trochanter" (but unrelated to the lesser trochanter of the femur in humans), is a ridge on the outer surface of the femur, near the femoral head. It was probably an insertion point for the iliofemoralis cranialis muscle which helps to raise the leg. Most archosaurs and archosaur relatives lack a distinct anterior trochanter, but ornithosuchids are an exception, along with most dinosauromorphs (dinosaurs and their close relatives). Much like the femur, the fibula (outer shin bone) also has a distinctive point for muscle insertion. The muscle in question is the iliofibularis, which helps to straighten the limbs. In most archosaurs, the iliofibularis is inserted onto the fibula by means of a tiny ridge on the proximal part of the fibula, near the knee. However, ornithosuchids have a much larger, knob-shaped iliofibularis insertion point located about midway down the shaft of the fibula. Phytosaurs and aetosaurs also share a knob-like attachment point midway down the fibula, so it is unclear whether the case in ornithosuchids is a unique case of convergent evolution, or alternatively the retention of a trait independently lost by several archosaur lineages. Unlike most other early archosaurs, the pedal unguals (the distalmost bones of the feet that form claws) are laterally compressed. They are sharp and recurved. The unguals are very deep, being taller than they are long, especially on the inner digits. This type of claw is not seen in any other Triassic archosaur except for pterosaurs. Major archosaur groups have often been distinguished from each other based on the structure of their ankles. In most crurotarsans, the astragalus has a convex projection that fits into a concave space in the calcaneum. This condition is often referred to as a "crocodile-normal" ankle, as it is the most common ankle type in crurotarsans. Ornithosuchids are unique among crurotarsans, and all other archosaurs, in their possession of a "crocodile-reversed" ankle. In a "crocodile-reversed" ankle, the placement of the concavity is reversed: instead of being on the calcaneum, it is on the astragalus. In ornithosuchids, the calcaneum bears a convex projection that is analogous to the convex projection on the "crocodile-normal" astragalus. Phylogeny Ornithosuchidae is generally considered to be within the larger clade Suchia, which includes aetosaurs, rauisuchians, and crocodylomorphs. Below is a cladogram based on Nesbitt (2011), showing the placement of Ornithosuchidae in Archosauriformes. References External links Palaeos Vertebrates Late Triassic pseudosuchians Late Triassic first appearances Rhaetian extinctions Prehistoric