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To renew call Telephone Center, 333-8400 UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN w^osm L161—O-I096 ILLINCk y BT jrbamac paign AUG 1 3 n% FIELDIANA Geology NEW SERIES, NO. 31 The Genus Placodus: Systematics, Morphology, Paleobiogeography, and Paleobiology Olivier Rieppel CO CO CD Information for Contributors to Fieldiana staff members and research associates, General: Fieldiana is primarily a journal for Field Museum although be considered as manuscripts from nonaffiliated authors may space permits. or fraction thereof. of at least 50% of The Journal carries a page charge of $65.00 per printed page Payment page research for which reduces the time. 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The corresponding normally can be made and answered. Only one set of of the edited manuscript on which deletions, additions, and changes queries be on the set of Changes in page page proofs will be sent. All desired corrections of type must made single page proofs. in can only be made if the proofs (as opposed to corrections) are very expensive. Author-generated changes page proofs author agrees in advance to pay for them. THIS PUBLICATION IS PRINTED ON ACID-FREE PAPER. tuami mm FIELDIANA Geology NEW SERIES, NO. 31 The Genus Placodus: Systematics, Morphology, Paleobiogeography, and Paleobiology Olivier Rieppel Department of Geology Field Museum of Natural History Roosevelt Road at Lake Shore Drive Chicago. Illinois 60605-2496 Accepted April 21, 1995 Published December 29, 1995 Publication 1472 PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY © 1995 Field Museum of Natural History ISSN 0096-2651 PRINTED IN THE UNITED STATES OF AMERICA Table of Contents 18. Skull of Placodus gigas 18 19. Skull of Placodus gigas 20 20. Rostrum of Placodus gigas 22 Abstract 1 2 1 . Skull of Placodus gigas 22 zusammenfassung 1 22. Skull of Macroplacus raeticus 23 Introduction 1 23. Skull of Placodus gigas 23 Systematic Paleontology 2 24. Skull of Placodus gigas 24 SKELBTAiL MOftPHOhOGY OF PlACODUS GIGAS .12 25. Holotype of Placodus hypsiceps 24 Skull 12 26. Skull of Placodus gigas 25 Lower jaw 20 27. Occiput of Placodus gigas 25 Dentition 24 28. Basioccipital of Placodus gigas 26 Postcranial skeleton 27 29. Mandibular symphysis of Placodus gi- The Systematic Position of the Genus gas 26 Placodus 33 30. Lower jaw of Placodus gigas 27 Stratigraphy, Paleobiogeography, and 3 1 . Lower jaw of Cyamodus sp 27 Paleoecology of the Genus Placodus 36 32. Lower jaw of Placodus gigas 28 Acknowledgments 39 33. Size distribution of the fourth (posteri- Literature Cited 39 or) dentary tooth plate 29 Appendix: Material Included in This Study 42 34. Dentary of Placodus gigas 29 35. Rostrum and lower jaw of Placodus gi- gas 30 36. Palate of Placodus gigas 30 List of Illustrations 37. Dorsal vertebra of Placodus gigas 31 38. Dorsal vertebra of Placodontia 31 39. Interclavicle of Placodontia 32 1 . Holotype of Placodus gigas 5 40. Pectoral girdle of Placodus gigas 32 2. Holotype of Placodus andriani 6 4 1 . Ilium of Placodus gigas 33 3. Proportions of the third (posterior) pala- 42. Stylopodium of Placodus gigas 33 tine tooth plate 7 43. Phylogenetic relationships of the Placo- 4. Hololype of Placodus bathygnathus 7 dontia 34 5. Holotype of Placodus bombidens 8 44. Interrelationships within the Placodon- 6. Holotype of Placodus pachygnathus .... 8 tia 35 7. Holotype of Placodus hypsiceps 9 45. Skull of Paraplacodus broilii 36 8. Holotype of Placodus quinimolaris 9 46. Skull of Paraplacodus broilii 37 9. Skull of Placodus gigas 10 47. Skull of Placochelys placodonta 37 10. Skull of Placodus gigas 11 1 1 . Skull of Placodus gigas 12 1 2. Skull of Placodus gigas 13 13. Holotype of Placodus antiquior 14 List of Tables 14. Holotype of Placodus antiquior 15 15. Dentary of Placodus antiquior 16 1 . of the humerus in 16. Proportions of the first (anterior) pala- Proportions Cyamo- tine tooth plate 16 dus/Placodus, Nothosaurus. and Simo- 33 17. Proportions of the fourth (posterior) saurus dentary tooth plate 17 m The Genus Placodus: Systematics, Morphology, Paleobiogeography, and Paleobiology Olivier Rieppel Abstract Placodus gigas Agassiz, 1833, from the Muschelkalk of the Germanic Triassic, is recognized as the only valid species of its genus. Placodus gracilis Jurcsak, 1976 (pending a revision of the original material), and Placodus impressus Agassiz, 1833, are considered nomina dubia. An amended diagnosis based on homologies (synapomorphies) is provided for the genus Placodus and its only known species. The skeletal structure of Placodus is described, and the phylogenetic interrelationships of the genus are discussed. Placodonts originated along the northern coast of the Tethys, and early during their phylogeny split into two lineages, the Placodontoidea and Cyamodontoidea. Placodus immigrated into the Muschelkalk Basin through an eastern seaway (possibly the Silesian-Moravian Gate) during early Anisian time. Zusammenfassung Placodus gigas Agassiz, 1833, aus dem Muschelkalk der Germanischen Trias wird als einziger Vertreter seiner Gattung anerkannt. Placodus gracilis Jurcsak, 1976 (eine Revision des Origin- almaterials steht noch aus) und Placodus impressus Agassiz, 1833, werden zu nomina dubia erklart.