Proterosuchus fergusi (Broom 1903 ) Early Triassic, ~250 mya, was derived from a sister to Youngoides UC1528 and preceded Fugusuchus at the bases of the Euarchosauriformes and the Archosauriformes. Junior synonyms include Elaphrosuchus and Chasmatosaurus. At 1.5 m it was substantially larger than Youngoides romeri.
Distinct from Youngoides UC1528, the skull of Proterosuchus was relatively smaller and provided with an antorbital fenestra and a mandibular fenestra. The premaxilla dropped anteriorly, giving the jawline a "hook". The lateral temporal fenestra was greatly expanded, as in BPI 3859 and AMNH 5561. In dorsal view the rostrum was slightly pinched and the occipital condyle was anterior to the jaw joints.The posterior frontals were laterally expanded. The palatines were reduced to strips. The ectopterygoids were bent posteriorly.
Distinct from BPI 3859, the cervicals were much more robust and gently arched. All the neural spines were taller.
The scapula was more robust and expanded dorsally. The manus was robust, but otherwise identical.
The ilium was lower with a small anterior process. The pubis and ischium were in complete contact and oriented medially. Metatarsal V was reduced and hook-shaped. Metarsal I was reduced such that mt II and III lined up with p1.1.
Lateral view image above courtesy of G. Paul and used with permission.