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A Pseudosuchian :Reptile from Arizona

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A Pseudosuchian :Reptile from Arizona A PSEUDOSUCHIAN :REPTILE FROM ARIZONA EDWIN H. COLBERT BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 99:ARTICLE 10 NE'W YORK: 1952 .. .. f. 00 A PSEUDOSUCHIAN REPTILE FROM ARIZONA A PSEUDOSUCHIAN REPTILE FROM ARIZONA EDWIN H. COLBERT Curator of Fossil Reptiles and Amphibians BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 99 : ARTICLE 10 NEW YORK: 1952 BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Volume 99, article 10, pages 561-592, text figures 1-35, plates 48-49, table 1 Issued November 14, 1P52 Price: $.50 a copy INTRODUCTION IN THE COLLECTIONS OF the American Mu- Owing to the exigencies of an extensive pro- seum of Natural History there is the partial gram, the specimen was never studied by Dr. skeleton of a small reptile, collected by Bar- Brown. It is the purpose of this present paper num Brown, Llewellyn I. Price, and Wil- to describe the specimen in some detail and to liam B. Hayden in the upper Triassic sedi- compare it with related forms of comparable ments of northern Arizona during the autumn age from other regions. It represents a new of 1929 and the late summer of 1930. The col- element in the upper Triassic faunas of North lecting of this specimen, which represents an America, and for this reason it is of particular FIG. 1. Map of the region south and east of Cameron, Arizona, showing the locality at which Hesperosuchus agilis was discovered. This is the lo- cality of the "Ward bone bed" near the old Tanner Crossing of the Little Colorado River. unusually rare pseudosuchian, was greatly fa- importance. Also this is a reasonably com- cilitated by the interest and assistance given plete skeleton, even though the skull is frag- to the Museum party by Mr. Hubert Rich- mentary and much of the tail is missing, and ardson and his family, of Cameron, Arizona, as such it adds some facts to our knowledge of to whom the Museum and the science of pale- the pseudosuchian reptiles. ontology are deeply indebted. The specimen The specimen was found about 6 miles was brought to the Museum, where it was southeast of Cameron, Arizona, near the old carefully prepared by Mr. Otto Falkenbach. Tanner Crossing of the Little Colorado River, This was a painstaking task, since the speci- a locality that has yielded numerous remains men is of rather small size and has delicate of Triassic vertebrates in past years. It came bones, but the work was completed by Mr. from a level about 160 feet above the Moen- Falkenbach with his usual skill. Many of the kopi formation, which places it in the lower bones were then drawn by the late Mr. Syd- portion of the Chinle formation. In this re- ney Prentice of the Carnegie Museum of gion the Little Colorado River flows through Pittsburgh. In addition to the drawings orig- a steep-walled canyon cut into the hard and inally made by Mr. Prentice, additional fig- very resistant Shinarump conglomerate and ures have been recently prepared by Mr. the heavy Moenkopi sandstones, the former John LeGrand and Mrs. Lois Darling of the above the latter. The Shinarump is rarely Museum Illustrators Corps. more than about 60 feet in thickness, and the 565 566 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY VOL. 99 top of this conglomerate layer marks the edge skull and partial skeleton of Machaeroproso- of the canyon rim. The extensive badlands pus gregorii described by the writer a few flanking the canyon are cut in the compara- years ago (Colbert, 1947). With the skeleton 0 c~ o .o X >~~~ .°~~~~~~~~~~~ 0~~~~~~~~~~~ Clay a Sandstone | < = := 3 ~~~Chinle | Shinarum Sandstone 7Moenkopi - 5,000 FIG. 2. Section showing the Triassic sediments along the Little Colorado River, about 6 miles southeast of Cameron, Arizona. The Ward bone bed and the Hesperosuchus locality are estimated to be about 100 feet, more or less, above the base of the Chinle formation. The locality is slightly less than a mile to the east of the Little Colorado River, which cuts a canyon in the Moenkopi and Shinarump. tively soft Chinle clays and sandstones, and to be described in this paper were found vari- it was from a clay facies of the Chinle that the ous teeth and bones indicating several asso- skeleton was collected. From this same facies ciated fishes and reptiles. These are discussed and not far distant there have been found a below. number of phytosaurs, including the gigantic SYSTEMATIC POSITION CLASS REPTILIA single individual. In addition there are frag- ORDER THECODONTIA ments of bones and teeth that may represent other individuals of the species under consid- SUBORDER PSEUDOSUCHIA eration, as well as other species of small rep- FAMILY ORNITHOSUCHIDAE tiles. Small pseudosuchians, lightly built and bi- HORIZON: Chinle formation, about 100 feet pedal in posture. The limbs are slender, with above the Shinarump