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Postosuchus kirkpatricki
Postosuchus kirkpatricki

[Image: Postosuchus031.jpg]

Fossil range: Carnian–Norian

Scientific classification 
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Reptilia
Subclass: Diapsida
Infraclass: Archosauromorpha 
(unranked): Archosauria 
(unranked): Crurotarsi
Order: Rauisuchia
Family: Rauisuchidae
Genus: Postosuchus

[Image: postosuchus.jpg]

Postosuchus, meaning "crocodile from Post", was a basal archosaur that lived in what is now North America during the middle through to the late Triassic period (228-202 million years ago). It was a rauisuchian, a cousin of crocodiles and came from the same ancestry as dinosaurs. Its name refers to Post Quarry, a place in Texas where many fossils of this genus were found. It was one of the apex predators of its area during the Triassic, larger than the small dinosaur predators of its time (such as Coelophysis). It was a hunter which probably preyed on dicynodonts and many other creatures smaller than itself.

Postosuchus was a quadrupedal reptile with a wide skull and a long tail. This carnivore attacked with its large curved claws. It was about 4–5 meters long (12–15 feet), and was held up by columnar legs (a quite uncommon feature in reptiles). A crocodile-like snout, filled with many large-sized dagger-like teeth, was used to kill its prey. Rows of protective plates covering its back formed a defensive shield.

[Image: postosuchusz12.jpg]

Postosuchus was one of the largest carnivorous reptiles during the late Triassic, reaching up to 4 meters (13 ft) in length and 2 meters (6.5 ft) in height. In life the animal would weigh about 250 to 300 kilograms (551 to 661 lb). It had a massively built and narrow skull bearing dagger-like teeth. The neck was elongated, expanding to a short body and long tail. Each forelimb was slightly over half the size of the hindlimbs. This characteristic of short forelimbs can usually be seen in bipedal reptiles. Chatterjee suggested that Postosuchus could walk in an erect stance, since the short forelimbs were probably used only during slow locomotion. However, in 1995 Long and Murry stated that Postosuchus was heavily built and quadrupedal. There is doubt whether Postosuchus was bipedal or quadrupedal and scientists aren't certain yet about the gait.

The skull of Postosuchus was constructed narrow in front and extended wide and deep behind. It was 55 cm in length and 21 cm broad and deep. There are many fenestrae (openings) present in the bones that lighten the skull, providing space for the muscles. Like more derived archosaurs, the lower jaw had mandibular fenestrae (openings at the lower jaw), formed by the junction of the dentary with other jaw bones (surangular and angular). Postosuchus had very good long distant sight, due to large orbits, supporting large and sharp eyes, and strong olfaction provided by elongated nostrils. Inside the skull, under the nostrils, there was a hollowing that probably was for the Jacobson's organ, an olfactory sensory organ sometimes referred as the "sixth sense". The jaws held large and sharp serrated teeth and in some points were developed even larger to operate as hooked sabers. A complete tooth found among Postosuchus remains in North Carolina measured about 7.2 cm in height. Postosuchus possessed heterodonty dentition, which means each tooth was different in size and shape from the others. The upper jaw composed of seventeen teeth, with each premaxilla bore only four teeth and each maxilla thirteen teeth. In the lower jaw were counted over thirty teeth. Replacement activity in Postosuchus was different from that of crocodiles, since the replacement tooth didn't fit directly in the pulp cavity of the old tooth, but grew until resorption of the old tooth was complete.

The neck of Postosuchus consists of at least eight cervical vertebrae followed by sixteen dorsals, while four co-ossified sacral vertebrae supported the hips. It is thought to be over thirty vertebrae in the tail decreasing in size to the end. The pelvis with the hooked pubis and the rod-like ischium looked like those of carnosaur dinosaurs. Along with remains of the skeleton, paleontologists also identify osteoderms, which were thick plates forming scales. These were on its back, neck, and possibly above or under the tail. The ribcage of Postosuchus had typical archosaur structure, composed of large and slender, curved ribs. In some discoveries ribs were found associated with gastralia, dermal bones which located in the ventral region of the body. The limbs were located underneath the body giving Postosuchus an upright stance. With the forelimbs being approximately 64% of the hindlimbs, Postosuchus had small hands bearing five toes. Only the first toe bore a large claw, which used as an offensive weapon, and the forelimbs were robust probably to hold the prey. Peyer et al. 2008, argued that the thick pectoral girdle served for locomotion of the forelimbs. However, this doesn't retract the theory that Postosuchus could walk bipedaly. The feet were much larger than the hands, with the fifth metatarsal forming a hook shape. The halluxes were more slender than the other toes and the marginal ones couldn't touch the ground. As crurotarsan, the heel and ankle of Postosuchus resemble those of modern crocodiles.

