-about three feet long. Pleuracanthus evolved during Carboniferous and Permian times, and invaded fresh water. Modern sharks are of cladoselachian ancestry. CLASS

4: OSTEICHTHYES (bony fishes) The oldest known bony fishes lived in fresh water and belonged to the order Palaeoniscoidea, for example, the Devonian Cheirolepis. From such an ancestry the subclass Actinopterygii passed through three stages, the primitive Chondrostei being Devonian through Permian in age with a few forms surviving into the Recent. The other two superorders, Holostei and Teleostei, are Mesozoic to Recent. SUBCLASS: CHOANICHTHYES ( air-breathers )-lungfishes or dipnoans and crossopterygians. These fishes comprise a subclass of Osteichthyes parallel to the subclass Actinopterygii. Both subclasses appeared in Devonian times; perhaps their most significant differences were the internal narial openings of the choanate fishes and the arrangement of the supporting bones of their limbs which CLASS

40

M . Y. WILLIAMS

"consisted of median or axial elements with lesser bones radiating either on the sides or distally from these central members" ( 3) . The choanate fishes were on the line of evolution of the higher vertebrates. Dipnoans or lungfishes. The earliest lungfishes are represented by Dipterus of middle Devonian age. "They reached their greatest variety in late Devonian and late Palaeozoic times" ( 3). Ceratodus was Triassic in age. Epiceratodus, extant in Australia, is a direct descendant of Ceratodus and the living genera Protopterus of Africa, and Lepidosiren of South America have evolved as side branches of the central stem of dipnoan evolution. Crossopterygians or lobe fins. Osteolepis of the middle Devonian represents the generalized Crossopterygian. Two suborders evolved: the Rhipidistia, primarily freshwater fishes and the Coelacanthini. Holoptychius was a late Devonian representative of a side branch of the Rhipidistia. Another Devonian genus, Eusthenopteron was "obviously on the direct line toward the early amphibians," as shown by its skull pattern ( 3) . The Rhipidistia disappeared with the Palaeozoic. The Coelacanths were predominantly marine and far removed from the main line of evolution. They were especially characteristic of the Mesozoic era but are still represented in the Indian Ocean by Latimeria discovered in 1938-9 and found again in 1952. SUPERCLASS: TETRAPODA CLASS

1:

AMPHIBIA

A supposedly amphibian footprint was found in the Upper Devonian rocks of Pennsylvania and bones of primitive amphibians occur in Upper Devonian sediments in east Greenland. These sediments may be transitional into Mississippian sediments. The ichthyostegid amphibia had solidly constructed skulls similar to those of the crossopterygian fishes, and according to Colbert, "at some time during the Devonian period . . . some of the crossopterygian fishes came out on the land." There followed the labyrinthodont or stegocephalian amphibians with teeth similar, in infolded enamel structure, to those of the crossopterygian fishes and with solidly roofed skull and underbody plating for protection against the rough land surfaces. Lungs, legs, eyes, and ears all developed in response to periodic land environment. Changes in the skeleton in shape and ossification provide strong support for the theory that this was a land-living animal. The mostly aquatic embolomeres of the Mississippian and Pennsylvanian were ancestral to the Permian reptiles and from them the anurans or tailless amphibia are probably descended. The rhachitomes, on the other hand, originated

EVOLUTION OF PALAEOZOIC LIFE

41

in the Mississippian and developed in the Pennsylvanian and Permian periods-Eryops of the Lower Permian was a typical example. The rhachitomes died out early in the Triassic leaving some specialized descendants to develop into the Triassic Stereospondyls. The Lepospondyls were characterized by vertebrae formed directly as spool-like, bony cylinders around the notochord. This varied group appeared in the Mississippian and reached its climax of development in the Pennsylvanian and Permian periods. They are ancestral to modern salamanders and coecilians. CLASS

2:

