Homology of the astragalus and structure and function of the tarsus of Diadectidae

Abstract

Superbly preserved tarsi of a new, undescribed, primitive member of Diadectidae and of Diadectes, the best known member of the family, are described. The major distinction between them is the retention of sutures in the astragalus of the former which clearly indicate an origin from the fusion of three separate ossifications considered homologues of the primitive amphibian tibiale, intermedium, and proximal centrale. Among the Diadectomorpha (includes also Limnoscelidae and Tseajaiidae) only Diadectidae possesses an astragalus, which is considered a synapomorphy of the family within this grouping. Furthermore, the sister-group relationship of the new, undescribed diadectid to the other diadectids demonstrates a transformational, phylogenetic homology of the astragalus via the ontogenetic fusion of the primitive amphibian tarsal bones. The astragalus of diadectids is identical to those of late Paleozoic terrestrial amniotes in structure and relationship to neighboring elements. This, plus the wide acceptance of a close relationship between Diadectomorpha and Amniota, is cited as suggestive of an identical developmental origin of their astragali.In diadectids, including fully mature individuals, an unusual reduction or absence of ossification of some central and distal tarsal bones has resulted in an unique tarsus with large unoccupied areas and a structural pattern in which the only bony link between the tarsus and the digits is via the fourth distal tarsal, producing a crude facsimile of the lacertilian mesotarsal joint. Such a joint would have permitted, as in lacertilians, a wide range of movements which may have served several important functions: 1) maintaining an anteriorly directed pes to maximize the force of its posterior thrust during limb retraction, 2) placement of the pes close to the body midline for greater stride length and more efficient support and greater maneuverability during locomotion.