Paracrocodylomorpha

Parrish (1993) defined Paracrocodylomorpha as the last common ancestor of "Poposauridae" and Crocodylomorpha and all of its descendants. He believed that Poposaurus and its allies were among the closest relatives of crocodylomorphs. Therefore, his definition was intended to exclude various groups of heavily-built Triassic pseudosuchians, collectively known as "rauisuchians". Weinbaum & Hungerbühler (2007) hypothesized that the relations of Pseudosuchians were reversed to those hypothesized by Parrish. They claimed that "rauisuchians" such as Postosuchus, Batrachotomus, and Saurosuchus were closer to crocodylians than Poposaurus was. They erected a subgroup within Paracrocodylomorpha which they called Paracrocodyliformes. This subgroup contained all pseudosuchians closer to those three "rauisuchians" (as well as the Nile crocodile) rather than Poposaurus.[3]

Nesbitt (2011) introduced the most recent and common definition of Paracrocodylomorpha in his broad phylogenetic analysis. His results supported Weinbaum & Hungerbühler (2007)'s claim that "rauisuchians" were closer to crocodylians than Poposaurus was. In fact, he found that crocodylomorphs were actually descended from the "rauisuchian" group, thus rendering "Rauisuchia" a paraphyletic grade of various reptiles leading to crocodylomorphs. Moreover, the name "Paracrocodyliformes" used by Weinbaum & Hungerbühler (2007) was assigned a new name, Loricata.[2]

Ticinosuchus, the sister taxon to paracrocodylomorphs under Nesbitt (2011)'s methodologies

The name "Paracrocodylomorpha" was re-defined by Nesbitt in his study, reacquiring a name similar to its original usage by Parrish (1993). Nesbitt found that poposauroids formed the sister group to loricatans, and that the clade encompassing poposauroids and loricatans required a new name. Therefore, he used Paracrocodylomorpha to connect Poposaurus and Crocodylus, and all descendants of their common ancestor. Unlike Parrish's vision for the clade, Nesbitt's Paracrocodylomorpha included the "rauisuchians", which were members of Loricata. The only "rauisuchian" not found to belong to Paracrocodylomorpha with absolute confidence was the former "prestosuchid" Ticinosuchus. Nesbitt found that his most parsimonious results indicated that this genus was the sister taxon of the paracrocodylomorphs, rather than an internal component of the group. However, due to its crushed skeleton obscuring crucial anatomical details, it is also conceivable that Ticinosuchus may actually belong inside Paracrocodylomorpha pending new info.[2] This result has already been recovered by Da-Silva et al. (2019), which places Ticinosuchus as the basalmost loricatan while also sometimes having the recently described suchian Mandasuchus lie outside of the group.[4]

According to Parrish (1993), this clade was nested within Rauisuchia, a group of crurotarsans that once included only extinct Triassic forms but is now generally regarded as paraphyletic. Rauisuchia traditionally included the families Rauisuchidae and Poposauridae to the exclusion of Crocodylomorpha, but Parrish found Poposauridae more closely related to Crocodylomorpha than to Rauisuchidae. This close relationship was recognized since the 1980s. The genus Gracilisuchus was also found to be a close relative of crocodylomorphs and poposaurids but was not placed within Paracrocodylomorpha. Below is a cladogram showing the phylogenetic relationships in Parrish's 1993 study:[1]

However, more recent analyses have doubted the idea that poposaurids (and by extension other poposauroids) were grouped with crocodylians to the exception of other groups. For example, a 2007 phylogenetic study by Weinbaum & Hungerbühler, found the rauisuchian families Rauisuchidae and Prestosuchidae to be more closely related to Crocodylomorpha than was Poposauridae. The authors erected the clade Paracrocodyliformes for the clade including Rauisuchidae, Prestosuchidae, and Crocodylomorpha.[3] In addition, several putative poposaurids such as Postosuchus and Teratosaurus were found to be rauisuchids. Sterling J. Nesbitt in 2011 found a similar topology, including Rauisuchidae and "Prestosuchidae" ( the latter of which he found to be polyphyletic) within Paracrocodylomorpha. The closest relative of Paracrocodylmorpha to not qualify as a member of the clade is Ticinosuchus, a large predator once considered to qualify as a prestosuchid.[2] The composition of Paracrocodylomorpha generally follows the analysis of Nesbitt (2011) in other recent studies.