Heptasuchus

Heptasuchus is an extinct genus of loricatan pseudosuchian known from the Middle or Late Triassic upper Chugwater Group of Wyoming, USA.[1] It contains a single species, Heptasuchus clarki, the first formally recognized "rauisuchian" or loricatan pseudosuchian from North America.

Discovery and history
Collected in the summer of 1977 at the newly discovered Clark locality northwest of Casper Wyoming, the specimens assigned to H. clarki were described in a brief article[2] and two master's theses at Wayne State University, Detroit. Heptasuchus was first formally described and named by Robert M. Dawley, John M. Zawiskie and J. W. Cosgriff in 1979 and the type species is Heptasuchus clarki. The generic name is derived from epta (ἑπτά), "seven" in Ancient Greek, and suchus (συχος) which is the Latinised form of the Ancient Greek word for the crocodile god of ancient Egypt. The specific name honors George Clark of Casper for discovering the type locality of Heptasuchus. UW 11562, a partial skull and associated postcranial skeleton was designed as the holotype and UW 11563 through UW 11565, representing partial cranial and postcranial remains not associated with the holotype were designed as paratypes.[1][2] Little research followed the initial report, although Heptasuchus has been listed in various publications summarizing fossils of Wyoming and the Triassic. Thought to be lost until 1997, most of the specimens now reside at the University of Wyoming, although some elements are still unaccounted for.

The type locality of H. clarki occurs within a succession of red beds and carbonates above the Crow Mountain Formation which in turn overlies the Alcova Limestone within the Chugwater Group. The Chugwater Group is exposed as an outcrop belt along the trend of the Casper Arch flanking the southeastern Big Horn Mountains. The age of the type locality is not precisely known, but since it is lithostratigraphically equivalent to the lower Popo Agie Formation, though possibly chronostratigraphically older, it is probably Carnian or late Ladinian.[1] Zawiskie et al. (2011) suggested that all of the pseudosuchian elements collected at the locality belong to Heptasuchus and the taxon is not a chimera as previously suggested by Wroblewski (1997).[3] About 50% of the osteology of the taxon is preserved from a minimum of four individuals with overlapping skeletal elements.

Description
Heptasuchus was a medium-sized rauisuchian, with a skull about 65 cm (25.6 inches) long and an estimated total length of about 4.9-5.2 meters (16–17 feet) based on comparisons to Batrachotomus. It is unknown whether this taxon was an obligate quadruped or faculative biped and the only limb elements represented are a fragmentary humerus, an ulna, a tibia, calcania, possible metapodials, and some phalanges that may be referrable. No complete skull is known, but preserved elements indicate a taxon very similar to Batrachotomus from Germany with an arched nasal, large naris, and a postorbital bar that enters the orbit. Reconstructions of Heptasuchus depicting this taxon with unusually large premaxillary teeth are based on a single premaxillary tooth that has partially slipped out of the alveolus. It had a long, straight pubis, curved in anterior view, with a pubic boot, and a gracile ulna lacking the strong olecranon process seen in Postosuchus.[4] Vertebrae are represented by cervical centra, some neural spines from the dorsals or caudals, and caudal centra. Fragments of ribs, gastralia, and possible scutes were collected as well, but have not been formally described or illustrated.