L I B RAFIY OF THE UNIVERSITY Of ILLINOIS MLRAL HISTORY -STREET 550.5 FI V.4,COpCL REMOTE STORAG . 1 I 1 s o o & at 9 1 S I 91 Field Museum of Natural History Founded by Marshall Field, 1893 Publication 297 Geological Series Vol. IV, No. 6 OCCURRENCE OF THE ALLIGATOROID GENUS ALLOGNATHOSUCHUS IN THE LOWER OLIGOCENE BY Bryan Patterson ASSISTANT IN PALEONTOLOGY Oliver Cummings Farrington CURATOR, DEPARTMENT OP GEOLOGY EDITOR CHICAGO, U. S. A. August, 1931 PRINTED IN THE UNITED STATES OF AMERICA BY FIELD MUSEUM PRESS OCCURRENCE OF THE ALLIGATOROID GENUS ALLOGNATHOSUCHUS IN THE LOWER OLIGOCENE BY BRYAN PATTERSON The extensive researches of Dr. Mook 1 on living and fossil croco- diles have brought out a "morphological sequence" of the forms Allognathosuchus heterodon (of the Wasatch), A. polyodon (of the Bridger), Alligator thomsoni (of the Snake Creek), and the recent A. sinense. Very recently Dr. Simpson 2 has continued this sequence into the Paleocene by giving a preliminary description of a new species of Allognathosuchus, A. mooki, from the Puerco. It is the purpose of the present paper to provide a further link in the sequence by describing what appears to be a new species of Allognathosuchus from the Titanotherium beds, the first recorded occurrence of the genus in a formation later than the Middle Eocene. The material at hand, consisting of a well preserved left mandible, 3 was collected in the Titanotherium beds of the South Dakota "bad lands" by Mr. Elmer S. Riggs, Associate Curator of Paleontology, in whose honor the specific name is given. The writer here wishes to express his thanks to Mr. Riggs and to Mr. Karl P. Schmidt, Assistant Curator of Reptiles, for valuable criticism and advice. Order Crocodilia Family Alligatoridae Genus Allognathosuchus Mook Allognathosuchus riggsi sp. nov. Plate XLI. Holotype: — P 12141, Field Museum, left mandible. Coll. Elmer S. Riggs, 1898. Horizon and locality: — Lower Oligocene, upper levels of Titano- therium beds, Phinney Springs, Cheyenne River, South Dakota. Description: — A. riggsi is a large form, larger than the types of the other species of the genus. There are nineteen or twenty mandib- 1 Mook, C. 0, 1921, Bull. Amer. Mus. Nat. Hist., Vol. XLIV, pp. 105-110, 1 pi.; 1923, Amer. Mus. Novit., No. 73; 1923, Bull. Amer. Mus. Nat. Hist., Vol. XLVIII, pp. 553-562, figs. 1, 2. * Simpson, G. G., 1930, Amer. Mus. Novit., No. 445. * Some vertebrae and limb bones are reported in the field records, but unfor- tunately these seem to have been lost. 223 224 Field Museum of Natural History— Geology, Vol. IV ular teeth, of which the fourth and thirteenth are much enlarged as indicated by the alveoli, and the first moderately so. The alveoli for the second and third teeth are of medium size. The fourth tooth is unfortunately broken off at the crown but enough of it remains to show that it was sharply conical with an antero-posterior carina and radiating rugosities. Posterior to the fourth tooth there are eight small alveoli which are arranged as in Mook's description 1 of A. polyodon, i.e., facing obliquely outward as well as forward, steadily decreasing in diameter to the sixth and then increasing gradually to the eighth, which is the largest of them. The alveolus for the thirteenth tooth is but little smaller than that for the fourth. The fourteenth tooth is preserved. It is small and conical with a fairly prominent posterior carina extending most of the way down the tooth. The anterior carina is weak and restricted to the apical portion of the tooth. There are the usual radiating rugosities. The fifteenth tooth has an independent alveolus with stout walls, as have all the teeth anterior to it. The last four (or five) 2 teeth have a common alveolus and seem to have been comparatively small. The splenial is strong and extends anteriorly to form a small part of the symphysis. (The anterior end is broken off but the suture indicates the extent of the anterior extension.) The symphysis extends to the seventh mandibular tooth. The external mandibular foramen is of average size. The dentary is strongly pitted anteriorly and the surangular and angular are very deeply sculptured posterior to the external mandibular foramen. The peculiar contours of the mandible are very similar to those described by Mook for A. polyodon, though to judge from his figures it would seem that the concavity between the fourth and thirteenth teeth is somewhat deeper in the specimen at hand. Simpson described the dental border of A. mooki as being flattened and forming a shelf internal to the first thirteen teeth, and with a sharp flange projecting above the inner surface of the jaw. The same condition may be observed in A. riggsi, but the flange is not so strong nor does it project out so much as is shown in Simpson's figures. The dentary exhibits two indentations caused by maxillary teeth between and external to the sixteenth and seven- teenth, and seventeenth and eighteenth teeth. The present specimen appears to be full grown. »Op. tit., 1921, p. 107. 