Systematics and Morphology of American Mosasaurs 9781933789453

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Systematics and Morphology of AMERICAN MOSASAURS

Systematics and Morphology of AMERICAN MOSASAURS

DALE A. RUSSELL With a foreword by Jacques A. Gauthier

Peabody Museum of Natural History Yale University

Distributed by Yale University Press NEW HAVEN AND LONDON

“Systematics and Morphology of American Mosasaurs” was first published in November 1967 by the Peabody Museum of Natural History as Bulletin of the Peabody Museum of Natural History Volume 23 Facsimile edition published online in 2005 Published in 2020 by the Peabody Museum of Natural History, Yale University, P.O. Box 208118, New Haven, CT 06520-8118 U.S.A. peabody.yale.edu on the cover: Disarticulated skeleton of the Late Cretaceous mosasaur Platecarpus tympaniticus (YPM VP 001350.A) in a slab of Niobrara Chalk from Butte Creek, Gove County, Kansas, U.S.A, collected by S. W. Williston and crew in August 1877. This specimen was originally O. C. Marsh’s holotype of Holosaurus abruptus Marsh, 1880. From an album of black-and-white photographs of fossil vertebrates on display in the early twentiethth century in the original Peabody Museum of Natural History building at Yale University in New Haven (YPM VPAR 002251). Photographer unknown, early 1900s. Courtesy of the Division of Vertebrate Paleontology, Yale Peabody Museum of Natural History. frontispiece: Tylosaurus by Charles R. Knight (1874–1953), under the direction of Henry Fairfield Osborn; gouache on canvas board, 1899. AMNH Photo Studio (photographer). Courtesy of the American Museum of Natural History Research Library, Digital Special Collections (image ptc-2425).

ISBN 978-1-933789-44-6 (pbk.) ISBN 978-1-933789-45-3 (ebook) © 2020 Peabody Museum of Natural History, Yale University. All rights reserved. This book may not be reproduced, in whole or in part, including illustrations, in any form (beyond that copying permitted by Sections 107 and 108 of the U.S. Copyright Law and except by reviewers for the public press), without the written permission of the publisher. Printed in the Unites States of America Distributed by Yale University Press | New Haven & London yalebooks.com | yalebooks.co.uk Library of Congress Cataloging-in-Publication Data Names: Russell, Dale A., author. Title: Systematics and morphology of American mosasaurs / Dale A. Russell ; with a foreword by Jacques A. Gauthier. Description: New Haven : Peabody Museum of Natural History, Yale University, 2020. | First published in 1967. | Includes bibliographical references and index. | Summary: “Presents the complete, classic 1967 monographic revision of the mosasaurs then known from North America, which was the foundation the modern era of mosasaur research, describing mosasaur form and function, their habits and habitats, and their phylogenetic relationships from the species level to more inclusive taxa”-- Provided by publisher. Identifiers: LCCN 2019054115 (print) | LCCN 2019054116 (ebook) | ISBN 9781933789446 (trade paperback) | ISBN 9781933789453 (ebook) Subjects: LCSH: Mosasauridae--United States. | Paleontology--Cretaceous. | Paleontology--United States. Classification: LCC QE862.L2 R87 2020 (print) | LCC QE862.L2 (ebook) | DDC 567.9/5--dc23 LC record available at https://lccn.loc.gov/2019054115 LC ebook record available at https://lccn.loc.gov/2019054116

CONTENTS LIST OF ILLUSTRATIONS

vii

FOREWORD ABSTRACTS (ENGLISH, RUSSIAN, FRENCH)

x

INTRODUCTION

4

HISTORY OF CLASSIFICATION

8

' 1

GENERAL DESCRIPTION OF MOSASAUR SKULL Muzzle unit Parietal unit Occipital unit Pterygoicl units Epipterygoid units Scapes units Quadrate units Anterior lower jaw Posterior lower jaw Marginal dentition

13 14 27 29 41 45 45 46 49 51 54

FUNCTIONAL ANATOMY OF CRANIAL SKELETON Sensory functions Cranial circulation Cranial kinesis Streptostyly Mandibular movements Conclusions