Primarily, it was thought that the fossils of Postosuchus belonged to a dinosaur. Remains of Postosuchus were discovered for the first time in Crosby County, Texas, and described by paleontologist Ermine Cowles Case in 1922. The fossils were composed only of an isolated braincase (UM 7473) and fragments of pelvic bones (UM 7244). Case then mistakenly assigned these specimens to the dinosaur genus Coelophysis. In the case of the braincase later assigned to Postosuchus, in 2002 paleontologist David J. Gower argued that the specimen is not complete and may belong to an ornithodire. Between 1932 and 1934, Case discovered other fossils of caudal vertebrae (UMMP 13670) in Rotten Hill, Texas, and a complete pelvis (UCMP V72183/113314) near Kalgary, Texas. Within the same period, paleontologist Charles Lewis Camp collected over a hundred 'rauisuchian' bones, from what is now the Petrified Forest National Park of Arizona, which belong to at least seven individuals (UCMP A296, MNA 207C). Later, more remains came to light. In 1943, Case again described a pelvis along with a pubis (UM 23127) from the Dockum Group of Texas, which dates from the Carnian through the early Norian stages of Late Triassic period. These early findings, from 1932 to 1943, were initially referred to as a new phytosaur reptile, but assigned forty years later to Postosuchus.

During an expedition in 1980, paleontologists of the Texas Tech University discovered a new geological site rich in fossils near Post Town, Garza County, Texas, where a dozen of well preserved specimens belonging to a new rauisuchian were found. In the following years further excavation in the Post Quarry, in Cooper Canyon Formation (Dockum Group), unearthed many remains of late Triassic terrestrial fauna. The holotype of P. kirkpatricki (TTUP 9000), representing a well-preserved skull and a partial postcranial skeleton, was described along with other findings of this new genus by paleontologist Sankar Chatterjee in 1985. Chatterjee named the species after Mr. and Mrs. Jack Kirkpatrick who helped during his fieldwork. The first articulated skeleton referred to P. kirkpatricki (CM 73372) was recovered by David S. Berman of the Carnegie Museum of Natural History, in Coelophysis Quarry at Ghost Ranch, New Mexico, between 1988 and 1989. This specimen was composed of a well preserved skeleton without skull and was described by Long and Murry in 1995, Weinbaum in 2002 and Novak in 2004. During more recent research, paleontologists suggested that some bones (such manus and toe bones) described by Chatterjee back in 1985, are eventually a combination of remains belonging to Chatterjeea, Lythrosuchus, and Postosuchus. Long and Murry also pointed out that many of the juvenile skeletons (TTUP 9003-9011), which Chatterjee assigned to P. kirkpatricki, belong to a distinct genus, named Chatterjeea elegans. Furthermore, in 2006 Nesbitt and Norell argued that Chatterjeea is a junior synonym of Shuvosaurus.

In 2008, Peyer et al., described a new species of Postosuchus, P. alisonae, which was discovered in 1992 in Triangle Brick Co. Quarry, Durham County, North Carolina. The remains were prepared and reconstructed between 1994 and 1998 by the Department of Geological Sciences at the University of North Carolina. The specific name is in reference to Alison L. Chambers, who worked to popularize paleontology in North Carolina. The skeleton of P. alisonae consists of a few cranial bones, seven neck, one back, and four tail vertebrae, ribs, gastralia ("belly ribs"), chevrons, bony scutes, much of the shoulder girdles, most of the forelimbs except the left wrist and hand, most of the hindlimbs except for the thigh bones, and pieces from the hip. Moreover, the well preserved remains of P. alisonae shed new light on parts of Postosuchus anatomy, which were previously not well known. Specifically, the differences between the manus bones of P. kirkpatricki and P. alisonae confirm the chimera theory (associated fossils belonging to different animals) suggested by Long and Murry. The holotype specimen of P. alisonae (UNC 15575) is also unusual in its preservation of gut contents: bones from at least four other animals, including a partial skeleton of an aetosaur, a snout, coracoid, and humerus of the traversodontid cynodont Plinthogomphodon, two phalanges from a dicynodont, and a possible temnospondyl bone. Furthermore, the Postosuchus was positioned on top of a skeleton of the sphenosuchian Dromicosuchus, which included tooth marks on the skull and neck. P. alisonae represents the largest suchian reptile recovered from the quarry and the first articulated specimen of 'rauisuchian' archosaur found in eastern North America.

