The tarsus and distal end of the tibia are described in Herrerasauridae, a family that includes the oldest known dinosaurs. This tarsal configuration is compared to those of more advanced dinosaurs and to other archosaurs. Through phylogenetic analysis of the morphological characters, a picture emerges of the evolutionary changes in the ankles of early dinosaurs. The tibia of the herrerasaurids has a quadrangular distal articular surface, with a shallow ventrolateral notch. This morphology is strikingly similar to that of the lagosuchid thecodonts Pseudolagosuchus and Lagosuchus and represents the most primitive tibial condition known for Dinosauria. Aside from the derived states possessed by Theropoda, Sauropodomorpha, and Omithischia, respectively, it was impossible to recognize synapomorphies in tibiotarsal anatomy shared by these groups exclusive of Herrerasauridae. The transverse broadening of the distal end of the tibia seems to have been attained independently by omithischians, theropods, and sauropodomorphs. The tarsus of herrerasaurids is characterized by an astragalus with a small but conspicuous lateroventral depression, by a pyramidal calcaneum with a ventromedial projection that articulates into the cavity of the astragalus just mentioned, and by a posterolaterally directed calcaneal tuber. These characters are also seen in Lagosuchus (a close relative of dinosaurs), in the prosauropod Riojasaurus and, insofar as the astragalus is concerned, in the primitive dinosaur Walkeria, which suggests that dinosaurs of different lineages shared the same tarsal condition. By definition, this type of articulation between the astragalus and calcaneum follows the "crocodile-reversed" tarsal condition, suggesting that the tarsus in lagosuchids and dinosaurs could be derived from the "crocodile-reversed" pattern present in Orith- osuchidae and Euparkeria. In contrast, the mesotarsal ankle of lagosuchids and dinosaurs lacks the synapomorphies of the "crocodile- normal" ankle present in Crocodylia, Rauisuchidae, Aetosauria, and other archosaurs. It is concluded that Herrerasauridae retained the primitive tibiotarsal condition for Dinosauria, from which those of the Omithischia, Sauropodomorpha, and Theropoda were derived. Furthermore, tibiotarsal anatomy supports monophyly of Dinosauria.
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