FIELDIANA • GEOLOGY Published by CHICAGO NATURAL HISTORY MUSEUM Volume 10 Dbgbmbbr 21, 1946 No. 2 FOSSIL SPECIMENS OF MACROCHELYS FROM THE TERTIARY OF THE PLAINS Rainer Zangerl Curator or Fossil RBmi.Bs Among the collections of fossil reptiles in Chicago Natural History Museum are two fragmentary skulls, both congeneric with the recent alligator snapping turtle, Macrochelys temminckii Troost. One fossil representative of this genus has been described from the Florida Peace Creek beds (Pleistocene) as M. floridana Hay, based on a few isolated fragments of peripheral bones. The present material consists of a fairly well-preserved skull, C.N.H.M. No. P26014, from the Marsland formation (early middle Miocene), Marsland, Nebraska, and an unusually well-preserved fragment of the right anterior half of a skull, C.N.H.M. No. P15823, from Big Spring Canyon (early Pliocene), Bennett County, South Dakota. The specimen from Big Spring Canyon has been identified as M. temminckii Troost, since its features fall well within the range of variation of recent individuals. The Marsland specimen, on the other hand, definitely represents a new species. I take pleasure in naming it for Mr. Karl Patterson Schmidt, Chief Curator, Departs ment of Zoology, whose wide interests in chelonian phylogeny have led him to describe a number of important forms, and who has done all of the preliminary work for the description of the specimen from Big Spring Canyon. I am indebted to the authorities of the United States National Museum, the American Museum of Natural History, the Museum of Comparative Zoology, and the University of Michigan Museum of Zoology for the loan of recent specimens for study. Order Testudinata Family Chelydridae Macrochelys schmidti sp. nov. Holotype. — Chicago Natural History Museum No. P26014, a skull. The present fossil (figs. 2 and 3) is somewhat crushed and the No. 676 6 6 FIELDIANA: GEOLOGY, VOLUME 10 postero-dorsal parts, the supraoccipital, the squamosals and parie- tals, the right frontal and postorbital and parts of the right quadrato- jugal are missing. A large number of fine cracks obscure the courses of most of the sutures on the ventral aspect. Remains of matrix inside of the braincase indicate that the fossil was embedded in a fine sandstone; it was deposited in a stream channel, in intimate asso- ciation with mammal bones. Horizon and locality. — Marsland formation, Hemingfordian (early middle Miocene); Marsland, Nebraska. Diagnosis and description.— The general habitus of the skull clearly indicates the generic affiliation of the specimen. It is dis- tinctly triangular, somewhat wider than long. The roof of the skull is flat, its sides stand nearly vertical, the orbits face laterad and the prefrontals reach forward to or slightly beyond the premaxillae, as in the recent M. temminckii. Unfortunately, the skull of the recent Devisia mythodes Ogilby, which seems to be closer to Macrochelys than to Chelydra, is unknown. It is, therefore, not possible at present to outline the skull characters of the three genera of the Chelydridae, or to determine the position of the Miocene fossil in regard to Macrochelys on the one hand and Devisia on the other. Geographical considerations, however (since the monotypic genus Devisia is known only from the Fly River of New Guinea, and there is fossil evidence of the presence of M. temminckii as far north as South Dakota in the Pliocene), strongly suggest that the Miocene form is more closely related to Macrochelys than to Devisia. I The determination of specific characters is not a very easy matter, because of the wide range of individual variation in the living species. A collection of twenty-five skulls of M. temminckii ranging from 96 mm. to 195 mm. skull length (occipital condyle to tip of snout) was studied and measured for comparison and the results are given (table, p. 11), C.N.H.M. U.S.N.M. M.C.Z. A.M.N.H. U.M.M.Z. Spec. no. Spec. no. Spec. no. Spec. no. Spec. no. 22059 La. 03770a .... a .... 7182 . . . S220 Okla. 22111 ... 03770 .... 46617 Miss. 7187 . . . 22412 . . . 029355 Ala. b .... 7165 . . . 39296 Ark. 016224 Miss. c .... 7720 . . . 37481 Ark. 03769 Miss. 59062 .... 46566 Miss. 46567 Miss, d .... e .... 