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.