conglomerate. Upper the epipodials slightly longer than the pro- Triassic. podials and with the hind limbs considerably LOCALITY: About 6 miles southeast of larger than the fore limbs. Skull of rather gen- Cameron, Arizona, on the north side of the eralized archosaurian type. The dermal plates Little Colorado River. are characteristically somewhat elongated. DIAGNOSIS: A large ornithosuchid with ex- ceptionally well-developed and strong hind HESPEROSUCHUS, NEW GENUS limbs and long, slender fore limbs. The bones TYPE SPECIES: Hesperosuchus agilis, new of the skeleton are hollow. The fore limbs are species. much reduced as compared with the hind DIAGNOSIS: The diagnosis is that given for limbs, but there is a relatively large manus. the species, below. Fore and hind feet with five digits. The verte- brae are amphicoelous, with the centra con- Hesperosuchus agilis, new species stricted in the middle, and with rather long TYPE: A.M.N.H. No. 6758, a partial skele- vertebral spines lacking any expansions at ton, consisting of the following elements: the their tops. The zygapophyses are long and fronto-parietal region of the cranial roof, oblique. The elongated scapula is strongly ex- much of the occiput, other skull fragments, panded dorsally. The fronto-parietal region of posterior fragment of right maxilla, left pre- the skull is broad and very flat, and there is maxilla and maxilla, right quadrate, portion no pineal foramen. The narial opening seem- of an articular (?), a large portion of the left ingly is large, as is the antorbital opening and mandibular ramus, part of the right mandibu- the orbit, and it is placed at the anterior ex- lar ramus, the first 10 presacral vertebrae, 24 tremity of the skull. The front of the skull is additional partial vertebrae not definitely rather pointed. The skull as a whole probably identifiable as to position, sacral vertebra, is rather deep and narrow. There are perhaps various fragments of vertebrae, various cer- 20 thecodont dentary teeth, and probably a vical and thoracic ribs, left scapula and proxi- similar number in the upper jaws. The third mal portion of coracoid, left humerus, left dentary tooth is enlarged, as are the third and radius and ulna, proximal portions of meta- fourth upper teeth, these latter probably be- carpals of right manus, left femur, a greater ing of premaxillary relationships. The teeth part of the left tibia, portion of left fibula, have anterior and posterior ridges, which are portions of right femur, proximal and medial serrated. The anterior teeth are rather long parts of right tibia, proximal end of right and slender, but the posterior teeth are shorter fibula, three metatarsals, various phalanges, and lanceolate in shape. There is a single row numerous fragments of foot bones and limb of dorsal scutes on either side of the midline, bones, several dorsal scutes. This material and the scutes are elongated. was associated and seemingly represents a 567 DESCRIPTION AND DISCUSSION GENERAL CONSIDERATIONS of locomotion. It is reasonable to think, how- THIS NEW ORNITHOSUCHID is on the whole a ever, that most of the progression of these lightly built reptile with a delicate skeleton, reptiles was accomplished in the bipedal pose, yet even so it is among the largest members of and that the fore limbs and hands were used the family. It is closely comparable to, but to a large extent for food gathering. In these somewhat larger than, the type skeleton of animals the hand might have been useful for Ornithosuchus woodwardi, and likewise than digging, for pulling down plants, for catching that of Saltoposuchus longipes. There is a sec- prey, and for defense. ond specimen of Ornithosuchus, described by The tail in Hesperosuchus must have been Boulenger in 1903, that is much larger than long, to serve as a counterbalance for the the type specimen and also much larger than weight of the body. A complete series of cau- the specimen of Hesperosuchus here described. dal vertebrae is not preserved, so the length Also there is a single vertebra from northern of the tail is conjectural. It has been restored Russia, described by von Huene in 1940 un- with 45 caudal vertebrae, which would seem der the name of Dongusia colorata, which is to be about the right number, if comparisons much larger than any vertebra of Hespero- with other archosaurians are valid. Von suchus. The Dongusia vertebra is character- Huene's restoration of Saltoposuchus, in which ized, among other things, by a well-developed there are about 70 caudals indicated, seems hyposphene, so its assignment to the ornitho- excessively elongated. suchids can be questioned. Therefore, except for the second specimen of Ornithosuchus, THE SKULL, JAWS, AND DENTITION Hesperosuchus is the largest of the known It is unfortunate that the skull and mandi- Ornithosuchidae. ble of Hesperosuchus are so imperfectly pre- The delicate nature of the skeleton is evi- served.
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