58 58 59 60 63 64 65

GENERAL DESCRIPTION OF POSTCRANIAL SKELETON Cervical vertebrae Dorsal vertebrae Caudal vertebrae Pectoral appendage Pelvic appendage

70 70 76 78 82 97

FUNCTIONAL ANATOMY OF POSTCRANIAL SKELETON Functional anatomy of the vertebral column Functional anatomy of the forelimb Functional anatomy of the hindlimb Swimming in mosasaurs

107 107 Ill 115 119

SYSTEMATICS Subfamily Mosasaurinae . Clidastes M osasaurus Amphekepubis Liodon Globidens Plotosaurus Foreign genera of Mosasaurinae Subfamily Plioplatecarpinae

121 123 124 131 141 142 144 145 147 148

vi

CONTENTS

Pla tecarpus Ectenosaurus Plioplatecarpus Prognathodon Plesiotylosaurus Halisaurus Foreign Plioplatecarpinae Subfamily Tylosaurinae Tylosaunts Foreign Tylosaurinae Mosasaurs of uncertain taxonomic position

148 156 1 58 162 167 108 169 170 170 176 176

STRATIGRAPHIC AND PALEOECOLOCIC CONSIDERATIONS Gulf Coast Cretaceous New Jersey Cretaceous Niobrara Formation Pierre Formation Additional occurrences of American mosasaurs Foreign occurrences of mosasaurs Ecological conclusions

185 185 186 186 187 189 190 191

EVOLUTIONARY CONSIDERATIONS Mosasaurs compared with platynotan lizards Phylogeny of platynotans Phylogeny of mosasaurs Evolution of mosasaurs

193 193 199 202 204

APPENDIX A—MEASUREMENTS

208

APPENDIX B—LOCALITIES OF PIERRE MOSASAURS

210

LITERATURE CITED

212

INDEX

225

CHARTS 1-7

231

PLATES 1,2

239

ILLUSTRATIONS TEXT FIGURES

I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

Diagnm of the functional units of a mosasaur skull Mosasaur premaxillae Frontal of Mosasaurus maximus Undersurface of frontals Prefrontals Right palatine of Platecarpus ictericus Orbitosphenoid of Platecarpus Parietal of Mosasaurus maximus Basioccipital-basisphenoid of Platecarpus, lateral view Basioccipital-basisphenoid of Plateca,·pus, dorsal view Basicranial circulation of Platecarpus Braincase of Clidastes propython, lateral view Braincase of Clidastes propytlwn, medial view Periotic lab)Tinth of Clidastes propython Prootic of Tylosaurus nepaeolicus Endocranial cast of Platecarpus Occipital view of skull of Platecarpus ictericus Occipital muscle insertions in Platecarpus Supraoccipital of Platecarpus Quadratic suspensorium of Platecarpus Pterygoid of Tylosaurus proriger Pterygoids Stapes and epipterygoid of Platecarpus ictericus Quadrates Quadrate of Platecarpus ictericus Anterior mandibular unit of Clidastes liodontus Anterior tip of dentary of Tylosaurus prnriger Splenio-angular articulation of Platecarpus Mandible of P/atecarpus ictericus Marginal tooth of Mosasaurus maximus Reconstructed cross section of skull of Platecarpus Recomtructed temporal region of skull of Clidastes liodontus Restored superficial mandibular musculature of Clidastes Restored deep mandibular musculature of Clidastes Kinesis in mosasaurs Streptostyly in mosasaurs Restored skull of Platecarpus ictericus with jaws opened and protracted Restored skull of Platecarpus ictericus ·wit h jaws closed and retracted Atlas-axis vertebrae of Clidastes propython Anterior portion of vertebral column of Platecarpus Areas of muscle attachment on anterior portion of vert ebral column of Platecarpus Vertebra of Clidastes showing zygosphene-zygantrum Scapula-coracoid of Clidastcs liodontus Scapula-coracoid of Platecarpus Scapula-coracoid of Tylosaurus

viii 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97.

