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Postosuchus alisonae

Postosuchus Chatterjee, 1985.

ARCHOSAURIA > CRUROTARSI > SUCHIA > RAUISUCHIDAE

Type Species—Postosuchus kirkpatricki Chatterjee, 1985.

Revised Diagnosis of genus—Differs from all other ‘rauisuchians’ in: infratemporal fenestra divided by a forward projecting quadratojugal; comparatively large, plate-like quadratojugal; lacrimal with a dorsomedial process that separates the prefrontal from
the nasal; axis centrum with two parallel ventral keels; cervical vertebrae that are constricted at mid-length to less than half the width of centrum articular surface; cervical vertebrae with welldeveloped keels that are developed into hypapophyses anteriory and posteriorly; cervical vertebrae with heart-shaped ‘spine tables’; very short, anterior cervical ribs; a subrectangular rather than ovate coracoid; proportionately more robust supraglenoid buttress of the scapulocoracoid; immediately narrowing scapula posterior to glenoid fossa instead of slowly grading into the scapular blade; comparatively deeper notch ventral to medioventral process of humeral head; comparatively small manus; reduced blunt unguals on manual digit III and I

INTRODUCTION
The partial skeleton here referred to a new species of Postosuchus represents the first articulated skeletal material of a ‘rauisuchian’ archosaur from the Upper Triassic of eastern North America. It was discovered in 1994 in the Triangle Brick Co. Quarry near Genlee in the Durham sub-basin of the Deep River Basin in North Carolina and was prepared and reconstructed in the Department of Geological Sciences at the University of
North Carolina at Chapel Hill, between 1994 and 1998. The present specimen is the only large suchian discovered at the site. The  bones of the skeleton are virtually uncrushed and, for the most part, fully or at least partially articulated. In terms of its preservation and completeness, this specimen is comparable only to the holotype of Ticinosuchus ferox Krebs, 1965 from southern Switzerland, and to the hypodigm of Batrachotomus kupferzellensis Gower, 1999 from Baden-Württemberg (Germany), and possibly the holotype and paratype of Postosuchus kirkpatricki (TTUP 9002) Chatterjee, 1985 from the Cooper Canyon Formation (Dockum Group) in Post, Texas. In addition to P. kirkpatricki,
other Upper Triassic, North American ‘rauisuchians’ have been described from the Dockum Group of Texas, the Bull Canyon Formation of New Mexico, the Chinle and Moenkopi Formations of Arizona and New Mexico, and the Popo Agie Formation of Wyoming. Middle Triassic North American ‘rauisuchians’ are known from the Moenkopi Formation of Arizona. These taxa are represented by Poposaurus Mehl, 1915, Arizonasaurus Welles, 1947, Heptasuchus Dawley, Zawiskie, and Cosgriff, 1979, Postosuchus Chatterjee, 1985, Lythrosuchus Long and Murry, 1995, Chatterjeea Long and Murry, 1995, and Effigia Nesbitt and Norell, 2006 (Nesbitt, 2005a; Nesbitt and Norell, 2006). Throughout much of the Middle and Late Triassic, ‘rauisuchians’ were the dominant terrestrial predators and they had a near global distribution. Their remains are known from North America, South America, Europe, Africa, and India, but not yet from Australia or Antarctica (Gower, 2000).
The abdominal region of the holotype of the new species of Postosuchus preserves gastrointestinal contents comprising remains of at least four different taxa of Late Triassic tetrapods including a partial skeleton of a small stagonolepidid (cf. Stegomus), a snout, left coracoid, and left humerus of the traversodont cynodont Plinthogomophodon herpetairus Sues, Olsen, and Carter, 1999, two articulated phalanges of a large dicynodont, and a fragment of an unidentified ?temnospondyl bone. These materials differ in color from the bones of UNC 15575, and some of the bones bear tooth marks and show periosteal erosion possibly caused by digestion. An articulated skull and partial postcranial skeleton of the crocodylomorph Dromicosuchus grallator Sues, Carter, Olsen, and Scott, 2003, was preserved underneath the skeleton of Postosuchus and bears tooth marks on the skull and neck.

