Suchia
Suchia is a clade of archosaurs containing the majority of pseudosuchians (crocodilians and their extinct relatives). It was defined as the least inclusive clade containing Aetosaurus ferratus, Rauisuchus tiradentes, Prestosuchus chiniquensis, and Crocodylus niloticus (the living Nile crocodile) by Nesbitt (2011).[1] Generally the only pseudosuchian group which is omitted from Suchia is the family Ornithosuchidae, although at least one analysis classifies ornithosuchids as close relatives of erpetosuchids (which are usually considered suchians) and aetosaurs (which are suchians by definition of the group).[2] Phytosaurs are also excluded from Suchia, although it is not certain whether they qualify as pseudosuchians in the first place.[1][3]
There is some controversy over which traits, if any, can be used to distinguish suchians from non-suchian archosaurs. Anatomical features which evolve at the base of a group, and can thus be used to characterize the group, are known as synapomorphies. The two most comprehensive recent studies on archosaur relationships, Nesbitt (2011)[1] and Ezcurra (2016),[3] each came to different conclusions on what synapomorphies characterize Suchia. Nevertheless, they did agree on one trait that likely qualified as a synapomorphy of Suchia: a calcaneal tuber which was significantly wider than it was tall.
As Suchia is a clade, it is defined by relationships rather than shared characteristics between its members. However, several traits are found in most suchians but not in ornithosuchids, phytosaurs, or other archosaurs and archosaur relatives, so they can be used to help determine whether a reptile is a suchian or not. These traits are spread out throughout the body, but different paleontologists disagree on whether they were true synapomorphies (derived distinguishing features).
While Benton & Clark (1988) found numerous characteristics for the clade later renamed Suchia, Paul Sereno's 1991 study on archosaur relationships argued that many of them were also present in ornithosuchids, phytosaurs, or avemetatarsalians, so they could not be considered traits which diagnose Suchia (under his definition of the group). Instead, Sereno listed a single synapomorphy for Suchia. The postorbital-squamosal bar, which separates the upper and lower temporal fenestrae at the rear part of the skull, is short. This has the effect of making the lower temporal fenestra triangular in shape. However, Sereno also noted that this feature was not present in Prestosuchus, which he excluded from the group.[4] Later studies found support for a placement of Prestosuchus deep within Suchia, so Sereno's synapomorphy is likely invalid.[1]
