| Susisuchus Temporal range:Early Cretaceous | |
|---|---|
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| S. anatoceps specimen (A) and cross sectioned samples (B-C) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Crocodylomorpha |
| Clade: | Neosuchia |
| Family: | †Susisuchidae Salisburyet al.,2003 |
| Genus: | †Susisuchus Salisburyet al., 2003 |
| Species | |
Susisuchus is anextinctgenus ofneosuchianmesoeucrocodyliancrocodyliform from theEarly Cretaceous ofBrazil. Fossils have been found from the Nova Olinda Member of theAptian-ageCrato Formation in theAraripe andLima Campos Basins of northeastern Brazil. Named in 2003,Susisuchus is the sole member of the familySusisuchidae, and is closely related to thecladeEusuchia, which includes livingcrocodilians. Thetype species isS. 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.
The genusSusisuchus was first erected with the description of the type speciesS. anatoceps in 2003.S. anatoceps was found in theAptian-ageCrato Formation in the Araripe Basin, and was the first crocodyliform to be described from the formation. Theholotype skeleton is the most complete known specimen ofSusisuchus, 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 115 kilometres (71 mi) from where the skeleton ofS. anatoceps was uncovered.[1]
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 thetrematochampsidCaririsuchus, on the basis of characteristics of thefibula and proportions of thefemur andtibia. Since the limb elements differ from all other known crocodyliforms from the Araripe Basin, the specimen was tentatively classified as cf.Susisuchus sp.[2]
Theholotype specimen ofSusisuchus anatoceps includes a skull and lower jaw, forelimbs, parts of theaxial skeleton, and someosteoderms. 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 thesquamosal andparietal bones that is situated on the posterior border of thesupratemporal fenestra, a hole in theskull table.[1]
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 atetraserial 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 asBernissartia 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 inSusisuchus.[3]Susisuchus also has two rows of accessory osteoderms to either side of the paravertebral osteoderms which are not part of the shield.[4]
The segmentation of the paravertebral shield allowed for a greater degree of lateral flexibility in the trunk ofSusisuchus. 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 ofSusisuchus. IfSusisuchus grew any larger than 50 kilograms (110 lb), the forces encountered during high-walking would have been too great for the paravertebral shield to counteract.[3]
Susisuchus is closely related toPachycheilosuchus fromGlen Rose, Texas,United States, andIsisfordia fromWinton, Queensland,Australia. All three genera are highlyderived Early Cretaceous mesoeucrocodylians.[4] 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, whileSusisuchus was placed outside Eusuchia as itssister taxon. Below is acladogram from Salisburyet al. (2006) showing the relationships ofSusisuchus andIsisfordia within Mesoeucrocodylia:[3]
Susisuchus hasamphicoelusthoracic,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 whichSusisuchus is the only member.[4]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 ofS. jaguaribensis as a second species ofSusisuchus, the genus has beenphylogenetically redefined as anode-based taxon including the last common ancestor ofSusisuchus anatoceps andSusisuchus jaguaribensis and all of its descendants.[1]
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