PAR-E-IASAURS VER-SUS PLACO- DONTS AS NEAR Ancevstors :TO the TURTLES
Total Page：16
File Type：pdf, Size：1020Kb
PAR-E-IASAURS VER-SUS PLACO- DONTS AS NEAR ANCEvSTORS :TO THE TURTLES WILL:IAM :KING- GREGORY .. R. , .e... .r ....7... --'''' '- "BULLETIN' -- ;OF THE -- AMERIC:AN MUSEUM OF NATURAL HISTORY fVOLUM-E---* 8:6:..ART-IC:LE...6v:: -0--NEW YORE: 1946; m PAREIASAURS VERSUS PLACODONTS AS NEAR ANCESTORS TO THE TURTLES PAREIASAURS VERSUS PLACODONTS AS NEAR ANCESTORS TO THE TURTLES WILLIAM KING GREGORY Curator Emeritus of Fishes and of Comparative Anatomy BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 86: ARTICLE 6 NEW YORK: 1946 BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Volume 86, article 6, pages 275-326, text figures 1-33, plates 34-35, tables 1-3 Issued May 8, 1946 CONTENTS INTRODUCTION: THE APPROACH TO THE PROBLEM . 281 COMPARISON OF DIADECTID, PAREIASAUR, AND TURTLE SKELETONS . 285 General Body Form and Habits . 285 Skull .................................. 285 Vertebrae and Ribs . 296 Pectoral Girdle and Limbs . 300 Pelvic Girdle and Limbs. 307 COMPARISON OF CAPTORHINOMORPH AND TURTLE SKELETONS .. 311 COMPARISON OF SEYMOURIAMORPH AND TURTLE SKELETONS .. 313 COMPARISON OF PLACODONT AND TURTLE SKELETONS .. 314 DISCUSSION: CONVERGENT EVOLUTION AMONG TESTUDINATE ANIMALS .. 319 CONCLUSIONS .. 321 REFERENCES ................................ 323 279 INTRODUCTION: THE APPROACH TO THE PROBLEM THAT THE TURTLES, especially the sea turtles, called Otocoelus and Conodectes, in which the were more or less closely related to the region of the auricular meatus (otic notch) plesiosaurs has been the view of certain "resembled those of the Loricata and Testu- authors, including Baur (1887b, pp. 97-99), dinata." On top of the neural spines were but, according to Williston (1907, p. 487), placed "transverse osseous arches," which "It is chiefly because of the external resem- even overlapped the scapular arch (as does blances of form and similarity in mode of the carapace of turtles), while the temporal locomotion in the water that it has been region of the skull was "complex and with no generally and indefinitely assumed from longitudinal bars." Cope (1898) therefore Buckland's time to the present that the proposed a new order, the Chelydosauria, to plesiosaurs were related to the turtles." In include this family (Otocoelidae) and sug- Diadectes FIG. 1. Skeleton of Diadectes phaseolinus Cope. X About 1/12. Data from American Museum material and from Case (1905, 1911). Length about 1.7 m. opposition to this view, Williston (loc. cit.) gested that "the order is probably ancestral pointed out many supposedly inadaptive to the Testudinata and the Pseudosuchia." differences between the plesiosaurs and the Eventually it was shown that the Otocoelidae turtles and concluded that "Whatever of (Dissorophidae) were rhachitomous laby- resemblance there may be in the form and rinthodonts, and Cope subsequently aban- habits of these two orders of animals has doned the name Chelydosauria, retaining the been due solely to parallel evolution, to similar name Cotylosauria for Diadectes and its aquatic adaptations. In their internal struc- relatives. ture they are really remote from each other Case (1905), restricting the "Chelydo- and neither could have been derived from the sauria" to the Diadectidae and Otocoelidae, other type, not even in a remotely ante- pointed out seven important points in which cedent stage." the diadectid skull indicates affinities to the A new phase in the development of this Testudinata and concluded: "In just the already old problem began with Cope's characters in which the Chelydosauria (Oto- discoveries in the Texas Permian (or Permo- coelidae, Diadectidae) approach the turtles Carboniferous) of the very primitive reptiles they are distinct from the Cotylosauria (fig. 1) which he named Diadectes, Empe- (Pareiasauridae and Pariotichidae) and so it docles, and Empedias and for which he erected seems very probable that we have in the (1880) the suborder Cotylosauria, of the Diadectidae forms very closely related to the "order Theromorpha." Case (1905, p. 127) ancestral stem of the turtles, which tell us states that in 1896 Cope described two other much regarding the development of the genera from the Permian of Texas, which he Testudinata directly from the Cotylosauria" 281 282 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY VOL. 86 [sic ?Chelydosauria]. The conclusion of Cope general conclusion that the turtles have been and Case that the chelonians have indeed derived from the cotylosaurian order, with- been derived from the Cotylosauria (sensu out making any serious effort to find out lato) has been sustained by Williston (1904; which cotylosaurs, if any, were the nearest 1907; 1917; 1925, pp. 209, 210), Watson to the undiscovered pre-Triassic line that led (1914b), Hay (1905), von Huene (1936b), to Eunotosaurus (fig. 4) and the chelonians. and others. Nor did a cursory first review of the evidence The older view that the turtles are some- reveal anything further. The Procolophonia, how related to the Sauropterygia has been which have been ably