Previous Work on Postosuchus
The first known specimens of Postosuchus are fragments of
two pelves described by Case (1922), and referred to Postosuchus
kirkpatricki by Chatterjee (1985). Case (1932, 1934) then
discovered a series of caudal vertebrae (UMMP 13670) from
Rotten Hill, Texas and a complete pelvis (UCMP V72183/113314) near Kalgary, Texas. Camp (1932) collected the articulated
surangular-articular of a ‘rauisuchian’ archosaur (UCMP
27492) in what is today the Petrified Forest National Park of
Arizona and, during the period from 1932 to 1934, he recovered
over 100 additional disassociated ‘rauisuchian’ bones representing
at least seven individuals (UCMP A296, MNA 207C). This
material was assigned to Postosuchus kirkpatricki (Long and
Murry, 1995). During the early 1980s, Chatterjee excavated a
dozen, mostly dissociated specimens of ‘rauisuchian’ archosaurs
near Post, Texas. He designated the largest specimen (TTUP
9000) as the holotype and a slightly smaller specimen (TTUP
9002) as the paratype of his new species Postosuchus kirkpatricki.
The remaining much smaller skeletons, considered juveniles
by Chatterjee, were also assigned by him to Postosuchus
kirkpatricki, but Long and Murry (1995) suggested that they are
instead the new taxon Chatterjeea elegans Long and Murry, 1995.
Gower (2000) and Weinbaum (2002) agreed with Long and
Murry that Chatterjeea elegans represents a distinct taxon, although
Nesbitt and Norell (2006) indicated that Chatterjeea is a
junior synonym of Shuvosaurus. Long and Murry (1995) also
pointed out that the hypodigm of Postosuchus kirkpatricki represents
a chimera of three distinct ‘rauisuchian’ taxa: Chatterjeea,
Lythrosuchus, and Postosuchus. The present authors found
(pers. obs. of TTUP 9000 and 9002) that the material assigned to
Postosuchus kirpatricki is at least in part composed of associated
rather than articulated bones. For example, the manus that was
originally described for Postosuchus kirkpatricki (Chatterjee,
1985:fig. 14i) was not found in articulation but was reconstructed
from various phalanges found in proximity to the Postosuchus
material (Weinbaum, 2002). When compared to the articulated
manus of the UNC Postosuchus it is evident that the left manus
of P. kirkpatricki is composed of left and right phalanges and,
furthermore, certain manual elements appear to belong to a different
taxon (e.g., PhI-1; PhI-2; PhV-1), possibly Chatterjeea.
Whereas most phalanges are of manual origin based on size, it is
also possible that some represent distal pedal phalanges (PhII-3
and PhIII-4). Some elements closely resemble certain phalanges
in the UNC Postosuchus whereas the placement of others remains
unclear.
The first articulated ‘rauisuchian’ skeleton referable to Postosuchus
kirkpatricki was collected by D. S. Berman (Carnegie
Museum of Natural History) from the Coelophysis (Whitaker)
Quarry at Ghost Ranch in 1988 and 1989 and was subsequently
described by Long and Murry (1995), Weinbaum (2002), and
Novak (2004). It is a fairly well preserved specimen (CM73372)
missing only the skull, part of the neck, most of the left scapulocoracoid,
the distal portion of the left humerus, pubis, and the
articular ends of the femur. CM 73372 differs from Postosuchus
kirkpatricki TTUP 9000 and TTUP 9002, respectively, in the
structure of the ilium (preacetabular process of CM 73372 is
longer than the pubic process), bone sculpturing, and development
of certain muscle attachments (Novak, 2004). Novak (2004)
concluded that the differences can be explained by the fact that
CM 73372 represents a much younger individual than TTUP
9002 or TTUP 9000.
Although many mostly disarticulated elements have been assigned
to Postosuchus kirkpatricki few bones can be confidently
assigned to this taxon. At this point, it is likely that at least the
skull first described by Chatterjee (1985) is referable to Postosuchus
kirkpatricki (see Gower, 2002). In the present paper, we
compare the North Carolina specimen to elements in the TTUP
collection (TTUP 9000; TTUP 9002; TTUP 9235) that are probably
referable to Postosuchus based on their overall similarities
to the homologous elements in the articulated specimen of Postosuchus
kirkpatricki from Ghost Ranch (CM 73372) (Novak,
2004) and the new find from North Carolina (UNC 15575). Features
in the pectoral girdle, manus, distal region of the hindlimb,
and pes of the North Carolina specimen indicate that it represents
a new species of Postosuchus, differing from Postosuchus
kirkpatricki CM 73372, TTUP 9000, and TTUP 9002.

Postosuchus alisonae Peyer, Carter, Sues, Novak, and Olsen, 2008

Etymology—Named for the late Alison L. Chambers, honoring her dedication to the popularization of paleontology in North Carolina.