ILLUSTRATIONS

Interclavicles Humeri of Plotosaurus and Mosasaurus, flexor aspect Humeri of Tylosaurus and Platecarpus, extensor aspect Humeri of Mosasaurus and Clidastcs, extensor aspect Forelimb of Clidastes Forelimb of Mosasaurus conodon Forelimb of Plotosaurus tuckeri Forelimb of Platecarpus Forelimb of Ectenosaurus clidastoides Forelimb of Tylosaurus proriger Pelvic girdle of Mosasaurus conodon Pelvic girdle of Platecarpus Pelvic girdle of Tylosaurus Femur of Mosasaurus hotf manni Hind limb of Clidastes liodontus Hind limb of Mosasaurus conodon Hind limb of Platecarpus Hind limb of Tylosaurus proriger Cross section of the trunk of a mosasaur in the pelvic and caudal region Scapula-coracoid of Clidastes showing muscle attachments Extensor musculature of forelimb of Clidastes Flexor musculature of forelimb of Clidastes Pelvis of Clidastes showing supporting musculature Pelvis of Clidastes showing appendicular musculature Hind limb of Chdastes showing arrangement of appendicular muscles :Muzzle of Clidastes sternbergi Dorsal view of skull of Clidastes liodontus Muzzle of Clidastes liodontus Munle of Clidastes projJython Clidastes parietals Clidastes quadrates Dentary of Mosasaurus conodon Dentary tooth of M osasaurus conodon :Muzzle of Mosasaurus ivoensis Quadrate of Mosasaurus rnaximus Pelvis girdle of Amphekepubis johnsoni Marginal teeth of l,iodon sectorius Dorsal view of skull of Platecarpus ictericus Ventral view of skull of Platecarpus icte1·icus Plalecarpus dentaries Dorsal view of skull of Ectenosaurus clidastoides Quadrate of Plioplatecarpus j1r£maevus 1'fandibular elements of Plioplatecarpus depressus Dorsal view of skull of Prognathodon overtoni Lateral view of skull of Prognatlwdon overtoni Quadrate of Prognathodon rapax Dorsal view of skull of Tylosaurus proriget Tylosaurus parietals Tylosaurus quadrates Lateral view of skull of Tylosaurus nepaeolicus l\fosasaur skulls from the Niobrara Chalk Phylogeny of platynotans

ILLUSTRATIONS

ix

98. Restored skulls of Clidastes liodontus, Opetiosaurus bucchichi and hypothetical ancestral platynotan 99. Diagrammatic family tree of the mosasaurs CHARTS

I. 2. 3. 4. 5. 6. 7.

vVestern gulf rnosasaurs Eastern gulf mosasaurs Kew Jersey mosasaurs Mosasaurs of the interior Belgian mosasaurs French mosasaurs English and Swedish mosasaurs

PLATES

Frontispiece-Life restoration of Tylosaurus proriger l. Restorations of Clidastes liodontus, Platecarpus ictericus, and Tylosaurus

