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Thoracosaurus Neocesariensis (De Kay , 1842) (Crocodylia: Crocodylidae) from the Late Cretaceous Ripley Formation of M Ississippi

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Thoracosaurus Neocesariensis (De Kay , 1842) (Crocodylia: Crocodylidae) from the Late Cretaceous Ripley Formation of M Ississippi ~ THE DEPARTMENT OF NATURAL RESOURCES • • • • 4( miSSISSIPPI ~ geology Bureau of Geology I 2525 North West Street Volume 4, Number 1 Jackson, Mississippi 39216 September 1983 ~-a Thoracosaurus neocesariensis (De Kay , 1842) (Crocodylia: Crocodylidae) from the Late Cretaceous Ripley Formation of M ississippi Kenneth Carpenter University of Colorado Museum Boulder, Colorado 80309 ABSTRACT saurs, turtles. and fishes at the Mississippi Museum A complete skull with lower jaws and postcrania of Natural Science, Jackson, and the Dunn-Seiler of the eusuchian Thoracosaurus neocesariensis are Museum, Mississippi State University, Starkville. Brief described. The specimen is from the Late Cretaceous mention of Late Cretaceous vertebrates from the state Ripley Formation of north-central Mississippi. It is the was made by Stephenson and Monroe (1940), Lull most complete specimen of Thoracosaurus known and Wright (1942), and Horner (1979). More detailed from North America and shows that Holopsisuchus descriptions have been given by Kaye and Russell ( =Holops) is a junior synonym. Furthermore, only one (1973), Meyer (1974), and Carpenter (1982). species, T. neocesariensis. is recognized in North Most Late Cretaceous vertebrates from Missis­ America. sippi come from marine strata. This is due mostly to the paucity of Upper Cretaceous terrestrial sed iments INTRODUCTION as discussed by Stephenson and Monroe (1940). Since Late Cretaceous vertebrates from Missis­ Fossil vertebrates from the Late Cretaceous of sippi are so poorly known, any new specimen is Mississippi are not well known, possibly because of potentially important. This is certainly true of the the paucity of exposures suitable for prospecting. complete crocodilian skull, lower jaw and partial Furthermore, many specimens which have been skeleton from the Ripley Formation in the fossil col­ found are either in private collections or remain un­ lections of the Dunn-Seiler Museum, Mississippi studied. Among the latter include numerous mosa- State University (MSU). SYSTEMATICS the modern Tomistoma schlegelii and Crocodilus cataphractus. This is only superficial, however, and is Class ARCHOSAURIA Cope. 1869 due to the long narrow muzzle. Most of the skull surface is sculptured, especially the skull table. Order CROCODYLIA Gmelin. 1788 On the dorsal portion of the rostrum, the paired Suborder EUSUCHIA Huxley, 1875 pr8maxillae completely surround tne external nares. Fam ily CROCODYLIDAE Cuvier. 1807 The nares arc considerably longer than wide, and are Subfamily THORACOSAURINAE Nopcsa. 1928 not divided by a bony septum as in Osteolaemus and Alligator. The premaxillae are raised at the postero­ Genus THORACOSAURUS Leidy, 1852 lateral rims of the external nares, causing the nares to Thoracosaurus neocesariensis (De Kay, 1842) slope forward. As with most crocodiles, the muzzle is Gavialis neocesariensis De Kay, 1842 pinched at the premaxillary-maxillary suture to Crocodylus (Gavialis) clavirostris Morton. 1844 accommodate the third? and fourth? dentary teeth. Posterior to the external nares. the premaxillae form Sphenosauchus clavirostris (Morton. 1844) a midline wedge separating the maxillae. The pre­ Crocodilus basifissus Owen. 1849 maxillae approach, but do not contact, the nasals. Crocodilus basitruncatus Owen. 1849 Ventrally, each of the premaxillae exhibit five teeth, the largest of which is the fourth. The large oval incisive Crocodilus dekayi Leidy, 1852 foramen are almost as large as the external nares. As Thoracosaurus grandis Leidy. 1852 with the dorsal surface of the rostrum. the premaxillae Crocodilus obscurus Leidy, 1856 form a midline wedge between the maxillae. This Crocodilus tenebrosus Leidy. 1856 wedge extends posteriorly to about the distance between the third and fourth maxillary teeth. Thoracosaurus brevispinus Cope, 1867 The maxillae. which make up most of the rostrum. Holops brevispinus Cope, 1867 are separated anteriorly by a midline wedge of the Holopsisuchus brevispinus (Cope, 1867) premaxillae and posteriorly by a midline wedge of the Holops cordatus Cope, 1869 nasals. The lateral margins of the maxillae are parallel for about half the length of the skull. after which they Holopsisuchus cordatus (Cope, 1869) diverge. It is not known how many teeth were present Holops glyptodon Cope, 1869 in the maxillae because the posterior-most portion of Holopsisuchus glyptodon (Cope. 1869) each is damaged. There are at least twenty-three teeth and alveoli on the lett maxilla, which is the more Holops pneumaticus Cope. 1869 complete of the two. Most of the teeth or alveoli are Holopsisuchus pneumaticus (Cope, 1869) widely spaced permitting an interfingering of the Thoracosaurus meirsanus Troxell. 