Ornithosuchus Experiments with Bipedalism and Develops a Heel
There must have been something in the water.
Earlier we looked at various experiments with bipedalism in reptiles. Here we’ll take a look at Ornithosuchus and its morphology as compared to another non-bipedal ornithosuchid, Riojasuchus.
Figure 1. Two ornithosuchids, Ornithosuchus and Riojasuchus to scale (above).
The first thing you’ll note is the size. Ornithosuchus is much smaller and therefore lighter on its feet than Riojasuchus. The limbs are more slender. You may also notice the smaller pectoral girdle, evidently because less weight is applied to this area. The shorter vertebral spines argue against a bipedal configuration, as these can provide greater support when the forelimbs are elevated. The feet were relatively longer in Ornithosuchus, providing a more stable base. The torso was shorter, keeping more weight centered over the pelvis. Perhaps oddly (or not) the calcaneal heel was longer in Ornithosuchus, paralleling the same development in bipedal poposaurid dinosaurs that we talked about earlier. Otherwise in basal crocs and dinos the calcaneal heel does not develop.
Ornithosuchus was apparently not a full time biped. By simply extending the forelimbs they could reach the ground. Riojasuchus looks to have been a full time quadruped with the ability to occasionally rise on its hind limbs due to the apparent centering of the weight over the feet due to the balancing abilities afforded by it long, deep tail.
Distinct from dinosaurs, the neck did not form an elevated S-curve with parallelogram-shaped centra, so the head would have been held lower, more in line with the torso, despite inherent flexibility.
Ornithosuchids and rauisuchids were both derived from a sister to Euparkeria (Fig. 2), a tiny erythrosuchid descendant. That might mean the transition from erythrosuchids to ornithosuchids, Vjushkovia and rauisuchids included a tiny detour, but IMHO, due to the large number of larger taxa involved, Euparkeria was a tiny sidebar leading nowhere else. Perhaps a larger species of Euparkeria would have been closer to the main lineage.
Figure 2. Euparkeria and Ornithosuchus. That’s Euparkeria to scale between the feet of Ornithosuchus (red arrow).
Ornithosuchus was a contemporary of bipedal fenestrasaurs (including pterosaurs), bipedal basal crocs, bipedal basal dinosaurs and bipedal tropidosuchids. So whatever was driving bipedalism in one clade was contagious : ) and convergent in the others.
It would be great to get a few more ornithosuchids. Unfortunately, at present, we have too few to create a decent ornithosuchid tree.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References
Bonaparte JF 1969. Dos nuevas “Faunas” de reptiles Triasicos de Argentina: I. Gondwana Symp., IVGS: 283-306.
Huxley TH 1877. The Crocodilian Remains found in the Elgin Sandstones, with Remarks on the Ichnites of Cummingstone. Memoires of the Geological Suvery, Monograph iii.
Walker AD 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 248(744): 53-134.
wiki/Riojasuchus
wiki/Ornithosuchus
2012-12-10 08:09:10
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