THE M&RPHOLOGICAL AND FUNCTIONAL EVOL'UTION OF THE "TARSUS IN' AMPHIlBIAN,S AN~D'REPTILES' BBOBB SCIHAEFFER, .,BUI-LETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOL. LX'XVIII, ART. -VIpp 395-472 New~ York I8aued November 7, 1941 4' '4 '','L' '4X's~~''1, 'p ,- a ' , ,,, X, X. A" ,; Ses t 4, .4i: t , r ;ts8 , {j ii f4 Waa rNt; /0n ;) f1 iba 4' 4';'j .4 44, 4 '4, 49 44 '4" ,"4 '4v, ;; ,;Kf,'#''t< i; t;2s,9,,th~~~~~~'1Q'b'K;oe!sI Article VI.-THE MORPHOLOGICAL AND FUNCTIONAL EVOLUTION OF THE TARSUS IN AMPHIBIANS AND REPTILES BY BOBB SCHAEFFER' FIGURES 1 TO 21 CONTENTS PAGE INTRODUCTION ...................................................................... 396 THE AMPHIBIAN TARSUS ............................................................. 397 The tarsal pattern of the primitive tetrapods-labyrinthodonts......................... 397 The tarsus of the lepospondyls...........................-........... 403 The tarsus of the caudate amphibians ............................................... 403 Comparison with the primitive tetrapod pattern .................................... 403 The centrale complex ............................................................ 406 Comparison of the caudate and anuran tarsus ...................................... 407 The relation of the tarsus to the origin of the Amphibia .............................. 408 Amphibian locomotion .............................................................. 409 Pressure distribution in the unspecialized amphibian foot and an analysis of the forces acting on the limbs during locomotion ................................................... 415 The extrinsic and intrinsic musculature of the amphibian hind-foot and its relation to locomotion .............................................. ................. 420 THE REPTILIAN TARSUS ............................................ ................. 425 The origin of the primitive reptilian tarsus ........................................... 425 The cotylosaurian tarsus and its function ............................................ 428 The seymouriamorph tarsus .................................................. 428 The captorhinomorph tarsus ................................................ 428 The diadectomorph tarsus ............. -.................... 430 The procolophonid tarsus ................................................... 432 The pareiasaurian tarsus ........... ........................................ 433 The structural and functional derivatives of the unspecialized cotylosaurian tarsus........ 434 Type I: The structure and function of the type found in the Mesosauria, Ichthyosauria, Protorosauria, and Sauropterygia ............................................... 434 Type II: The structure and function of the type found in the Chelonia and the Eosuchia and the reptilian orders derived from the latter; the elaboration of the mesotarsal joint 435 The chelonian tarsus .......................................................... 435 The eosuchian tarsus and its derivatives ......................................... 436 The rhynchocephalian and lacertilian tarsus ....... ............................... 438 The thecodont tarsus.......................................................... 441 The crocodilian tarsus......................................................... 442 The locomotion of the alligator................................................. 443 The tarsus of dinosaurs, pterosaurs, and birds..................................... 446 Type III: The structure and function of the pelycosaurian tarsus and of the types de- rived from it; the elaboration of the crurotarsal joint............................ 447 The pelycosaurian tarsus and its derivatives ...................................... 447 The therapsid tarsus........................................................... 449 The dromasaurian tarsus..................................................... 450 The dinocephalian tarsus...................................................... 451 The dicynodont tarsus.......................................... 452 The gorgonopsian tarsus ....................................... ...............452 The method of phalangeal reduction as illustrated by the Gorgonopsia ........ 454 The therocephalian tarsus..................................................... 454 The cynodont tarsus......................................................... 454 The bauriamorph tarsus ..................................................... 454 The functioning of the therapsid foot............................................ 455 Astragalar superposition ........................................................ 455 The extrinsic and intrinsic musculature of the reptilian hind-foot and its relation to locomo- tion........ 457 SUMMARY........ 