Susisuchus

Susisuchus is an extinct genus of neosuchian mesoeucrocodylian crocodyliform from the Early Cretaceous of Brazil. Fossils have been found from the Nova Olinda Member of the Aptian-age Crato Formation in the Araripe and Lima Campos Basins of northeastern Brazil. Named in 2003, "Susisuchus" is the sole member of the family Susisuchidae, and is closely related to the clade Eusuchia, which includes living crocodilians. The type species is "S. anatoceps", known from a single partial articulated skeleton that preserves some soft tissue. A second species, "S. jaguaribensis", was named in 2009 from fragmentary remains.
Discovery.
The genus "Susisuchus" was first erected with the description of the type species "S. anatoceps" in 2003. "S. anatoceps" was found in the Aptian-age Crato Formation in the Araripe Basin, and was the first crocodyliform to be described from the formation. The holotype skeleton is the most complete known specimen of "Susisuchus", including most of the bones except for those of the hindlimbs, and even traces of soft tissue. The specimen is thought to have been a desiccated carcass before it was buried and later fossilized. A second species, "S. jaguaribensis", was named in 2009 from the Lima Campos Basin about from where the skeleton of "S. anatoceps" was uncovered.
A new crocodyliform specimen from the Crato Formation was described in 2009. It can be distinguished from the two other crocodyliforms known from the formation, "Araripesuchus" and the trematochampsid "Caririsuchus", on the basis of characteristics of the fibula and proportions of the femur and tibia. Since the limb elements differ from all other known crocodyliforms from the Araripe Basin, the specimen was tentatively classified as cf. "Susisuchus" sp.
Description.
The holotype specimen of "Susisuchus anatoceps" includes a skull and lower jaw, forelimbs, parts of the axial skeleton, and some osteoderms. There are traces of soft tissue around both forelimbs and the digits of the right hand. "S. jaguaribensis" is known from far less material, but the fragmentary remains are enough to diagnose the species. Both species have a contact between the squamosal and parietal bones that is situated on the posterior border of the supratemporal fenestra, a hole in the skull table.
Osteoderms.
"Susisuchus" has a dorsal shield of osteoderms that is similar to those of the more derived clade Eusuchia, which includes living crocodilians. "Susisuchus" was one of the first mesoeucrocodylians to have a tetraserial paravertebral shield, which is formed from the dorsal osteoderms to make the back more rigid. In a tetraserial paravertebral shield, there are four separate rows of paravertebral osteoderms (osteoderms near the midline of the back). Earlier mesoeucrocodylians such as "Bernissartia" have biserial paravertebral shields, in which there are only two rows of paravertebral osteoderms. These two rows of large osteoderms became segmented into four smaller rows in "Susisuchus". "Susisuchus" also has two rows of accessory osteoderms to either side of the paravertebral osteoderms which are not part of the shield.
The segmentation of the paravertebral shield allowed for a greater degree of lateral flexibility in the trunk of "Susisuchus". This flexibility would have enabled lateral undulation while swimming, resulting in greater efficiency in locomotion. Because the width of the paravertebral shield was not compromised by this segmentation, the back would remain rigid enough to overcome the forces encountered during high-walking, or walking semi-erect. However, the ability to high-walk would have restricted the size of "Susisuchus". If "Susisuchus" grew any larger than , the forces encountered during high-walking would have been too great for the paravertebral shield to counteract.
Classification.
"Susisuchus" is closely related to "Pachycheilosuchus" from Glen Rose, Texas, United States, and "Isisfordia" from Winton, Queensland, Australia. All three genera are highly derived Early Cretaceous mesoeucrocodylians. In the initial description of Susisuchidae, "Isisfordia" was considered a possible member of the family, although the genus had not yet been named. However, with its naming in 2006, "Isisfordia" was placed within Eusuchia as the most basal member of the clade, while "Susisuchus" was placed outside Eusuchia as its sister taxon. Below is a cladogram from Salisbury "et al." (2006) showing the relationships of "Susisuchus" and "Isisfordia" within Mesoeucrocodylia:
"Susisuchus" has amphicoelus thoracic, lumbar and caudal (tail) vertebrae, meaning that the ends of the centra of these vertebrae are concave. The presence of this feature in a crocodyliform with a tetraserial paravertebral shield is unusual, and has been used to erect the family Susisuchidae, of which "Susisuchus" is the only member. "Susisuchus" can be considered a transitional form because it has a tetraserial paravertebral shield characteristic of eusuchians, but retains more primitive amphicoelus vertebrae characteristic of more basal crocodylomorphs.
With the description of "S. jaguaribensis" as a second species of "Susisuchus", the genus has been phylogenetically redefined as a node-based taxon including the last common ancestor of "Susisuchus anatoceps" and "Susisuchus jaguaribensis" and all of its descendants.