1925 dentary teeth. Posteriorly, however, the alveoli and Thoracosaurus muflicensis Troxell, 1925 teeth are more closely set and lie with a groove. The toothed portion of the maxillae extend at least to MATERIAL: MSU 3293 skull, jaws, four cervicals. below the middle of the orbits. eleven dorsals. two sacrals. six caudals, left scapula. Both nasals are long slivers of bone which taper left and right coracoids. right ilium. left ischium, left anteriorly separating the maxillae along the midline, and right femur, and approximately forty-five scutes. and extend almost to the premaxillae. Posteriorly. Much of the postcrania is fragmentary. they are separated by a medial wedge of the frontals. Sutures with the prefrontal are obscured by matrix, OCCURRENCE: Ripley Formation. Oktibbeha plaster and fractures. County. Mississippi. Exact locality data on file at the The orbits are damaged. but between them it is Dunn-Seiler Museum. Mississippi State University. possible to reconstruct their original shape as sub­ Collected by Steve Newman, 1973. triangular. almost subrectangular. Sutural contacts for many of the preorbital bones are indistinct due to DESCRIPTION: The skull (Fig. 1, a-e; Fig. 2) is fractures. matrix and damage. It appears. however. complete except for damage sustained during collect­ that the lacrimals extend anteriorly from the front of ing and subsequent preparation. This has resulted in the orbits and taper medially against the nasals. The the loss of both quadrate condyles. right pterygoid. sutural contact between the prefrontals and nasals is ectopterygoid. quadratojugal. and jugal (partially). obscured. Laterally, the frontals form the postero­ The skull is slightly crushed dorsoventrally as indicated lateral margins of the orbits. and posteriorly, the by the deformed braincase. longitudinal fractures in anteromedial border of the supratemporal fenestrae. the jugal. and deformed postorbital bars. Crushmg, Neither the prefrontals nor the frontals are raised however, has not resulted 1n much distortion of the around the orbits as in many crocodiles. The supra­ pterygoid flanges. temporal fenestrae are large and oval. and are wider The skull is long and slender. resembling those of than long. MISSISSIPPI GEOLOGY 2 Figure 1. Skull and lower jaws of Thoracosaurus neocesariensis, MSU 3293. Skull in dorsal (a), ventral (b), and lateral (c) views; lower jaws in lateral (d) and dorsal (e) views. Abbreviations: an - angular; d - dentary; ec - ectopterygoid; f - frontal; j- jugal; 1- lacrimal; mx - maxilla; n - nasal; p- parietal; pal - palatine; pf - prefrontal; pm­ premaxilla; po - postorbital; pt - pterygoid; qj- quadratojugal; sa- surangular; sq- squamosal. Bar = 10cm. 3 SEPTEMBER 1983 The postorbitals connect the skull table to the On the palate. two very large triangular. suborbi­ cheeks via a descending process to the jugals. Dor­ tal fenestrae lie medial to the posterior part of the sally, the postorbitals separate the orbits from the maxillae. These fenestrae are formed by the maxillae supratemporal fenestrae, and laterally, from the lateral anteriorly and laterally, by the ectopterygoids and (infratemporal) fenestrae. As in most eusuchians, the pterygoids posteriorly. and by the paired palatines postorbital bar is recessed from the outer configura­ medially. tion of the skull. The long and narrow palatines connect the max­ Posterior to the postorbitals are the squamosals, illae with the pterygoids. Anteriorly. the palatines which form most of the lateral margins of the skull wedge between the maxillae: posteriorly, they under­ table. Viewed dorsally, the squamosals are L-shaped lap the pterygoids for a few millimeters. and form the lateral and posterior margins of the Both pterygoids are damaged: the left one is supratemporal fenestrae. The posterolateral corners almost complete except near the choanae. Damage of the squamosals are elongate processes which and matrix obscure the sutural contact between the overlap the inclined quadrate. A deep pocket, the pterygoids and braincase. The pterygoid flange is external auditory meatus, is present ventral to the large and is underlapped at the lateral margin by the squamosal and posterior to the supratemporal fenes­ descending process of the ectopterygoid. As in trae. The one on the right is damaged, and the one Crocodylus, Tomistoma and Gavialis, the posterior on the left is filled with matrix. However. it appears margins of the pterygoids lie at the same level as the that both were similar to the external auditory meatus lower edge of the basitubera. The posterior margin of of other crocodiles. One minor difference is that the the choanae is damaged, so it is not known if this postorbital bar does not form the anterior limit, or region was crested as in alligators (lordansky, 1973). margin, for the meatus as in Crocodylus
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