"Although birds and theropods have a long triangular ossification in front of the tibia and attached to the proximal tarsals, the morphological relationships of this bone are fundamentally different in the two groups."
"The structure of the avian tarsus has recently been cited as evidence for the derivation of birds from theropod dinosaurs. Although birds and theropods have a long triangular ossification in front of the tibia and attached to the proximal tarsals, the morphological relationships of this bone are fundamentally different in the two groups. In modern birds and in all Mesozoic birds, this "pretibial" bone is a high, narrow structure associated primarily with the calcaneum, but independently ossified. The corresponding structure in dinosaurs is a broad extension [ascending process] of the astragalus" [The astragalus is also called the talus bone].
(L.D. Martin et al)
"The Neognathae represent the rest of the modern birds. Their palette is much smaller and their ankles have, instead of a process on the astragalus, it’s the calcaneum that has the process."
"The extremely well-preserved tarsus of the tapejarid Tapejara sp. and the anhanguerid Anhanguera piscator (Pterosauria, Pterodactyloidea) are described and regarded as representative of the ankle structure of Pterosauria. The pterosaur ankle joint (PAJ) shows the following features: astragalus mediolaterally elongated forming a hemicylinder; proximal part of the astragalocalcaneal contact characterized by a ridge bordered on each side by a depression on the astragalus that has a perfect counterpart in the calcaneum, and distal part that is concavo-convex, with the concavity present in the astragalus; calcaneum extremely reduced not reaching the posterior portion of the tarsus; absence of an astragalar posterior groove, perforating foramen, calcaneal tuber, and astragalar ascending process; proximal tarsals fusing very early in ontogeny, forming a tibiotarsus.
The main movement between the crus and foot in the PAJ occurs between the proximal and distal tarsals as in the advanced mesotarsal-reversed joint (AM-R). The main differences from the latter are the lack of an ascending process and the extreme reduction of the calcaneum that make the PAJ unique. The absence of an astragalar groove and the reduction of the calcaneum reinforce the hypothesis that pterosaurs are basal ornithodirans and closely related to the Dinosauromorpha. As has been demonstrated by this and other studies, the ankle structure (a complex of characters) is phylogenetically informative and, in the light of characters from other parts of the animal's body, can contribute to a better understanding of archosaur relationships."
It would not tax the imagination to engender a long list of obstacles for the now dominant model of a theropod origin of birds, including, but not limited to: the fact that early theropods (e.g., Triassic Herrerasaurus) are highly specialized obligate bipeds (with arms reduced to 1/2 the length of the hindlimbs); the fact that the stratigraphic sequence of bird-like theropods has been almost the reversal of the expected evolutionary sequence leading to birds; the fact that the earliest described “feathered dinosaur” is the unbird-like compsognathid Sinosauropteryx, devoid of any preserved structures that can be shown to be feather-like; the fact that any downy-like integumentary covering in a terrestrial theropod would be maladaptive; the fact that flight feathers arranged precisely on the hand as in modern birds are present in microraptors and the basal oviraptosaur Caudipteryx; the fact that many of the derived characters or synapomorphies linking birds and theropods are in question, including notably but not limited to: the sliding lower jaw joint of theropods (absent in birds), the theropod ascending process of the astragalus (distinctive from the avian pretibial bone), and the digital mismatch (1,2,3 theropod vs. 2,3,4 bird hand), etc., to mention a few" (Feduccia, et al., 2005, 266:126, emp. added).
In birds and theropods, a sheet of bone that braces the anterior face of the tibia is usually called the “ascending process of the astragalus” or simply the “ascending process.” It is less evident in adult neornithines than in juvenile (or embryonic) neornithines and Mesozoic birds. This sheet of bone is particularly conspicuous in basal birds, including Archaeopteryx. This common feature has consistently been regarded as one of the most striking homologies shared by birds and theropods (e.g., Paul 2002), but comparative anatomical research reveals that establishing the homologies of the ascending processes of theropods and birds is difficult.PTEROSAUR ANKLE JOINT
In neornithines a triangular, late-developing cartilage appears, after fusion of the proximal tarsals, on the lateral face of the tibia, dorsal to the calcaneum (Martin et al. 1980, and references therein). Subsequently, this cartilage fuses with the calcaneum, with which it is primarily associated in both Mesozoic and modern birds (Martin et al. 1980; Fig. 6). Morse (1872) called this structure the “pretibial.” Ostrom (e.g., 1976a, 1985) argued that this structure is homologous with a similar structure in the tarsus of theropods (see also Paul 2002), but according to Martin et al. (1980:88) “differences in placement and (the pretibial's) late appearance during development suggest that it is a uniquely derived character for birds and is properly termed a pretibial bone, rather than an astragalar process.”
In contrast to the situation in neornithine and Mesozoic birds, the ascending process of theropods is usually a broad sheet of bone, continuous and exclusively associated with the astragalus (compare Fig. 6A and B).
"The metamorphosis of the ankle joints of pterosaurs during development is interesting. In juvenile specimens, the proximal row consists of astragalus and calcaneum which are both reduced in length proximo-distally to become essentially only caps over the lower ends of the tibia, with the loss of the calcaneal tuber.
In adult [pterosaur] specimens, these two bones [the calcaneum and the astragalus] are fused with the tibia to form a rolling hinge joint at their distal condyles as seen in modern birds."
(Sankar Chatterjee, R. J. Templin)
Description: "Fore-limb and hind-limb compared. H., Humerus; R., radius; U., ulna; r., radiale; u., ulnare; C., distal carpals united to carpo-metacarpus; CC., the whole carpal region; MC.I., metacarpal of the thumb; I., phalanx of the thumb; MC.II., second metacarpus; II., second digit; MC.III., third metacarpus; III., third digit.
F., femur; T.T., tibio-tarsus; Fi., fibula; Pt., proximal tarsals united to lower end of tibia; dt., distal tarsals nited to upper end of tarso-metatarsus (T.MT.); T., entire tarsal region; MT.I., first metatarsal, free; I.-IV., toes." -Thomson, 1916Source: Thomson, J. Arthur Outlines of Zoology (New York: D. Appleton & Company, 1916) 650
Further complicating the issue, tibiotarsi of a variety of pterosaurs are even more bird-like and have been frequently misidentified as bird fossils.