9099 La. The skull is approximately 72 mm. long (condyle to tip of snout), therefore representing a relatively small specimen compared to ZANGERL: FOSSIL MACROCHELYS 7 large individuals of the recent species, in which the length of the skull approximates 200 mm. in adults. In many of its features the fossil resembles these giants of the recent species, rather than speci- mens more nearly its own size. The most outstanding difference between the living and the fossil species lies in the shortness of the antorbital part of the snout of the latter (table, p. 11). Ccwn pared to skulls of the living species ranging in length from 96 to 123 mm., the fossil skull surpasses the recent ones in its overall width, the width Fig. 2. Macrochelys schmidti sp. nov. A, dorsal, B, ventral, and C, lateral view of type. Approx. X 5/7. across the quadrates, the width of the pterygoids, the posterior width of the alveolar surface, the length of the orbit, and the width of the snout at the anterior and posterior rims of the orbits. In all of these characters (with the exception of the length of the orbit) the fossil exhibits proportions more nearly like those seen in skulls of M. temminckii of two and one-half times its size (table, p. 11). The relatively large size of the orbit in the fossil specimen is probably an age character. There are no significant differences in the courses of the sutures between the living and the fossil forms, so far as can be determined 8 FIELDIANA: GEOLOGY, VOLUME 10 in the latter. The pattern of the surface relief, however, is more pronounced in the fossil than in both small and large specimens of M. temminckii and resembles more nearly the condition in Chelydra serpentina; the jugular bone in particular exhibits a system of fine, bony ridges radiating in all directions from a point in the anterior half of the bone. Fig. 3. Reconstruction of the skull of Macrochelys schmidti. Outlines for which there is no evidence in the specimen are represented by dotted lines. Bsph, basisphenoid; Fr, frontal; Jug, jugal; Mx, maxilla; Pa, parietal; Pfr, prefrontal; Po, postorbital; Q, quadrate. While the generic identity of M. schmidti is unmistakable, the skull does show some resemblances to the genus Chelydra. Such characters are the short antorbital part of the snout, the relatively great width of the snout at the levels of the anterior and posterior rims of the orbits, the comparatively short "tooth" formed by the premaxillae, the low alveolar ridge of the maxilla, and the surface sculpture of the roof bones of the skull. Although few specimens listed in the table (p. 11) were sexed, the indices of the greatest width of the skull seem to indicate that skulls of female individuals of 150 mm. and more in length are ZANGERL: FOSSIL MACROCHELYS 9 distinctly narrower than others of comparable size. Pending con- firmation it is here suggested that there is a sexual dimorphism expressed in the relative width of the skull in large individuals of this species. Since the type specimen of M. schmidti differs greatly in its proportions from specimens of comparable size of the recent form, it seems doubtful that the proportional changes due to age as deter- mined for the recent species apply in a similar manner to the fossil and the question of its sex must therefore be left unanswered. Measurements ^^ mm. Total length (condyle to tip of snout) 72 Greatest width ' 76 Width acrass quadrates 78 Width across pterygoids 9.5 Width of alveolar surface posteriorly 12 Height of snout at anterior end 16 Least height of snout (just anterior to orbit) 13.6 Width of suborbital bar 4 Height of snout at orbit 19.5 Length of orbit 17 Width of snout at anterior rim of orbit 22.6 Width of snout at posterior rim of orbit 44 Length of prefrontal suture 15 Distance between anterior orbital rim and nasal notch 8.5 Macrochelys temtninckii Troost Referred speamen.— C.N. H.M. No. P15823, Big Spring Canyon, Bennett County, South Dakota; Clarendonian (early Pliocene). Fig. 4. Macrochelys temtninckii from Big Spring Canyon (early Pliocene), South Dakota. C.N.H.M. P15823. Approx. X 7/10. 10 FIELDIANA: GEOLOGY, VOLUME 10 An exceedingly well-preserved fragment, of the right half of the snout of a large individual (fig. 4) belonging to the genus Macro- chelys was obtained from a poorly consolidated, rather fine sand- stone.^ The fragment suffered no distortion and can therefore be accurately compared with specimens of equal size of the recent species; its indices are included in the table on page 11. If sufficient numbers of specimens of the recent species are considered, no signi- ficant differences can be found between the fossil and the living forms at least in so far as the anterior part of the skull is concerned. Judging from the posterior width of the alveolar surface, which seems to be narrower in females than in males, in the recent form, the fossil remnant appears to have belonged to a female individual at the size when disparity between the sexes begins to increase. Measurements ^^ mm. Width of alveolar surface posteriorly 22.6 Height of snout at anterior end 39.5 Least height of snout (just anterior to orbit) 31.8 Width of suborbital bar 10.5 Height of snout at orbit 40.0 Length of orbit 24.6 Width of snout at anterior rim of orbit " 43.6 ■ Width of snout at posterior rim of orbit 79.0 