REPTILIA

SUBCLASS 1: ANAPSIDA (no temporal opening in the skull behind the eye). Order 1: Cotylosauria. Seymouria found near the top of the lower Permian of Seymour, Texas, is typically reptilian in skull, digits, etc., and yet it is in many ways intermediate between reptiles and the embolomerous amphibians, for example, in skull bones and teeth, including those on palatine bones. There is no proof that it originated from an amniote egg. It is considered a basic member of the cotylosaurs. From this order the Chelonia (turtles) and eunotosaurs are descended (Cenozoic). Other reptiles springing from the cotylosaur stem are the aquatic mesosaurs, appearing during the Pennsylvanian and the eosuchians (Petrolacosaurus, from the Pennsylvanian of Kansas and Youngina from the Permian of South Africa) . They were ancestral to the diapsid reptiles ( dinosaurs and modern reptiles) . SUBCLASS 2: SYNAPSIDA (mammal-like reptiles). Synapsids occur in rocks of late Pennsylvanian age, and bridged the gap between the primitive reptile and the mammal before the close of the Triassic period. "There was an early trend toward a differentiation of the teeth into anterior incisors, enlarged canines, and laterally placed cheek teeth" ( 3) . Order 1. Pelycosauria. The earliest synapsids known are the pelycosaurs of the late Pennsylvanian and lower Permian of North America, particularly of Texas, Oklahoma, and New Mexico. Of the three suborders, Ophiacodonts, Sphenacodonts, and Edaphosaurs, the first were generalized lizard-like forms of Permian age, largely fish-eating and reaching five to eight feet in length; the second suborder developed into large terrestrial carnivores, with highly differentiated teeth and elongated vertebral spines supporting a great dorsal fin, for example, Dimetrodon; and the third suborder Edaphosaurs were terrestrial plant feeders some of which had dorsal fins with cross-bars on the vertebral spines.

42

M. Y. WILLIAMS

Order 2: Therapsida. In contrast with the Pelycosaurs, the Therapsids were of middle and late Permian and Triassic age and are known especially from the Karroo sediments of South Africa. Their skulls tended more and more towards mammalian characters in temporal opening, jaw attachment, palate, and differentation of teeth. The neck became differentiated from the body, the legs were drawn under the body and became adapted to walking and running rather than sprawling. Suborder A. Aromodontia. The Permian Tapinocephalians were archaic, massive, upland herbivores reaching a probable weight of 1,000 pounds. The Dicynodonts appeared in the middle Permian and became in the late Permian some of the most common reptiles. In Triassic time they attained world-wide distribution and large size. The jaws were covered with turtle-like beaks and except for tusks in some of the skulls, the teeth were reduced to tiny remnants or were absent altogether. Suborder B. Theriodontia. The Theriodonts were small to large carnivorous reptiles, which occurred in middle Permian deposits and developed during the remainder of the Permian period and into the early and middle Triassic period. Known widely in the Old World, they are most common in the Karroo beds of South Africa. These were the most mammal-like of the therapsids and are represented by Jonkeria of the Permian with heavy body, stout limbs, elongated snout armed with large incisors, and long piercing canines. The Permian genus Lycaenops is typical of the gorgonopsians in which many trends are shown which culminated in later theriodonts, for instance, large dentary bones, differentiated but not highly specialized teeth, lack of secondary palate, and single occipital condyle. Mammalian trends are clearly demonstrated in the lower Triassic genus Cynognathus. About the size of a wolf, this reptile had a long, narrow skull, an enlarged temporal opening, highly specialized and differentiated teeth ( quite dog-like), nasal passage separated from the mouth, jaw hinged to quadrate bone, a double condyle formed by exoccipital bones, and a strongly differentiated vertebral column, etc. CONCLUSION

The successive occurrence of orders and genera of plants and animals during the Palaeozoic era is clearly shown by the fossil record, closely co-ordinated with land and sea relations accompanying the building of continents. Pelagic and deep-water forms were least affected and changes were slow. Shore life, land, and freshwater forms were subject to great variations, climatic and otherwise, and here evolution was