proriger 2. Restorations of Tylosaurus proriger and Plotosaurus

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FOREWORD It is difficult to imagine a lizard more magnificent than a mosasaur. By the standards of the typical lizard—a small terrestrial insectivore—even the earliest mosasaurs are conspicuous for being large-bodied apex predators with uniformly aquatic habits. Indeed, the 45-foot-long Tylosaurus proriger has justly become known as the “Tyrannosaur of the Seas.” Predatory mosasaurs appear in the fossil record of the circumequatorial Tethys Sea during a hot-house episode in Earth history when sea-surface temperatures at low latitudes climbed above 100 °F, or more than 37 °C. These oceanic reptiles rose to preeminence in marine ecosystems toward the end of the Cretaceous as other important groups of Mesozoic sea monsters—the long-necked plesiosaurs and dolphin-like ichthyosaurs—dwindled in diversity and abundance. Mosasaurs disappeared 66 million years ago, presumably extirpated by the Manhattanisland-sized asteroid that also killed off the giant dinosaurs (apart from a few small terrestrial birds). I must confess to being relieved that mosasaurs are long gone, as they most certainly would ruin a holiday at the beach. One could hardly have missed such sea monsters in the world’s oceans, so fossil mosasaurs played central roles in convincing the scientific community by the early 1800s of the fact of extinction. Mosasaurs thereby contributed to the growth of evolutionary thought. How could God’s perfect design for an aquatic lifestyle— including flippers, tail fins, and live birth at sea—have been found wanting? Darwin had a better answer for that apparent conundrum. Mosasaurs have captured our imaginations for centuries. We have learned so much more about them in the past few decades with the discovery of many new species, and better-preserved specimens of known species, from around the world. Just as important, there have been significant conceptual advances in the theory and methods used to infer mosasaur phylogeny. These advances have together revolutionized our understanding of mosasaur evolution. We now know, for example, that adaptations in the limbs and girdles for life in the open ocean, rather than on land, evolved twice within Mosasauria, with the two main branches diverging from less modified aquatic species (formerly known as “aigialosaurs”) with limbs still proportioned much like those of terrestrial lizards. From the very beginning, mosasaurs have been compared almost exclusively with varanids (the smallest crown clade including Lanthanotus and Varanus) among the

xii

FOREWORD

more than 10,000 species of lizards alive today. And mosasaurs are in some ways quite similar to Miocene to Recent Varanus, especially in having long-snouted skulls with expandable jaws and prehensile dentitions. But that resemblance could be due to convergence related to their shared predatory habits, rather than to propinquity of descent. Recent analyses of a broader range of lizards and more disparate sources of data seldom place mosasaurs close to crown varanids (much less Varanus). If accurate, that would explain the curious discontinuity in their respective fossil records: mosasaurs are exclusively Mesozoic in age (Late Jurassic(?) to Late Cretaceous), whereas crown varanids are known only from the Cenozoic (Eocene to Recent). Firm conclusions on these and many other questions in mosasaur biology and evolution remain elusive. Mosasaurs and their marine sister clade, the long-necked dolichosaurs, deservedly continue to be subjects of active research. Dale A. Russell’s classic 1967 monographic revision of the mosasaurs then known from North America—spoils of the “Bone Wars” waged by O. C. Marsh of Yale College and E. D. Cope of the Philadelphia Academy of Sciences—was the springboard for the modern era of mosasaur research. Russell’s careful description of mosasaur form and function, thoughtful consideration of their habits and habitats, and his perceptive views of their phylogenetic relationships from the species level to more inclusive taxa, laid firm foundations for all later research, including my own.

Jacques A. Gauthier Professor of Geology and Geophysics, Yale University Curator-in-Charge of Vertebrate Paleontology and Curator of Vertebrate Zoology, Yale Peabody Museum of Natural History