2 There are three clearly defined tooth spaces in the common groove in which the posterior teeth were placed and there is certainly space for a fourth. Owing to the anterior end of the surangular being slightly broken off, the writer cannot definitely state whether or not there was a fifth tooth present in this groove. If it were present it would have been exceedingly small. Alligatoroid from the Lower Oligocene 225 Diagnostic characters: — The latest known species of the genus. Size large. Nineteen (or twenty) mandibular teeth. Fourth and thirteenth teeth much enlarged. Last four (or five) teeth implanted in a groove and almost certainly fairly small. Jaw somewhat con- stricted near the end of the tooth row. Measurements Mil. Total length of mandible 314 Total length of dental series 142 Length posterior to dentary 172 Length of symphysis (estimated) 45 Antero-posterior diameter of fourth alveolus 13 Length of row of eight small teeth 46 Antero-posterior diameter of thirteenth alveolus 11 Depth of symphysis 20 Breadth of ramus at posterior end of symphysis 36 Height of ramus at bottom of first concavity 18 Height of ramus at superior process of dentary 37 Maximum height of ramus in angular surangular region 57 A. riggsi may be distinguished from A. mooki by its more slender jaw, by the last four (or five) teeth being in a groove, by the con- striction near the end of the tooth row and by the difference in the flange described above. From A. polyodon it may be separated on the basis of the abrupt inward bend of the post-articular portion of the jaw in the Bridger species, which can be plainly seen in Mook's figure. In A. riggsi no such abrupt bend is present; also there is no constriction of the jaw near the end of the tooth row in A. polyodon. A. heterodon (Cope) has never been adequately described or figured. The figures and description given by Cope 1 are based on fragments, and Mook 2 gives only a brief summary of characters. Cope, however, mentions that it was an extremely small form, possibly not exceeding three feet in total length, even when adult. The small size of this form, combined with its geological age, suffi- ciently distinguishes it from the species under consideration. A. wortheni Case 3 is based on fragmentary material and is appar- ently characterized by the thirteenth, fourteenth and fifteenth teeth being of equal size. This distinction sharply sets off this species from any other of the genus. The writer must confess to some hesitancy in the generic reference of this specimen, having been somewhat undecided between Allog- ^ope, E. D., 1884, Tertiary Vertebrata, Rep. U. S. Geol. Surv. Territories, Vol. VIII, p. 164, pi. XXIIIA, figs. 11-18. 'Op. cit., 1921, p. 109. 'Case, E. C, 1925, Cont. Mus. Geol. Univ. Michigan, Vol. II, pp. 93-97, 1 pi., 1 fig. 226 Field Museum of Natural History — Geology, Vol. IV nathosuchus and Alligator. The contemporary genus Caimanoidea, as described by Mehl, 1 cannot include the specimen under consider- ation, for in it the thirteenth tooth is not enlarged and the lateral border of the mandible is not as undulating in outline. Mook (1923 B) in describing Alligator thomsoni, stressed the close approach of that species to Allognathosuchus in mandibular characters, and stated that "it may be considered as intermediate between these (the Wasatch crocodilians) and the living alligators." The writer, in referring the present species to Allognathosuchus, has been guided chiefly by the robust character of the splenial and its forward ex- tension and participation in the symphysis. Yet, in one of Mook's specimens of Alligator thomsoni, the splenial extends forward to within one millimeter of the symphysis. This similarity, however, merely serves to emphasize the "morphological sequence" already alluded to. A. riggsi may possibly be referred to Alligator on the discovery of further material, but, in this event, the results would be essentially the same, the vertical range of Alligator being lowered instead of that of Allognathosuchus being elevated, while the present species retains its intermediate position. Dr. Simpson, 2 however, does not regard Allognathosuchus as being closely related to the genus Alligator and considers Caima- noidea to be nearer the ancestry of the existing genus. He stresses the close similarity between Caimanoidea and Allognathosuchus and concludes that Allognathosuchus represented a divergent line rather far from the present day alligators, while Caimanoidea, representing a more conservative line, was nearer to the stem of Alligator. The writer is inclined to hold an opinion the reverse of this. He regards Allognathosuchus as structurally nearer to Alligator and Caimanoidea as a conservative form not especially close to the existing genus. This, of course, does not imply that Alligator is a direct descendant of Allognathosuchus. To suggest this, in the present state of knowl- edge, would be very premature; but the presence of Allognathosuchus riggsi as a contemporary of Caimanoidea in the Titanotherium beds, stresses more strongly the resemblance between Alligator sinense, A. thomsoni and Allognathosuchus. The exact relationships of these early alligatoroids can only be settled by the discovery of much more complete material, but, in the meantime, the publication of Dr. Mook's study of the skeleton of Allognathosuchus mooki should contribute much of interest to the problem. 1 Mehl, M. G., 1916, Jour. Geol., Vol. XXIV, pp. 47-56, figs. 1-4. *Op. cit, 1930.