EVOLUTION OF PALAEOZOIC LIFE

43

rapid. The passing of the graptolites, stromatoporoids, tetracorals, most of the brachiopods, the trilobites, primitive fish, stegocephalian amphibia, ancient reptiles, seed-bearing ferns, most of the coal-age forest, and the primitive insects indicates changes in world-wide conditions, marking the close of the era. The early air-breathers-the myriapods, scorpions, and insects-were obviously derived from marine worm-like forms and arthropoda. Some of the eurypterids were much like scorpions in shape. The evolutionary ladder from fish to amphibian to reptile is clearly demonstrated in general character by Palaeozoic fossils and the theriodont reptiles of the Permian have characters suggestive of mammalian ancestry. The reptilian ancestors of the birds are to be sought in Triassic formations. In the plant world, the marine algae have come down from Cambrian time with little recorded change. The spore bearers became established on land in Silurian time and formed the major part of the forests during the great coal age-Carboniferous. The conifers were well established in Permian time. Compared with the low lying, half submerged continents of Ordovician time, the highland masses bordered by Appalachian and contemporaneous mountain chains, common at the close of the Palaeozoic Era, stood in as great a contrast as did the life of the two periods. From the foothills, Late Permian mammal-like reptiles and their associates looked out over the emergent cemeteries of ancient marine life, past forests of ginkgos, cycads, primitive conifers and their associated vegetation, to a sea where relic and more recent forms mingled. The sunrise of tomorrow would usher in the Age of Reptiles, toothed birds, higher forms of life both plant and animal, which would inhabit the sea, the rivers and lakes, the land, and even the air. REFERENCES 1. ARNOLD, CHESTER A. (194 7).

2. 3. 4.

5. 6.

An introduction to paleobotany. 1st ed. McGraw-Hill. BUEHLER, EDWARD J. (1955). The morphology and taxonomy of the halysitidae. Peabody Museum of Natural History, Yale University Bull., 8. COLBERT, EDWIN H. (1955). Evolution of the vertebrates. John Wiley & Sons. HAMADA, T . (1957). On the septa) projection of the halysitidae. On the classification of the halysitidae, I, II. Jour. Fae. Sci., University of Tokyo, Section II, X: 383-430. MOORE, RAYMOND C. ( 1958). Introduction to historical geology, 2d. ed. McGraw-Hill. RAYMOND, PERCY E. (1947). Prehistoric life. Harvard University Press.

44

M. Y. WILLIAMS

7. SHIMER, HERVEY W. and SHROCK, ROBERT R. (1949). Reference index fossils of North America. 4th printing. 8. STOVALL, J. WILLIS and BROWN, HOWARD E. (1954). The principles of historical geology. Ginn and Co. 9. TwENHOFEL, WILLIAM H. and SHROCK, ROBERT R. (1935). Invertebrate paleontology. McGraw-Hill.

EVOLUTION OF DENTAL PATTERNS IN THE LOWER VERTEBRATES

Gordon Edmund IN THE STUDY of the evolution of mammals, changes in the structure of the cheek teeth have been used probably more than any other criterion. Indeed, some palaeomammalogists have been accused of studying the evolution of molars, not mammals. However true this may be, little has been done with the study of the evolution of the dentition of lower vertebrates. Richard Owen's Odontography ( 6) is still the only encyclopaedia of vertebrate dentition, but it of course does not contain any evolutionary proposals. Romer ( 7) gives an excellent summary of the dentition of most reptilian types, with a general introduction to problems of implantation, replacement, etc. Edmund ( 3) described the sequence in which teeth were laid down in embryos and adults, and the order in which they were replaced, stressing the basic alternating replacement pattern found in both fossil and modern forms. His bibliography contains most of the pertinent references. Some attempts have been made to use such features as mode of implantation or mechanism of replacement (2; 5), and the dental problems arising from the transition from therapsid reptile to mammal are currently being investigated. Probably the main reason for the lack of an evolutionary scheme based on dentition in the lower vertebrates is the similarity of tooth form in most types. The simple conical tooth is found from fish to reptile, and is usually lacking the various crests, lophs, cingula, etc., characteristic of the mammalian molar. Teeth of various diverse reptilian groups often bear close resemblance to one another, as for example, those of certain plesiosaurs, ichthyosaurs, and mosasaurs. Under close examination they are clearly distinct, but the characteristics used to distinguish them are not necessarily superficially obvious. Similarly, in smaller groups such as the lizards, there may be a close resemblance between species in rather distantly related forms (for example, some anguids resemble some teiids) or specialized types may approach each other as examples of convergence ( the durophogous dentitions of Varanus niloticus [family Varani-