S YSTEMATICS AND MORPHOLOGY OF AMERICAN MOSASAURS* (Reptilia, Sauria) BY DALE A. RUSSELL

ABSTRACT Mosasaurs were large, marine platynotan lizards which became abundant and diversified during the latter half of Cretaceous time, but disappeared at the close of the period. Three subfamilies are recognized within the Family Mosasauridae: the Mosasaurinae (including the new tribes, Mosasaurini, Globid­ensini and Plotosaurini), the Plioplatecarpinae (including the new tribes, Plioplatecarpini and Prognathodontini), and the Tylosaurinae. At present thirteen genera and thirty species are diagnosed from the North American Cretaceous, with one new genus (Ectenosaurus) and one new species (Plioplate­carpus primaevus). 0£ the older names available, nineteen generic and twenty­nine specific names have been placed into synonymy, two generic and eighteen specific names are based on indeterminate material, and an additional generic and six specific names are only of dubious validity. The osteology of mosasaurs is described in detail and with reference to the soft anatomy. Mosasaurs possessed a good sense of sight and a poor sense of smell. A calcified tympanum, present in all three subfamilies, was probably useful in transmitting waterborne sound to the middle ear and is not indicative of deep-diving habits. Streptostylic quadrates permitted anteroposterior move­ment of the mandibles, which in turn facihtated the underwater swallowing of prey. l\fosasaurs swam by lateral undulations of the body, the flippers and rela­tively long neck serving as organs of equilibration. They fed on smaller mosa­saurs, chelonians, fish, ammonites, belemnites, echinoderms and pelecypods, and for the most part were highly active aquatic carnivores. l\fosasaurs inhabited subtropical epicontinental seas of less than 100 fathoms (about 180 meters) depth and of variable salinity. Individual forms had a ·wide geographic distribution that was little affected by changes in depositional en­ vironments. The range of many species of mosasaurs inhabiting the ·western edge of the l\"orth Atlantic basin probably extended into western European waters. Those from the eastern edge of the Pacific basin appear to have belonged to a distinct zoogeographic province. Mosasaurs descended from primitive middle Cretaceous varanoids (aigialo­saurs) possessing many cranial characteristics of rnosasaurs but with a post­cranial morphology similar to that of the modem Varanus. Ancestral mosasaurs seem to have been of two basic types; forms with long bodies and short dilated tails giving rise to the mosasaurines, and forms with short bodies and long pointed tails giving rise to the plioplatecarpines and tylosaurines. During their relatively brief geologic existence mosasaurs exhibited a few clearly progressive trends, such as a tendency to increase in overall size, to telescope the frontals over the anterior edge of the parietals (consequently suppressing kinesis), to in­crease the number of pygal vertebrae, and to alter the primitive ·webbed paddles into long, hyperphalangic flippers. *Published with the aid of a National Science Foundation Publication Grant, Ko. GN-528.

