The rise of scaley bird legs, aka podotheca

New podotheca evidence just in!

Hooray for scaley bird legs! Image of Grey parrot feet by sarniebills1, via Wikipedia.

Back in 2013, I wrote about podotheca (the scales covering the lower leg and foot) in modern birds, and a fossil theropod described by Casa et al. (2013). The authors examined the holotype specimen of Aniksosaurus darwini (a coelurosaurian theropod) for taphonomic features. They found that the upper leg bone (femur) had suffered some post-mortem weathering damage and had tooth scratches from a small scavenging crocodyliform, but the lower leg (tibia, fibula, and metatarsals) had remained relatively unscathed and articulated. They suggested that this could be evidence for a protective podotheca-like covering in A. darwini, which may have been present in many basal coelurosaurians. But Casa et al. (2013) did not find any direct evidence of a fossilised podotheca with the A. darwini holotype.

Their hypothesis now has some new evidence to back it up: a paper by Cuesta et al. (2015) details the first fossil evidence of podotheca in non-avian dinosaurs, specifically, theropods. Cuesta et al. (2015) examined the holotype specimen of Concavenator corcovatus (MCCM-LH 6666) and found fossil podotheca-like scale impressions around the right foot. During decay the claw sheathes and soft tissue had detached from the bone and moved slightly away from the foot, but the authors were able to calculate the degree of skin-slippage and reconstruct the foot as it most likely originally appeared.

Photograph and reconstructions of the right foot of the holotype of C. corcovatus. Images A and B show the original position of the fossil bones and podotheca/claw sheathe outlines. Images C and D show the corrected original alignment of soft tissue on the fossil bone. Figure from Cuesta et al. (2015).

The research by Cuesta et al. (2015) confirms that a podotheca was present in C. corcovatus and likely were present in other non-coelurosaurian theropods. The authors suggest that avian podotheca first appeared in the common ancestor of Avetheropoda or perhaps even Dinosauria. These clades include both coelurosaurian and non-coelurosaurian theropods, meaning that Casa et al.’s (2013) hypothesis on the presence of podotheca in A. darwini is even more likely to be correct!

I think we should conduct more taphonomic research into the decay and disarticulation of podotheca-covered legs. I’ll get right onto it, right after I whip-up this PhD thesis…

Distribution of podotheca in avian and non-avian dinosaurs (crocodyliformes do not have podotheca). The green lines indicate the distribution of podotheca in all Avetheropodans according to Cuesta et al. (2015). They indicate the possible podotheca found in the Neornithischian Kulindadromeus may push back the origin of podotheca to common ancestor of Dinosauria. Figure from Cuesta et al. (2015).

References

Casal, G. A., Martinez, R. D., Ibiricu, L. M., Riga, B. G., Foix, N. 2013. Taphonomy of the theropod dinosaur Aniksosaurus darwini, Bajo Barreal Formation, Late Cretaceous of Patagonia (Argentina) [In Spanish, English abstract]. Ameghiniana, 50: 6.

Cuesta, E., Díaz-Martínez, I., Ortega, F., Sanz, J.L., 2015. Did all theropods have chicken-like feet? First evidence of a non-avian dinosaur podotheca. Cretaceous Research, 56: 53–59.