Suchia

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. Later studies found support for a placement of Prestosuchus deep within Suchia, so Sereno's synapomorphy is likely invalid. Ezcurra (2016)'s synapomorphies ''. Note the rounded ridge on the jugal bone beneath the eye (one of Nesbitt [2011]'s synapomorphies) visible in A, and the posterodorsal process of the maxilla (one of Ezcurra [2016]'s synapomorphies) visible in E (pdpm) Martin Ezcurra's 2016 analysis of archosauromorphs provided an alternative diagnosis for Suchia. Ezcurra found that Koilamasuchus was a member of the group, but also argued that Suchia excluded Nundasuchus. As a result, his diagnosis for Suchia depended primarily on several traits visible in Koilamasuchus, which was known from significantly less fossil material than most other Suchia. These traits are the presence of large, well-rimmed pits on either side of the dorsal (back) vertebrae, a humerus (upper arm bone) which has a symmetrical proximal portion when seen from the front, and a preacetabular process (front blade) of the ilium (upper hip bone) which has a moderate length, longer than it is high, but not longer than the pubic peduncle. He also noted that in some suchians, the tip of the maxillary bone's posterior process (rear branch), near the rear lower tip of the antorbital fenestra, is actually taller than the middle portion of the rear branch. Despite the broad distribution of this characteristic, its absence in some suchians makes it ambiguous whether it qualifies as a synapomorphy of Suchia, or alternatively evolved in several independent lineages within the group. Ezcurra also agreed with one of Nesbitt (2011)'s synapomorphies of the calcaneum, namely the calcaneal tuber being wider than tall. ==Classification==

Suchia was originally formalized by German paleontologist Bernard Krebs in 1974. His intention was to link the Triassic pseudosuchians (which at that time were considered to have left no descendants) with the true crocodilians that would evolve later in the Mesozoic. Suchia was named as an order united by the possession of a crurotarsal ankle, with Pseudosuchia and Crocodilia as independent suborders within it. However, newer studies starting in the 1980s have significantly altered the traditional classification scheme of crocodilian ancestors. Rather than pseudosuchians and crocodilians being sister groups as Krebs believed, the crocodilians are now considered to be a subset of Pseudosuchia. This necessitated Suchia to acquire a new definition, as Kreb's original classification scheme was invalidated. Sereno (1991) assigned the name Suchia to a clade containing most pseudosuchians, which had previously been found by Benton & Clark (1988). Under this new definition, Suchia included Gracilisuchus stipanicicorum, Aetosauria, "Rauisuchia", Poposauridae, Crocodylomorpha, and all descendants of their common ancestor. Phylogeny Below is a phylogenetic cladogram by Butler et al. in 2011 showing the cladistics of Archosauriformes, focusing mostly on Pseudosuchia: Clade names follow Nesbitt 2011. ==References==