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. References Neosuchians Early Cretaceous crocodylomorphs of