2 AMEP11KAHCKvIE M03A3ABPbl )l3Jlb PYCCEJTb

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KYPATOP OT.tl.EJJA l1CK0flAEMblX f10380HO'lHhlX, HALJ,HOHAJlbHblf1 MY3Ef! KAHA)J,bl AECTPAKT Mo3a3aBpbl 6wm: lrnKaM11 Ha aHaroM1110 11x Tena. Mo3a3aBpbl 06.�a,a.a.�11 xopowo pa3B11TbIM 3peH11eM 11 flJIOX0 pa3B11TblM OOOHSIHHeM. OKaMeHeBrnaS[ 6apa6aHHaSI -no.rncTb, J:!MeJOlll_aSlcSI y BCex -rpex no,a.ceMeHCTB, DOBl1JJ.11MOMY 6b1J1a noJie3Ha AJISl nepe.n.a'lli B cpeAHee yxo Tex 3ByKos, KO,0pb1e pacnpocrpaHS1J111c1, B sop.e, 0Ha He yKa3b!Baer Ha TO, 'ITO )IU1B0THOe MOrJJO Hblpj!Tb rny6oKO. CTpenTOCTl1JlbHble Kpa,11.paTbl fl03B0JISIJ111 JI.BH)f{eH11e BepxHllX 1Ie.,t0CTeH Bnepe.n: 11 Ha3aA, 'ITO B CB0IO O'iepe.n:1, 06,,eI"la.�o npornaTblll3Hl110 ,11.06wm no,,i, ll0)1.0IO. Mo3a3al!pbl n.13BMH nocpe,a.cTll0M BOJIH006pa3Hb!X JJ.B11)KeHHH TeJla B ,11.mtHy, n.1aBHl1Kl1 11 0TH0CHTeJtbHO Jl..1MHHair rneSl CJIY)Kl1JIH )I.JlSI paBH0Becm1. n11TaJ11iCb 0HH 6onee MCJIKHM11 M03a3aBpa,-.111, 'lCpenaxa:,111, pb!60H., aMM0Hl1T3Ml1, 6ene,\1Hl1T3MH, l1rJI0K0)Kl1M11, neneu11no)l.aMl1j OHl1 OblJ111 0'ieHb 3KTHB­ HblMl1 ll0,11.HblMli rtJI0T0ll,ll.Hb!,\\11 )Kl1B0THblMH. OH HaCeJIS!JIH cy6Tpon11qecK11e 3fll1K0HTl1HeHTaJll,Hh!e MOPS! rJ1y611HOIO MeHee qeM CTO ,¼0pCKHX: Ca)f{eHe11 (OK0,10 CTa B0CbMl1)1.eCS!Tl1 MeTpOB) Ii H�le!OlUl1e pa3Jil1'1HOe co,11.ep)l{aH11e co,1ei1. OT,a.enbH1,1e 11x cpopMbt 11Men11 6onhrnoit reorpacp111IeCK1111 apean, Ha Korop1,1if Mano BJ!i-lS!Jll1 pa3Jil1'1Hll B OKpy,ttaJOlUen cpe.n.e. ApeaJI MHOf11X BHJJ.OB M03a3aBp0B, Haceml.Bllll1lC 3anaJJ.HYIO OKpa11Hy Cesepo-ATJl3HTl1'ieCK0ro 6acceiiHa, AOX0)UIJI nOBl1JJ.11M0MY )1.0 111opea 3anaAHOH EBpOnbl. Hx B11)1.bl 113 BOCTO'iHOH 0Kpa11Hbl 6accei1'Ha T11xoro OKeaHa l10B11.A,11M0MY np11Ha,11.J1e)l{aJ111 K .n.pyro11 300reorpacpH'ieCKOH np0B11HUHH. Mo3a3aBphl np0H301llJIH OT npHMl1THBHb\X BapaHOB cpe..n.Hero MCJla (a1-1rnaJ103aBp1,1), oco6emIOCTl1 'iepena nocne.n.HHX 61,JJIH nOX0)!(11 Ila Te Jfonornsi 6MJia KaK y C0BpeMeHHblX BapaHOB. npe,ll.KII !II03a3aBp0B nOB11,ll.11M0My 61,1,111 JJ.BYX OCHOBHblX TWI0Bj 6bl.1H cpopMbl, HMeBrnHe ,11.JIMHHblC Te..1a M pacwHpeHHble XB0CTbl, OT HHX npo1130lliJJH M03a3ayp�1Hbl; 6b!,1H cpopMbl C KOp0TKHMH TCJl3MH 11 JJ.JJl1HHl,IMl1 OCTpOK0He'I­ HblMU XB0CTaMl-1, - 3TO 61,rnu npe)i.Kl1 HJll10f!JlaTeKapmrnoB H T11,,03ayp11HOB. 8 Te'ieHHe csoero KOpOTK0ro nep11o;i:a )Kl13Hl1 B J,!CTOp1111 3eMJil1 M03a3aBpbl noKa3aJJH HeCK0JihK0 nporpeCCl1BHl,IX TeH)l_eRUHH, KaK, Hanpl-!Mep, TeH,U.eHUl1!0 K yaeJl11'ieHl1IO o6mero pa3Mepa, x noMeUJ,eH1110 nepe;i:Heil noJIOBHHbt TeMeID! Ha,11. nepe.U.HHM KpaeM Ja,11.neii noJio­ BHHbl TeMeH11 (BCJie,D.CTBl1e 'iero yMeHbilll1JlaCb nOAB11)1{HOCTb KOCTen 'il)f)ena), yseJil1'ieHHe '!fl1C/la n11raJ1bHhIX rt03B0HKOB H ll3MeHem1e np11MHTl1BHblX nepen0H'laTblX nnaBHllK0B B A.JIHH­ ltb1e, C 60llbWHM 'lllCJlOM q>MaHr,

• Sistcmatika i morlo!ogiiaamcrikanskikh mozazavrov (Reptilia, Sauria)

