Sillosuchus

''Sillosuchus'' is a genus of shuvosaurid poposauroid archosaur that lived in South America during the Late Triassic period. Shuvosaurids were an unusual family of reptiles belonging to the group Poposauroidea; although their closest modern relatives are crocodilians, they were bipedal and lightly armored, with dinosaur-like hip and skull structures. Based on skull remains from members of the family such as ''Effigia'', they were also toothless and likely beaked herbivores.

Discovery and naming

The holotype specimen of ''Sillosuchus'', PVSJ 85, is a partial skeleton discovered in sediments of the Ischigualasto Formation, Cancha de Bochas Member in the Ischigualasto-Villa Unión Basin in northwestern Argentina. The skeleton includes various vertebrae, ribs, and pelvic (hip) bones. A shoulder girdle, humerus (upper arm bone), and partial tibiae (inner shin bones) were also associated with the skeleton but not featured in the original description of the specimen.

Possible additional remains were first described by Dr. William Sill in 1974, though he referred them to ''Saurosuchus'', a giant quadrupedal predator distantly related to ''Sillosuchus''. One specimen, PVL 2472, includes fragments of the tibia, ankle, and a large isolated cervical (neck vertebra), long. In comparison, the cervicals of the ''Sillosuchus'' holotype are only long. Another specimen, PVL 2267, includes hip and hindlimb material. Nesbitt (2011) briefly reconsidered these large fragments, and found that they showed several distinctive features akin to the ''Sillosuchus'' holotype, mostly differing in their much larger size.

The generic name of ''Sillosuchus'' refers to Dr. William Sill, and ''suchus'', from the Greek word meaning "crocodile". The specific name, ''longicervix'', refers to the elongated cervical vertebrae of the genus. Later studies have shown that, while the neck of ''Sillosuchus'' was long compared to most pseudosuchians, this trait was par for the course for shuvosaurids. A reconstruction of the fossil was presented in 2008 by the National University of San Juan in Argentina, in an homage to Dr. William Dudley Sill who was an important promoter of paleontology in San Juan. ''Sillosuchus'' is the only named genus of bipedal poposauroid currently known from outside North America. Other bipedal poposauroids included the related shuvosaurids ''Shuvosaurus'' and ''Effigia,'' as well as the carnivorous ''Poposaurus''.

Description

The individual of ''Sillosuchus'' to which the original specimen belonged had an estimated length of about . However, other remains could possibly indicate that it could grow much larger, up to in length. If these fossils are properly attributed, they would indicate that ''Sillosuchus'' was among the largest terrestrial pseudosuchians (crocodilian-line archosaurs).

''Sillosuchus'' had several unique features compared to its relatives. The neck and back vertebrae had large excavations or pockets on the side, an unusual trait that assisted paleontologists in assigning the giant cervical vertebra to ''Sillosuchus'' rather than ''Saurosuchus''. The left and right ischia (rear-pointing hip bones) were short and fused to each other. They were also flattened in a dorsoventral (top-to-bottom) direction, unlike the case in most other reptiles in which they were flattened in a mediolateral (side-to-side) direction. Otherwise, the hip is practically identical to that of ''Effigia'' and ''Shuvosaurus'' (also sometimes known as "''Chatterjeea''").

References

{{Portal bar, Paleontology, Argentina

Poposauroidea
Carnian genera
Late Triassic reptiles of South America
Triassic Argentina
Fossils of Argentina
Ischigualasto Formation
Fossil taxa described in 1997
Prehistoric pseudosuchian genera