Sillosuchus longicervix Alcober and Parrish, 1997

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 29-31

Sillosuchus longicervix Alcober and Parrish, 1997

AGE: Late Carnian, Late Triassic ( Rogers et al., 1993, adjusted for the new Triassic timescale of Muttoni et al., 2004).

OCCURRENCE: Ischigualasto Formation, Argentina.

HOLOTYPE: PVSJ 85, postcranium consisting of parts of five cervical vertebrae, the last four dorsal vertebrae, five sacrals, and the first eight caudal vertebrae (the dorsal, sacrals, and caudal vertebrae preserved in articulation), partial right ilium, both pubes (nearly complete), both ischia preserved in articulation, both femora (complete), various pieces of ribs, and indeterminate fragments. Additionally, a partial left coracoid and scapula, the proximal portion of the left humerus, and the proximal portions of both tibiae were also collected with the holotype but not described in the original description.

REMARKS: Sillosuchus longicervix was described from an incomplete postcranial skeleton that is poorly preserved and crushed ( Alcober and Parrish, 1997). Although rare in the Ischigualasto Formation, other specimens have been found, but await description. The deep pockets on the lateral side of the cervical and the anterior dorsal vertebrae are unparalleled among pseudosuchians. The anteroposteriorly elongate and oval pockets stretch for much of the length of the centrum and only a thin lamina of bone at the midline separates the lateral pockets. Much of the morphology of the pelvis of Sillosuchus is very similar to Shuvosaurus and Effigia . All three taxa share coossified ischia, a dorsally expanded ilium, a thin, anteriorly arching crest dorsal to the supraacetabular crest, anteroposteriorly elongated cervical centra, and four or more sacral vertebrae (Nesbitt, 2007).

The coracoid, part of the scapula, and the proximal portion of the humerus were collected with the holotype, but were not described in the original description. The coracoid bears an elongated postglenoid process like that of Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9001), but does not bear a deep fossa on the dorsal surface of the process as do Effigia and Shuvosaurus . The scapula is anteriorly expanded into a thin sheet of bone just like that of Effigia (AMNH FR 30587). Furthermore, the proximal portion of the head is poorly expanded, and even though the bone is not complete, the proximal portion of the humerus was probably not expanded more than twice the midshaft, another synapomorphy with Effigia and Shuvosaurus (Nesbitt, 2007) . In summary, the undescribed forelimb material of the holotype of Sillosuchus is remarkably similar to that of Effigia and Shuvosaurus . Furthermore, the gracile humerus of Sillosuchus suggests that the forelimb of Sillosuchus was possibly similar to the short forelimbs of Effigia , and it may have had a similar forelimb to hind limb length. Therefore, Sillosuchus possibly was another bipedal taxon.

As remarked by Alcober and Parrish (1997), the presence of Saurosuchus and Sillosuchus in the Ischigualasto Formation, two relatively closely related taxa, adds ambiguity to the identification of isolated specimens of both taxa. For example, Sill (1974) tentatively assigned PVL 2267, an isolated cervical vertebra, to Saurosuchus . The presence of deep lateral pockets and anteroposterior elongation of the centrum indicate assignment to Sillosuchus rather than Saurosuchus . Furthermore, the anteroposteriorly short and dorsally tall cervical vertebrae found with the nearly complete skull of Saurosuchus (PVSJ 23) preclude assignment of PVL 2267 to Saurosuchus .

Sillosuchus is one of the larger pseudosuchians from the Triassic as indicated by the holotype (femur length 5 47 cm) and the larger isolated cervical vertebra (PVL 2267) referable to Sillosuchus (see preceding paragraph). Each of the cervicals in the holotype measures about 8 cm long, whereas PVL 2267 measures 20 cm long. The length of PVL 2267 suggests that Sillosuchus could have reached an estimated length (from extrapolation of data from the holotype of Sillosuchus and Effigia ) of 9–10 meters. This is further supported by other large isolated elements (PVL 2267; partial left ilium).

Nesbitt (2007) confirmed the hypothesis of Alcober and Parrish (1997) that Sillosuchus and Shuvosaurus (5 Chatterjeea ) are closely related, and both are closely related to Poposaurus .

Sillosuchus longicervix possesses deep pockets (5 pneumatic recesses) on the lateral side of both the cervical and dorsal centra and coossified ischia with a highly dorsoventrally compressed distal end that differentiates it from all other archosaurs. Sillosuchus also possesses a unique suite of characters including: five coossified sacral vertebrae; small pubic boot; dorsally expanded ilium with a thin, anteriorly arching crest dorsal to the supraacetabular crest (5 rim). Alcober and Parrish (1997) listed two characters in the diagnosis: elongated cervical vertebrae and relatively short ischia. The cervical vertebrae of Sillosuchus are indeed elongated but proportionally are not more elongated than those of Shuvosaurus (5 Chatterjeea ) and Effigia (AMNH FR 30587). Furthermore, the short ischia are not complete, and therefore the length cannot be assessed with certainty. That said, the preserved length of the ischia are probably relatively short relative to the pubis.

KEY REFERENCES: Alcober and Parrish, 1997; Nesbitt, 2007.