Baurusuchidae - Phylogeny

Phylogeny

The family Baurusuchidae was named by Brazilian paleontologist Llewellyn Ivor Price in 1945 to include Baurusuchus. In 1946, American paleontologist Edwin Harris Colbert erected the group Sebecosuchia, which united Baurusuchidae with the family Sebecidae (represented by the genus Sebecus). Both Baurusuchus and Sebecus have deep snouts and ziphodont dentitions (teeth that are serrated and laterally compressed). Other forms were later found that had a close appearance to these two genera, among them Cynodontosuchus, Stratiotosuchus, and Wargosuchus. Several features were used to unite these groups: a deep snout, a ziphodont dentition, a curved tooth row, an enlarged canine-like dentary tooth that fits into a deep notch in the upper jaw, and a groove on the lower jaw.

Many phylogenetic analyses within the past decades have supported a close relationship between the two families. Baurusuchids and sebecosuchids are both early members of the clade Metasuchia, which includes the subgroups Notosuchia (mainly terrestrial crocodyliforms) and Neosuchia (larger, often semiaquatic crocodyliforms, including living crocodylians). Sebecosuchians, which include both baurusuchids and sebecosuchids, were found to be closely related to notosuchians in several studies. The new genera Iberosuchus and Eremosuchus were later assigned to Baurusuchidae, and phylogenetic analyses encompassing these taxa continued to find Baurusuchidae to be closely related to Sebecidae. Both families were allied with notosuchians in the larger group Ziphosuchia, composed of ziphodont crocodyliforms. More recently, sebecosuchians - including baurusuchids - have been placed within Notosuchia as derived members of the clade. Below is a modified cladogram from Ortega et al. (2000) placing baurusuchids within Notosuchia:

In 2004, the superfamily Baurusuchoidea was established to include baurusuchids and sebecids. Phylogenetically, Baurusuchoidea was defined as the most recent common ancestor of Baurusuchus and Sebecus and all of its descendants while Baurusuchidae was defined as the most recent common ancestor of Baurusuchus and Stratiotosuchus and all of its descendants.

In a 2005 analysis, Sebecidae was found to be a paraphyletic grouping, or a grouping that includes some descendants of a common ancestor but not all. Sebecids formed an assemblage of basal sebecosuchians, while baurusuchids remained a valid grouping of derived sebecosuchians. Below is a modified cladogram from Turner and Calvo (2005):

Later studies noted many features that distinguished baurusuchids from sebecosuchids. Sebecosuchids were often considered to be more closely related to Neosuchia, a group that includes modern crocodylians, while baurusuchids were thought to be a more distantly related clade. In a 1999 phylogenetic analysis, Baurusuchus formed a clade with notosuchians to the exclusion of other ziphosuchians. This placement has been upheld by recent analyses, which place Baurusuchus within Notosuchia.

In 2007, a new clade called Sebecia was erected. Sebecia included sebecids and peirosaurids. Peirosauridae, a family of small terrestrial crocodyliforms, had often been placed in or near Neosuchia in previous studies. The assignment of sebecids to Sebecia placed the family closer to Neosuchia than Notosuchia. In this study, baurusuchids were split up, with Baurusuchus placed as a more basal metasuchian and the remaining baurusuchids (Bretesuchus and Pabwehshi) placed as sebecians. Therefore, the family Baurusuchidae was paraphyletic. Below is a modified cladogram from Larsson and Sues (2007):

More recent studies have nested Baurusuchus deep within Notosuchia, just as the larger group Sebecosuchia once was, while the remaining sebecosuchian genera have been placed more distantly in Metasuchia. A new baurusuchid called Pissarrachampsa was named in 2011, and a comprehensive phylogenetic analysis of baurusuchids was conducted along with its description. Montefeltro et al. (2011) found Baurusuchidae to be a monophyletic group with the genera Baurusuchus, Cynodontosuchus, Pissarrachampsa, Stratiotosuchus, and Wargosuchus. They adopted the name Baurusuchia in a phylogenetic sense to distinguish baurusuchids from related crocodyliforms. Baurusuchia was first erected as an infraorder in 1968, but in the 2011 analysis it was found to be in an identical position to Baurusuchidae in the final tree. The only difference between Baurusuchidae and Baurusuchia is that the former is a node-based taxon and the latter is a stem-based taxon. Baurusuchidae is defined as the least inclusive clade containing Baurusuchus, Cynodontosuchus, Pissarrachampsa, Stratiotosuchus, and Wargosuchus. As in all node-based clades, there is a most recent common ancestor; these genera are all of its known descendants. Baurusuchia is defined as Baurusuchus pricei and all Crocodyliformes that share a more recent common ancestor with B. pricei than with Notosuchus terrestris, Mariliasuchus amarali, Armadillosuchus arrudai, Araripesuchus gomesi, Sebecus icaeorhinus, Bretesuchus bonapartei, Peirosaurus torminni, or Crocodylus niloticus.

In contrast to the node-based Baurusuchidae, the stem-based Baurusuchia does not include a common ancestor and all its descendants, but rather all forms more closely related to a specific baurusuchid than a non-baurusuchid. As a stem-based taxon, Baurusuchia is more inclusive than Baurusuchidae; a new taxon could potentially be placed outside Baurusuchidae because it is not a descendant of the most recent common ancestor of baurusuchids, but would still be a baurusuchian because it is more closely related to baurusuchids than it is to other crocodyliforms. For now, however, Baurusuchidae and Baurusuchia are essentially identical in scope.

Montefeltro et al. (2011) also divided Baurusuchidae into two subfamilies, Pissarrachampsinae and Baurusuchinae. Pissarrachampsinae includes Pissarrachampsa and Wargosuchus while Baurusuchinae includes Stratiotosuchus and Baurusuchus. Cynodontosuchus is not a member of either of these subfamilies, but the most basal baurusuchid. Many of the unique features that separate Cynodontosuchus may also be associated with a juvenile individual. The material that Cynodontosuchus is based on has been suggested to be a juvenile form of Baurusuchus, and the two taxa may be synonymous.

Below is a cladogram from Montefeltro et al. (2011):

A sixth genus of baurusuchid, Campinasuchus, was named just a few months before Pissarrachampsa, and was not included in the analysis.