reviewed by Colbert defended by Jaekel (1902, pp. 135-144) (1946), were a distinct suborder of cotylo- and independently by Broom (1924). Both saurs, running their course to their ex- these authors cite the turtle-like skull tinction in the Upper Triassic, retaining the characters of the Upper Triassic placodont great otic notch of Diadectes but remaining Placochelys (see p. 314), and Jaekel also quite different from the contemporary am- attempted to homologize certain parts of the phichelydian turtles. The captorhinomorphs, carapace and plastron of Placochelys with another suborder of cotylosaurs, had lost the those of chelonians. Jaekel (ibid., p. 143) otic notch and the curved quadrate which explained the contemporaneity of the heavily were characteristic of Diadectes (fig. 6A) and toothed placodonts and the virtually tooth- which would - be expected in an ancestral less beaked chelonians in Upper Triassic chelonian, nor did either the rest of the skull times by the assumption that the ancestors or the postcranial skeleton indicate a very of the chelonians had undergone a severe close connection with the turtles. For that transformation together with a change in matter, Diadectes itself, although quite turtle- food habits. The traces of some such trans- like in its large otic notch and curved quad- formation were indeed evident in the much rate, was well provided with highly special- later discovered skulls of Henodus chelyops ized jaws and dentition; its very large from the Lower Jurassic of Europe, described vertebrae (fig. 16) bore complex hyposphene- by von Huene (1936a, 1938). In this genus hypantral articulations, while a bony cara- only the last upper and lower crushing teeth pace and plastron were either lacking or not of the placodonts remained. The oral borders sufficiently coherent to be preserved. were evidently covered with a horny beak When I turned my attention to the which, on the evidence cited by von Huene, pareiasaurs (figs. 2, 3), I was surprised to may have borne parallel ridges (fig. 31) for find that apart from their gigantic size they sifting food from the water, as in cer- seemed on the whole to afford an excellent tain ducks. The carapace and plastron of starting point for the chelonian line,, both in Henodus in general form were remarkably their general construction and in many like those of certain turtles. Nevertheless features of the skull, vertebrae, ribs, girdles, von Huene showed that Henodus is only a limb bones, hands, and feet. highly specialized placodont and the last of Seeley (1887-1895) made many compari- its order. The earlier placodonts, including sons between pareiasaurs and anomodonts, Paraplacodus (Peyer, 1935), are far less pareiasaurs and protorosaurs, pareiasaurs turtle-like and, on the other side, I shall and mammals, and the like, but, strange to presently cite decisive evidence that in the say, neither his text nor his figures compare girdles and limbs as well as in the construc- pareiasaurs and chelonians. Haughton and tion of the temporal region the older placo- Boonstra's thorough systematic and com- donts were related to the nothosaurs and parative studies on South African pareiasaurs plesiosaurs. Von Huene, therefore, even in (1929, 1930, 1932, 1933) have supplied in- his latest phylogenetic diagrams (1936b, dispensable material for the present study, 1940) continues to place the Testudinata as but they do not touch upon the major prob- a derivative of the Cotylosauria, its line lem of the possible relationships of pareiasaur starting between the pareiasaurs and the to chelonian. seymouriamorphs. The present paper is in part a continuation Until recently I was content to accept the of the series on comparative osteology and 1946 GREGORY: PAREIASAURS AS ANCESTORS TO TURTLES 283 z(ore Scutosaurus FIG. 2. Skeleton of Scutosaurus karpinskyi (Amalitsky). Based on American Museum skeleton mounted by Mr. Charles Lang under the direction of Dr. Barnum Brown. Length about 3.05 m. Pareiasaurus karpinnskyi FIG. 3. Skeleton of "Pareiasaurus karPinskyi." Based largely on unpublished lithographic plates of Amalitsky's, with changes indicated by comparative study. The line along the middle of the back indi- cates the position of the neural or middorsal bony scutes. The transverse rows of scutes are not indi- cated. 284 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY VOL. 86 myology by the author and his collabora- transformation of organic designs: a review tors, especially L. A. Adams (1919), Charles of the origin and deployment of the earlier L. Camp (Gregory and Camp, 1918), R. W. vertebrates" (1936). Miner (1925), G. K. Noble (Gregory and The skeletons of various chelonians in the Noble, 1924), A. S. Romer (1922), H. 0. American Museum have been studied with Elftman (1929), H. C. Waterman (1929), special reference to the skull, vertebral B. Schaeffer (1941), H. C. Raven (Gregory column, girdles, and limbs, especially in their and Raven, 1941), and F. G. Evans (1939), adjustments to the presence of a fixed cara- A Eunofosauru-s FIG. 4. Sketches of thoracic skeleton of Upper Permian chelonian, Eunotosaurus See- ley.