Last Friday I wrote about a new study by paleontologist Phil Senter that revised the arrangement of bones in the front feet of Stegosaurus. Despite being only a distant relative of the sauropod dinosaurs, Stegosaurus had convergently evolved a semi-circular pattern of bones which would have given it semi-tubular forefeet similar to that of sauropods like
Towards the end of the paper Senter suggested that ankylosaurs, too, may have had sauropod-like forefeet. If correct, this condition may have been shared among the armored dinosaurs, though Senter stated that further research was required to investigate this idea. That research—conducted by Senter himself—has recently been posted as an in-press paper at Acta Palaeontologica Polonica.
As with stegosaurs, the forefeet of ankylosaurs have been traditionally portrayed with the metacarpals—the bones of the forefoot just behind the fingers—being configured in a shallow arc shape. This would have spread out the fingers and suggested the presence of a pad of flesh to help support the weight of the animal. In rare, articulated ankylosaur skeletons, however, the forefeet have the semi-tubular arrangement seen in some sauropod dinosaurs, and the bones actually have to be articulated incorrectly to give the forefeet a splayed appearance.
A study of the forefeet of the Lower Cretaceous ankylosaur Peloroplites cedrimontanus from Utah's Cedar Mountain Formation confirmed Senter's hypothesis. When articulated naturally, the bones formed a semi-tube which would have made the metacarpals, rather than the fingers, the main weight-bearing bones. Furthermore, Senter cites the skeleton of the Late Cretaceous ankylosaur Saichania chulsanensis from Mongolia as being found articulated in the rock with metacarpals in a semi-tube shape and therefore supporting the idea that this was a natural configuration.
Senter's findings have implications for the evolution of the armored dinosaurs, as well. Stegosaurs and ankylosaurs were sister groups and, together with their closest early relatives, composed a group called the Thyreophora. Linked by common ancestry, stegosaurs and ankylosaurs were more closely related to each other than other kinds of dinosaurs. This presents two alternatives. Either this forefoot arrangement evolved independently in each group, or it was a characteristic inherited from the last common ancestor of the two.
Frustratingly, however, we don't know very much about the early history of armored dinosaurs. Perhaps the best known early form is the approximately 200-million-year-old Scutellosaurus. The trouble is that this dinosaur had forelimbs that were shorter than its hindlimbs, and so it was probably not regularly walking on all fours. If the semi-tube arrangement of metacarpals was an adaptation to support the bulk of these animals, then the characteristic may have been absent in Scutellosaurus.
If Scutellosaurus can be taken as representative of what the last common ancestor of stegosaurs and ankylosaurs was like, then I have to wonder if the semi-tubular metacarpal pattern evolved in each group due to anatomical constraints present in that common ancestor. Rather than inheriting the semi-tubular arrangement directly, perhaps there was something about the forefeet of the last common ancestor that constrained the way the bones could articulate when early stegosaurs and ankylosaurs began walking on all fours. Evolution is not entirely open-ended, and the characteristics of ancestral species place limits on the ways in which their descendants can be adapted.
Furthermore, in the diagram provided by Senter in the paper, the metacarpal arrangement of the ankylosaur Saichania does not form as much of a semi-tube as in Stegosaurus or Pelorolites. Perhaps Saichania, despite being geologically younger than these other dinosaurs, preserves a more archaic condition hinting at independent modification of the forelimb. One data point isn't enough to know for sure, though. Forelimbs from other ankylosaur species, as well as those of the earliest armored dinosaurs, must be found and studied to investigate the evolution of this trait.
Senter, P. (2010). Evidence for a sauropod-like metacarpal configuration in ankylosaurian dinosaurs Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0041