Postosuchus lived in a tropical environment. The moist and warm region consisted of ferns, such as Cynepteris, Phelopteris and Clathropteris, gymnosperms, represented by Pelourdea, Araucarioxylon, Woodworthia, Otozamites and Dinophyton, and cycads like Sanmiguelia. Plants of the Dockum Group are not well known since the oxidizing of the environment has destroyed most of the plant fossils. However, some of them may provide information about the climate in Dockum Group during the late Triassic period. For example, the discovery of large specimens belonging to Araucarioxylon determine that the region was well watered. The fauna found in Dockum Group confirm that were lakes and/or rivers which contain fish like the cartilaginous Xenacanthus, the lobe-finned Chinlea and the dipnoan Ceratodus. On the shores of these rivers lived labyrinthodonts (Latiscopus) and reptiles such as Malerisaurus and Trilophosaurus. Also living in the margin of the lakes were the archosaurs Parasuchus, Nicrosaurus and Rutiodon. Postosuchus lived in the uplands along with Coelophysis and other archosaurs such as Desmatosuchus and Typothorax. Postosuchus was one of the largest animals in that ecosystem and exploited the fact of the blended communities praying upon herbivores such as Trilophosaurus and Typothorax.
[Image: wildcat10-CougarHuntingDeer.jpg]
Temnospondyl Wrote:Reptile evolution
[Image: postosuchus588.jpg]
Postosuchus kirkpatricki (Chatterjee 1985) Late Triassic ~210 mya) 4-5 m long, was derived from a sister to Prestosuchus and was a sister to Saurosuchus within the Rauisuchidae. It was originally considered a close ancestor of the dinosaur Tyrannosaurus.
Distinct from Saurosuchus, the skull of Postosuchus was shorter with a prefrontal crest anterior to the orbits and a reduction of the lateral temporal fenestra resulting from further invasion of the quadratojugal in contact with the postorbital. The anterior mandible was strongly upturned.
All the neural spines were longer, but those of the cervical series were especially tall.
The pectoral girdle was relatively smaller. The carpal elements were coosified. The manus had the standard phalangeal formula. Digits II and III were the most robust.
The ilium was substantially taller. The pubis had a large posteriorly directed "boot." The slender ischium had a smaller "boot." The entire pes was substantially longer, due in large part to the elongated metatarsals. Metatarsal I was more gracile.
[Image: wildcat10-CougarHuntingDeer.jpg]
Lateral and top view of the skull from Weinbaum (2011). Chatterjee (1985) made one as well, but the snout was apparently much too narrow (his reconstruction can be seen here).

[Image: Restoration-of-Postosuchus-kirkpatricki-...le-bar.ppm]

Chatterjee (1985) describes metacarpal I of Postosuchus as having an ungual much larger, sharper, and more recurved than those on the other digits. Peyer et al. (2008) corroborate this claim; the digit I manual ungual was mediolaterally compressed, recurved, and from Figure 8 of their paper, evidently sharp-tipped. This has been interpreted as an offensive weapon by Chatterjee, and while Peyer et al. suggest that claws were probably not the primary means of subduing and dismembering prey (considering the fact that the manus was short, and that only one useful claws existed on each manus), they do suggest that the robust forelimbs armed with these claws could have been used to hold onto prey that was being ripped apart by the jaws.

[Image: Postosuchus-alisonae-UNC-15575-holotype-...langes.png]

Interestingly enough, Chatterjee (1985) also described the pedal unguals as slightly recurved and compressed sideways. Again, Peyer et al. (2008) seem to corroborate this claim at least somewhat, describing the digit I pedal ungual as a large, mediolaterally compressed ungual, the digit II pedal ungual as also large, and the digit II pedal ungual as somewhat shorter. Figure 9 of their paper shows the left pes, and the claws are evidently recurved and sharp at the tip (although, the digit I ungual looks blunter-tipped than the rest; perhaps this is due to taphonomic wear?). Hmmmm...I wonder if these might have seen some raptorial use as well...

[Image: Postosuchus-alisonae-UNC-15575-holotype-...sus-in.png]
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