Earlier I used
Greg Paul and José Bonaparte drawings of the basal bipedal croc Pseudhesperosuchus Bonaparted 1969) for data on this taxon. The specimen has some traits that lead toward the secondarily quadrupedal Trialestes. Together they are part of a clade that is closer to basal dinosaurs than traditional taxa paleontologists have been working with.
The drawings were great,
but I wondered what the real material looked like…and more importantly, what was real and what was not.
A recent request to
the curators at Miguel Lillo in Argentina was honored with a set of emailed jpegs from their museum drawers (Fig.1), for which I am very grateful. These were traced in line and color and reassembled with just a few unidentified parts left over (Fig. 2).
Pseudhesperosuchus jachaleri (Bonaparte 1969 Norian, Late Triassic ~210mya, ~1 m in length, was derived from a sister to Junggarsuchus and Lewisuchus and was at the base of a clade that included Trialestes on one branch and the Dinosauria on the other branch.
Much larger and distinct from Lewisuchus,
the skull of Pseudhesperosuchus had a smaller antorbital fenestra, an arched lateral temporal fenestra, a deeper maxilla and a large mandibular fenestra. The seven cervicals were attended by robust ribs.
The scapula and coracoid were each rather slender and elongated. An straight interclavicle was present. The forelimbs were long and slender. The radiale and ulnare were elongated, a croc trait. Only three metacarpals and no digits are known.
The ilium was relatively small, but probably longer than tall and not perforated. The femur remained longer than the tibia. The tarsus, if that astragalus is identified correctly, included a simple hinge ankle joint. Only two conjoined partial metatarsals are known.
There is a small box
full of little sometimes interconnected squares among the Pseudhesperosuchus material (Fig. 2, aqua colored). I’m guessing that those are osteoderms, and if so, were probably located along the back. These would have helped keep that elevated backbone from sagging in this new biped.
The improvements in the Pseudhesperosuchus data
changed a few scores, but did no change the tree topology. The large reptile tree (LRT) can be seen here.
It’s good to see what Pseudhesperosuchyus really looked like,
— or at least get a little closer to that distant ideal. Size-wise and morphologically, this largely complete specimen is closer to the basal dinosaur outgroup than any other currently included in the LRT. And yet it is also distinctly different as it shares several traits with Trialestes unknown in any dinosaur. As a denizen of the Late Triassic, Pseudhesperosuchus represents a radiation that occurred tens of millions of years earlier, probably in the Middle Triassic. None of this clade survived into the Jurassic, as far as we know.
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.