Figure 1. Diandongosuchus (above) compared to Teraterpeton (below). Note the similar scapula shapes and the way the posterior dorsal ribs terminate in a line. Both lack the flaring cheeks of parasuchians and Youngina. Teraterpeton, with so few teeth, could well have been a plant eater or anything but a carnivore. Hopefully we’ll find more of this genus someday.
From the Pritchard and Sues 2016 abstract (abridged)
“Teraterpeton hrynewichorum, from the Upper Triassic (Carnian) Wolfville Formation of Nova Scotia, is one of the more unusual early archosauromorphs, with an elongate edentulous snout, transversely broadened and cusped teeth, and a closed lateral temporal fenestra. Initial phylogenetic analyses recovered this species as the sister taxon to Trilophosaurus spp. (1). New material of Teraterpeton includes the first-known complete pelvic girdle and hind limbs and the proximal portion of the tail. These bones differ radically from those in Trilophosaurus, and present a striking mosaic of anatomical features for an early saurian (2). The ilium has an elongate, dorsoventrally tall anterior process similar to that of hyperodapedontine rhynchosaurs. (3) The pelvis has a well-developed thyroid fenestra, a feature shared by Tanystropheidae, Kuehneosauridae, and Lepidosauria. (4) The calcaneum is ventrally concave, as in Azendohsaurus. (5) The fifth metatarsal is proximodistally short, comparable to the condition in Tanystropheidae. (6) Much as in the manus, the pedal unguals of Teraterpeton are transversely flattened and dorsoventrally deep. Phylogenetic analysis of 57 taxa (7) of Permo-Triassic diapsids and 315 characters supports the placement of Teraterpeton as the sister-taxon of Trilophosaurus in a clade that also includes Azendohsauridae and, rather unexpectedly, Kuehneosauridae.(8) In the current phylogeny, the aforementioned amalgam of characters in Teraterpeton were all acquired independently from the other saurian lineages. We partitioned the dataset based on anatomical region to examine metrics of homoplasy across early Sauria. The CI of the partitions are not markedly different, but the RI of the pelvic girdle and hindlimb partitions are markedly higher than the others. Although the characters in the hindquarters partitions underwent a similar number of homoplastic changes, a higher proportion of them contribute to the overall structure of this phylogenetic reconstruction. The mosaic condition in Teraterpeton underscores the importance of thorough taxon sampling for understanding the dynamics of character change in Triassic reptiles and the use of apomorphies in identifying fragmentary fossils.”
Figure 2. Paleorhinus also has a tall ilium with an anterior process like that of rhynchosaurs, AND it is closely related to Teraterpeton in the LRT. The ventral pelvis is unknown but no phytosaur or any basal archosauriform has a thyroid fenestra, but then none of these are toothless, like Teraterpeton.
Pritchard AC, Sues H-D 2016. Mosaic evolution of the early saurian post cranium revealed by the postcranial skeleton of Teraterpeton hrynewichorum (Archosauromorpha, Late Triassic). Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.