Scleromochlus taylori (Woodward 1907) Late Carnian, Late Triassic ~217 mya, 18 cm long, was derived from a sister to Gracilisuchus and Saltopus.
Padian (1984), Sereno (1991), Bennett (1996), Benton (1999), Senter (2003) and Hone and Benton (2007, 2008) reported that Scleromochlus was the closest taxon to pterosaurs, but this larger analysis found otherwise. One look makes it easy to see why. Scleromochlus has tiny hands, little to no fourth (wing) finger and no pedal digit V, among dozens of other traits listed by Peters (2002). Besides, the fenestrasaurs, Cosesaurus, Sharovipteryx and Longsiquama, provide a series of taxa documenting a gradual evolution of pterosaur characters. No series of archosaurs, archosauriforms or archosauromorphs can do the same. Ironically, Hone and Benton (2007) argued against bipedalism in pterosaurs and their ancestors, but Scleromochlus would have been an obligate biped!
Overall smaller than and distinct from Gracilisuchus, the skull of Scleromochlus was wider and lower. The angle of the quadrate cannot be directly observed so when Benton (1999) reconstructed Scleromochlus he provided with a quadrate that leaned posteriorly, but that produced a mandible with an extremely long retroarticular process and made the palate autapomorphic. Here the quadrate is reconstructed to lean anteriorly, as in otherwise established sister taxa, like Gracilisuchus and the quadrate now articulates with the articular, as in other tetrapods. Moreover, the palate was very much like that in Gracilisuchus with pterygoids meeting medially and with a raised transverse rim. The posterior skull crest is absent here and all later crocodylomorphs.
Only six cervical vertebrate were present, down from eight. A lumbar section remained. The tail was reduced. All neural spines were shorter. Smaller rib, without costal processes, produced a wider, shallower torso.
The forelimb was longer. The manus was tiny.
The pelvis was relatively shorter and smaller. The hind limbs were relatively longer and more gracile. The metatarsus was longer.
Skull image above
from Bennett 2020 reassessed Scleromochlus, but his cladogram included no basal bipedal crocodylomorphs, so the nesting of Scleromochlus was not settled. |