Type Locality and Horizon—Mudstone of Lithofacies Association II sensu Hoffman and Gallagher (1989), south-central region of Durham sub-basin of Deep River Basin, Newark Supergroup. West Genlee, Durham County, North Carolina, U.S.A. Age: late Carnian or early Norian, Late Triassic.

Holotype—UNC 15575, partial skeleton comprising a few fragmentary cranial bones: nasal, frontal, squamosal, prootic, supraoccipital, left and right opisthotic, articular, angular, prearticular, and isolated teeth. The postcranial skeleton includes seven cervical, one dorsal, and four caudal vertebrae, with associated ribs and chevrons; partial sacral rib; cervical, dorsal, and caudal osteoderms; gastralia; right and partial left coracoid; partial left and right scapulae; interclavicle; clavicle; left and right humeri, radii, and ulnae; right carpus; nearly complete right and partial left manus; distal ends of left and right pubes; left and right tibiae, fibulae, tarsi, and pedes.


Skeletal reconstruction of Postosuchus alisonae based on the preserved elements of UNC 15575 (holotype). Preserved bones are shown in black. The reconstruction of the skull and pelvic bones are based on Postosuchus kirkpatricki (TTUP 9002). Scale bar equals 50 cm.

Diagnosis—Postosuchus alisonae differs from all known ‘rauisuchians’ in: proximal portion of metacarpal I grooved for contact with metacarpal II. Distinguished from Ticinosuchus ferox in: strongly constricted axis centrum that has about the same length as the postaxial cervical vertebrae; no accessory caudal neural spines; small, possibly ventral or distal dorsal osteoderms that are not waisted and concave posteriorly; a subrectangular coracoid; more prominent olecranon process; proportionately shorter manus; more reduced manual unguals. Distinguished from Fasolasuchus tenax in: much smaller overall size; less expanded spine tables on the dorsal vertebrae; less pronounced
olecranon process. Distinguished from Yarasuchus deccanensis in: presence of only a single median keel on the ventral surface of the cervical centra. Distinguished from Prestosuchus chiniquensis in: presence of a longer postglenoid process on the coracoid. Distinguished from Rauisuchus tiradentes in: more pronounced nasal ridge; scapular blade narrows immediately posterior to the glenoid fossa; presence of posterior lip on the proximal end of the fibula; single prominent insertion for M. iliofibularis. Distinguished from Batrachotomus kupferzellensis in: presence of a dorsally flat frontal; nearly flat medial portion of the posterodorsal surface of the supraoccipital; no median ridge on the dorsal surface of the small osteoderms; a subrectangular coracoid; absence of a well-developed muscle insertion on the scapula; more strongly developed olecranon process.

Description - A partial but largely articulated skeleton of a ‘rauisuchian’ archosaur from Late Upper Triassic strata of the Durham sub-basin, Deep River basin, Newark Supergroup, North Carolina, represents a new species of Postosuchus Chatterjee, 1985. It represents the first record of this taxon from eastern North America. The well preserved specimen includes cranial bones, a largely articulated right manus, right and left pedes, pubes, axis, several postaxial cervical, dorsal, and caudal vertebrae, chevron bones, osteoderms, interclavicle, clavicles, cervical ribs, a sacral rib, and a complete set of gastralia. The skeletal elements are described and compared to those of other ‘rauisuchians’. An apparant autapomorphy
of Postosuchus alisonae includes a well developed flange on the proximal portion of metacarpal II fitting into strongly proximally grooved metacarpal I. The new specimen includes many bones previously unknown for Postosuchus and it allows a more complete differentiation of Postosuchus from other ‘rauisuchian’ genera. Diagnostic features of this genus include an axis with two ventral keels; postaxial cervical centra with strongly developed single ventral keels that are anteriorly and/or posteriorly extended into hypapophyses; short ribs on the anterior cervical vertebrae; heart-shaped cervical neural ‘spine tables’; a subrectangular, relatively short coracoid; and proportionately short manus with dorsoventrally compressed, reduced, blunt unguals on manual digits III and IV. Morphological comparisons indicate a close relationship between Postosuchus and Batrachotomus and possibly also Tikisuchus.

References
Peyer, Carter, Sues, Novak, and Olsen, 2008. A NEW SUCHIAN ARCHOSAUR FROM THE UPPER TRIASSIC OF NORTH CAROLINA. Journal of Vertebrate Paleontology 28(2):363–381
Category: "Thecodontia" | Added: dinosauria
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