46

GORDON EDMUND

dae], and Dracaena guianensis [ family Teiidae]). Regional differentiation is rare in reptiles, though enlarged teeth in the canine area are occasionally seen, and there are a few types with specialized cheek teeth ( for example, Diadectes [ order Cotylosauria]). Some features which can be used in a study of the evolution of lower vertebrate dentition are as follows: (i) Form of individual teeth . The simple conical form is often modified in various ways. Sometimes these modifications are examples of convergence, being specializations appearing here and there in various taxonomic divisions (for example, the leaf-like form of teeth in the ornithopoda) . (ii) Method of attachment. While standard terms are used to denote specific sites of implantation ( 7) they do, in fact . intergrade subtly with one another. Teeth are retained in the jaw either by bony fusion (ankylosis) or by non-calcified connective tissue. (iii) Method of replacing individual teeth. Virtually all lower vertebrates possess the happy faculty of polyphyodonty, the ability continually to replace elements of their dentition. In the case of a young alligator, Edmund ( 3) found that the average life of a tooth was twelve months, but other creatures, for example snakes and lizards, may replace teeth even more rapidly. The method by which the old tooth is lost, the position of the replacement during both early and later growth, and the development of various accessory structures during the replacement process all vary considerably, and modifications in the whole mechanism of individual replacement can be traced from group to group. (iv) Highly specialized innovations. Fundamental changes have taken place in certain groups, resulting in the development of new types of dental apparatus. Such changes may be characteristic of whole genera or families, and are thus of fundamental importance in the establishment of new higher taxa. The most striking example of this is the dental battery of the hadrosaurian dinosaurs. (v) The sequence in which the teeth are replaced. The teeth of lower vertebrates are replaced throughout life, not haphazardly, but in a definite, orderly sequence. In the embryo, teeth are produced on the dental lamina, a plate of specialized oral epithelium invaginated below the surface near the jaw bones. As shown in Figure 1, the first tooth is laid down at the anterior end of the lamina, and subsequent teeth appear in caudad sequence. Since the anterior teeth are older than the more posterior, the series produced is thus graded in size, decreasing from anterior to posterior. When the teeth in one such series have reached a certain size, production of a second series is initiated, then

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POSITION 1

FIRST ANLAGE,

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POSITION 4

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SECOND ANLAGE, "'-o---.____POSITION 1 _ • _ _ _•___.__ _ __

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10 ' - - ~ - ~ - ~ - ~ ~ - ~ - ~ - ~ ~ - - , - - - ~ ~ ~ ~ - - , - ~ 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

MASS

NUMBER

FIGURE 2. A plot of the abundances of heavy nuclei according to the mechanism of their formation. The points represent those nuclides which have been made predominantly by one of the three mechanisms of nucleogenesis effective in this region.

this way it is possible to sort out three mechanisms of formation in the heavy element region. The abundance level associated with each of the three groups of nuclei is shown in Figure 2. It may be seen that there are interesting finer details associated with each of the processes. Let us now conduct a brief survey of the chemical evolution of a star which is much more massive than the sun and which runs through the various stages of its active life history very quickly, sometimes in just a few million years. All the stars start with a composition in which hydrogen forms the majority of the mass. The first major source of energy in the star involves the conversion of this hydrogen to helium. The

231

THE ORIGIN OF THE ELEMENTS

CONVERSION OF HYDROGEN TO HELIUM PROTON - PROTON

CHAINS:

H1 (p,,8+v)D 2 (p,y) He 3 (He\2p) He 4

PP I

"'(

~