Proterosuchus, Chasmatosaurus, Polymorphodon, Archosaurus, Teyujagua and Sarmatosucbus

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.

Proterosuchus was the earliest and most primitive member of the Euarchosauriformes herein defined as Passer, Proterosuchus, their last common ancestor and all of its descendents. These are the traditional Archosauriformes sans the Pararachosauriformes, Youngina, Youngoides and pterosaurs.

Proterosuchus is traditionally considered the first archosauriform to have an antorbital fenestra, but it actually appeared four times in the Reptilia, including in Chroniosuchus, Cosesaurus, Proterochampsa and Proterosuchus. Click here to enlarge family tree.

Several Proterosuchus (Chasmatosaurus) skulls and their kin to scale and in phylogenetic order. The resemblance of derived specimens to Archosaurus indicates that this genus had its origin in the Late Permian (or the Late Permian date for Archosaurus is in error.)

Polymorphodon adorfi (Sues et al. 2020; Middle Triassic; SMNS 91343, SMNS 91400) is known from scattered and crushed bones. Here it nests basal to all members of the Pararchosauriformes and therefore basal to the proterosuchids.

Archosaurus rossicus (Tatarinov 1960) Late Permian ~250 mya, ~21 cm skull length, is the taxon which gave its name to the entire clade.

Sarmatosuchus otschevi (Sennikov 1994) early Triassic, ~250 mya, ~35 cm skull length, was originally considered a sister to Proterosuchus, but Gower and Sennikov (1997) considered it a basal archosaur. Here Sarmatosuchus appears to be a sister to Proterosuchus. Archosaurs are restricted to the last common ancestor of birds and crocs and all of its descendants.

Teyujagua paradoxa (Pinheiro et al. 2016, De-Oliveira 2024; Early Triassic) was originally described as, "transitional in morphology between archosauriforms and more primitive reptiles...as the sister taxon to Archosauriformes." Instead the large reptile tree recovered Teyujagua as a phylogenetically miniaturized Proterosuchus, phylogenetically nesting deep within that clade. The secondary reduction of the antorbital fenestra, also seen in Youngina BPI 2871 , no doubt caused some of the earlier mistake and the rest can be blamed on taxon exclusion along with inaccurate tracings.

Synaptichnium ichnites (Avanzini and Mietto 2008) can be rather closely matched in shape and chronology to the genus Proterosuchus. This type of foot is the last in this lineage to have digit 4 longer than 3. It also extends proterosuchids into the Middle Triassic.

Avanzini M and Mietto P 2008. The occurrence of the vertebrate ichnogenus Synaptichnium in the Anisian (Middle Triassic) of Southern Alps. Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 259-265.

Broom R. 1903. On a new reptile (Proterosuchus fergusi) from the Karroo beds of Tarkastad, South Africa. Annals of the South African Museum 4: 159–164.

De-Oliveira TM, Da Silva JL, Kerber L and Pinheiro FL 2024. The postcranial skeleton of Teyujagua paradoxa (Reptliia: Archosauromorpha) from the early Triassic of South America. The Anatomical Record DOI: 10.1002/ar.25391

Gower DJ and Sennikov AG 1997. Sarmatosuchus and the Early History of the Archosauria. Journal of Vertebrate Paleontology 17(1):60-73.

Pinheiro FL, França MAG, Lacerda MB, Butler RJ and Schultz CL 2016. An exceptional fossil skull from South America and the origins of the archosauriform radiation. Nature Scientific Reports 6:22817 DOI: 10.1038/srep22817.

Sennikov AG 1994. Pervyj srednetriansovyj proteroscchid iz Vostochnoy Evropy. Doklady Akademii Nauk 336:359-661.

Sues H-D, Schoch RR, Sobral G and Irmis RB 2020. A new archosauriform reptile with distinctive teeth from the Middle Triassic (Ladinian) of Germany. Journal of Vertebrate Paleontology Article: e1764968 (advance online publication)
doi: https://doi.org/10.1080/02724634.2020.1764968
https://www.tandfonline.com/doi/abs/10.1080/02724634.2020.1764968

Tatarinov LP 1960. Otkrytie pseudozhukhii v verkhnei permi SSSR: Paleontologischeskii Zhurnal, 1960, n. 4, p. 74-80.

wiki/Proterosuchus
wiki/Sarmatosuchus
wiki/Teyujagua

Euarchosauriformes

Pararchosauriformes

Proterosuchus

Choristodera