466 REFERENCES ........ 468 1 Department of Zo6logy, Columbia University. 395 396 Bulletin American Museum of Natural History [Vol. LXXVIII INTRODUCTION The possibility of tracing satisfactorily motion in recent types in the form of mo- the phylogeny of any part of the vertebrate tion pictures, together with experiments on skeleton is naturally limited by the for- the action of individual muscles and tuitous discovery of the proper material groups of muscles that tend to act to- that will enable one to define the critical gether. Such information is of great value stages in the transition from a more primi- in interpreting and supplementing the con- tive morphological type to a more ad- clusions drawn from the fossil material. vanced. Although the true nature of the Whereas such data have been determined tarsus of many extinct tetrapod groups is for the caudate amphibian and the alliga- either unknown or at best but poorly tor, it is realized that comparable data known, either due to lack of ossification or should also be gathered for a typical preservation, there has become available lizard, a bird, and a primitive mammal in within recent years a wealth of material connection with further studies on this which makes it practicable to follow the general topic. general evolution of this region in consider- The evolution of the tarsus and its ef- able detail. fect on the method of locomotion are ob- The purpose of this paper is to describe viously intimately associated with the the morphological and the functional evolu- evolution of the crus, the femur, and the tion of the tarsus in the amphibians and pelvis. Although it has not been possible reptiles, and to present a synthetic picture within the space of this paper to integrate of the steps leading to the various defini- all of this information, the more important tive types of tarsal structure. While the changes in the crus, the femur, and the morphological discussion is based as much knee-joint are briefly considered. as possible on actual material, it has been As might be suspected, the literature necessary to resort to the literature for relating to the tetrapod tarsus (excluding several important structural stages that the mammalian) is very large. Most of are only represented by material in collec- the accounts are purely descriptive, some tions in Europe and South Africa. Al- comparative, while very few make any at- though conclusions drawn from published tempt at discussing the functional im- descriptions and diagrams may not be as plications. In the descriptive category, accurate as those based on actual speci- the work of Boonstra, Broom, Gregory, mens, it is felt that constant cross-check- Haughton, Holmgren, von Huene, Rabl, ing through papers on the same subject Romer, Schmalhausen, Sewertzoff, and but by different authors has reduced to a Watson is of the greatest importance. minimum most of the inaccuracies and From the functional point of view, the misconceptions. contributions of Elftman, Gray, Gregory, The functional aspects have been treated Hirsh, von Huene, Haughton and Boonstra, in considerable detail for the primitive Morton, Nauck, and Romer are the most tetrapod tarsus and many of the types illuminating of the very few ventures into derived from it. Some of the functional this field. The study of the mechanics of interpretation is admittedly more or less locomotion in the lower tetrapods has superficial, as it has not been possible with been sadly neglected and any information the time and the material available to in this field is very desirable. treat each important functional change The writer wishes to acknowledge his with the same degree of thoroughness. deep gratitude to Professor W. K. Gregory In studying the functioning of the tarsus for suggesting the problem and for his con- and its relation to the movements of the stant interest and guidance throughout the hind-limb and body as a whole, it is highly investigation. The writer is also in- desirable to have experimental data on the debted to Mr. H. C. Raven and to Professor angulation and movement of the various A. S. Romer for helpful suggestions, and to limb segments and on the method of loco- Mr. Howard Pearson for lending his knowl- 1941 ] Schaeffer, Evolution of the Tarsus in Amphibians and Reptiles 397 edge of mechanics to many discussions ance in making the motion pictures. The on locomotion. He also wishes to thank drawings have been made by Mrs. Helen Curators Barnum Brown and Walter Granger for permission to use the vertebrate Zisa witha herusa sil canthe wie paleontological collections in the American wishes to thank her for her cooperation and Museum, and Dr. A. J. Ramsay for assist- patience. THE AMPHIBIAN TARSUS THE TARSAL PATTERN OF THE PRIMITIVE TETRAPODS-LABYRINTHODONTS