3

SYSTEMATIOUES ET IvlORPHOLOGIE DES MOSASAURES AMERICAINS (Reptilia, Sauria) DALE A. RUSSELL

RESCJ\tE

Les mosasaures etaient des hydrosauriens ayant la forme de grands lezards qui se sont multiplies et diversifies clans la seconde moitie du Cretan� pour disparaitre a la fin de cette periode. La famille des Mosasaurides se subdivise en trois sous-familles: les mosasaurines, comprenant les nouvelles tribus des mosa­ saurins, des globidensins et des plotosaurins, les plioplatecarpines, comprenant les nouvelles tribus des plioplatccarpins et des prognathodonlins, et les tylo­ saurines. On a jusqu'ici categorise 13 genres et 30 especes du Cretan� en Amerique du Nord, y compris un nouveau genre (Ectenosrmrus) et une nouvelle especc (I'lioplatecarpus primaevus). Parmi les noms deja existants, dix-neuf noms generiques et vingt-neuf noms specifiques ont ete places en synonymie, cleux norns generiques et dix-huit noms specifiques sont bases sur des donnees in­ -suffisantes, et la valeur d'un autre nom generique et de six noms specifiques est contestable. L'osteologie des mosasaures est decrite en detail et en rapport avec la partie non-osseuse de leur anatomie. Les rnosasaures etaient doues d'une bonne vue, mais leur odorat etait faible. Un tympan calcifie, que l'on a retrouve chez !es trois sous-familles, devait servir a transmettre les sons produits dans l'eau a l'oreille mcdiane, et n'indique pas que ccs sauriens avaient l'habitude de plonger en profondeur. Les os carres streptostyliques permettaient le rnouvement antero­ posterieur des mandibules, ce qui permettait d'avaler facilement des proies sous l'eau. Les mosasaures nagcaicnt par mouvcmcnts ondulants lateraux, se main­ tenant en equilibre grace a leurs nagcoires et a !cur long cou. Grands carnivores aquatiques, ils se nourrissaient de mosasaures plus petits qu'eux, de chelonicns, -de poissons, d'amrnonites, de belemnites, d'echinoderrnes et de pelecypodes. Les mosasaures habitaient les mers semi-tropicales d'une profondeur de rnoins ,de 100 brasses (environ 180 metres) et d'une salinite variable. Des formes partic­ ulieres de ces anirnaux suivaient une repartition geographique etendue qui n'etait que peu influencee par les depots sedirnentaires presents. Plusieurs especes de mosasaures habitant la section occidentale du bassin de l'Atlantique Nord se sont probablement repandues ,iusque dans les eaux de !'Europe de l'Ouest. Les mosasaures de la section orientale du bassin du Pacifique seroblent avoir app,u-tcnu a une province zoogcographiquc distincte. Les mosasaures dcsccndaient de varanides primitifs (aigialosaures) du Crctace moyen qui possedaient plusieurs caracteristiques du crane que l'on retrouve chez les mosasaures, mais conservaient une morphologie post-cranienne semblable a celle du Varanus moderne. A l'origne il sernble y avoir existe deux types -de base de mosasaures: un type au corps allonge et a la queue courte, mais deployee, qui a donne naissance aux mosasaurines, et un type de plus petite taille, mais a la queue longue et effilee, qui a donne naissance aux plio­ platecarpines et aux tylosaurines. Au cours de leur presence relativement breve clans l'histoire geologique, les mosasaures ont donne quelques signes evidents -d'une evolution: leur taille a eu tendance a s'accroitre, leur frontal s'est recouvert sur la section anterieure du parietal (eliminant ainsi toute possibilite de la cinetique crinienne), le nombre de leurs vertebres a la base de la queue a augment# Virchow, H,, 108 Vomer, 25-26, 151 Vomeronasal oigan. See Jacobson's organ Walls, G.L., 58 Williston, S.W., 5, 12, H, 70, 119, 132, 136, 192 Wiraan, C., 127 Zygantrum, 75, 75 Zygopophyses, function, 108 Zygosphene, 75j 75

.231

CHART I WESTERN GULF MOSASAURS Correlation after Stephenson et. al. 1942; Young 1963; Jeletzky 19 65 personal c ommunication Time scale after Kulp 1961