Length of prefrontal suture 37.3 Distance between anterior orbital rim and nasal notch 24.6 The discovery of true alligator snapping turtles in the middle Miocene of Nebraska and the Pliocene of South Dakota is highly interesting. The range of distribution of the recent M. temminckii is now well restricted to the southern part of the Mississippi drainage basin, but apparently extended far north and west of its present boundaries in Pliocene times. The common snapping turtle Chelydra serpentina L. is widely distributed at the present time on the North American continent east of the Rocky Mountains and in Central America, but oddly enough, there are no. fossil remains of this animal prior to the Pleistocene time in North America. Instead, the Miocene deposits of central Europe (Oeningen) have furnished a number of skeletons of chelydrid turtles that have been described as Chelydra murchisoni Bell, and C decheni H. v. Meyer was described from the upper Oligocene of the Braunkohlengrube Krautgarten near Rott in the Siebengebirge. Unfortunately, the skulls in these skeletons are not sufficiently well preserved to permit accurate ' 1 The specimen was found in association with an extensive assemblage of mammalian remains. See J. T. Gregory, Pliocene Vertebrates from Big Spring Canyon, South Dakota, Univ. Calif. Publ., Bull. Dept. Geol. Sci., 26, (4), pp. 307-446, 1942. ZANGERL: FOSSIL MACROCHELYS 11 Measurements and Indices Absolute length measurement's and various indices of skull measurements of Macrochelyg temminckii compared with those of the fossil specimens described in this report. All indices were arrived at in the following manner: Values under A, B, C, D, E, F, and G times 100 divided by total length. A, greatest width of skull; B, width across quadrates; C, width of alveolar surface posteriorly; D, length of orbit; E, width of snout at anterior rim of orbit; F, width of snout at posterior rim of orbit; G, distance between anterior rim of orbit and nasal notch. SPaCUIBN NUMBBR C.N.H.M. P260n U.S.N.M. 03770a C.N.H.M. 22059 C.N.H.M. 22111 M.C.Z.-a U.S.N.M. 03770 M.C.Z. 46617 . U.M.M.Z. S220 C.N.H.M. 22412 C.N.H.M. P1582S C.N.H.M. 39296 M.C.Z.-b U.S.N.M. 029356 A.M. N.H. 7182(9) M.C.Z.-c U.S.N.M. 016224 A.M.N.H. 7187 U.S.N.M. 03769 A.M.N.H. 7165(9). M.C.Z. 46566 C.N.H.M. 37481 . A.M.N.H. 7720(9). A.M.N.H. 9099 U.S.N,M. 59062(9) M.C.Z. 46567(9).. M.C.Z.-d M.C.Z.-e comparison with recent chelydrid turtles, and their generic identity is somewhat doubtful. In 1852 H. V. Meyer described C. decheni and additional speci- mens of C. murchisoni. Meyer compares these specimens with Chelydra Schweigger and with the kinosternid Staurotypus Wagler, but apparently not with Macrochelys Gray. Judging from H. v. Meyer's plates and description and also from illustrations given by Winkler (1869) of further specimens of C. murchisoni, it is rather apparent that the fossils under consideration exhibit features of all three genera. The general outlines of the skulls and the dorsal aspect of the mandibles in Meyer (1852, pi. 26) and Winkler (1869, pis. 16 and 19) do not conform well with either Macrochelys or Chelydra, but rather with Staurotypus, and the same must be said of the 12 FIELDIANA: GEOLOGY, VOLUME 10 anterior lobe of the plastron, if Meyer's reconstructions are correct (op. cit., pi. 30). On the other hand the carapace compares well with Chelydra, except for the strong serration of its rear edge, which is a distinctive feature of the shell of Macrochelys. In size C. mur- chisoni seems to have surpassed slightly even the largest specimens of C. serpentina and might well have reached the size of Macrochelys temminckii. Apparently the chelydrid turtles of the European Oligocene and Miocene cannot be placed in a recent genus as clearly as their more or less contemporary ally M. schmidti. A thorough reexamina- tion of the European material seems to be necessary before the question of the ancestry of the genus Chelydra can be discussed properly. REFERENCES BOULENGER, G. A. 1889. Catalogue of the Chelonians, Rhynchocephalians and Crocodiles in the British Museum, London. ix+ 311 pp., 6 pis., 73 figs. (Illustrates skull and shell of Staurotypus salvinii.) Hay, O. p. 1908. The fossil turtles of North America. Carnegie Inst. Wash. Pub., 75, iv + 568 pp., 704 figs., 113 pis. Meyer, H. v. 1852. Ueber Chelydra Murchisoni und Chelydra Decheni. Palaeontographica, 2, pp. 237 ff. 1856. Ueber den Jugendzustand der Chelydra Decheni aus der Braunkohle des Siebengebirges. Paleontographica, 4, p. 61. 1856-68. Zur Chelydra Decheni aus der Braunkohle des Siebengebirges. Palae- ontographica, 15, p. 41. Winkler, T. C. 1869. Des tortues fossiles conservees dans la Musee Teyler. 151 pp., 33 pis. Paris (Gautier-Villars) ; Leipzig (G. E. Schulze).