New perspectives on morphological variation in tridactyl footprints: clues to widespread convergence in developmental dynamics

Abstract

It is well-known that the phalangeal formulae of the feet of dinosaurs and other vertebrates are relatively consistent within any given clade. Indeed, such similarities are part of the basis for alpha taxonomy (e.g., bird and non-avian theropod relationships). Developmental studies of the vertebrate foot support the view that morphological similarity owes as much to intrinsic (formal) developmental dynamics as to extrinsic (functional) influences. Thus, the morphologies of ornithopod, non-avian theropod and most bird feet are convergent (especially with respect to digits II-IV) despite significant differences in size and functional adaptation. Despite these "general" convergences, "detailed" variation in the morphology of tridactyl, mesaxonic, Triassic through recent non-avian dinosaur and bird tracks has allowed the diagnosis of hundreds of ichnotaxa, as well as the recognition of nearly continuous subtle variation in modern bird tracks. Several studies characterize this variation by measuring how far digit III projects anteriorly beyond lateral digit IV and medial digit II, creating an "anterior triangle" between the tips of digits II, III and IV (Weems, 1992). Differences in this projection of digit III highlight a polarity between strong mesaxony (strong central tendency) and weak mesaxony (weak central tendency). Early studies (Olsen, 1980) suggested that as theropod tracks in the Grallator-Eubrontes plexus increase in size, they shift from narrow to wide and from strongly to weakly mesaxonic. However, such polarities also reiterate (independent of size) among medium-sized and diminutive theropod tracks (e.g., Minisauripus). Such polarity also recurs among ornithopods, and is equally striking in extant birds (e.g., between passerines and members of the charadriiformes). Such "intrinsic" differences can only be partially attributed to functional variation in digit divarication caused by foot-substrate interaction. Moreover, such polarities in foot morphology give important clues to whole limb and whole body proportions, and suggest an intrinsic "lawfulness" to patterns of convergence.