From Flat Foot to Fat Foot: Structure, Ontogeny, Function, and Evolution of Elephant “Sixth Toes”

Abstract

Several groups of tetrapods have expanded sesamoid (small, tendon-anchoring) bones into digit-like structures ("predigits"), such as pandas' "thumbs." Elephants similarly have expanded structures in the fat pads of their fore- and hindfeet, but for three centuries these have been overlooked as mere cartilaginous curiosities. We show that these are indeed massive sesamoids that employ a patchy mode of ossification of a massive cartilaginous precursor and that the predigits act functionally like digits. Further, we reveal clear osteological correlates of predigit joint articulation with the carpals/tarsals that are visible in fossils. Our survey shows that basal proboscideans were relatively "flat-footed" (plantigrade), whereas early elephantiforms evolved the more derived "tip-toed" (subunguligrade) morphology, including the predigits and fat pad, of extant elephants. Thus, elephants co-opted sesamoid bones into a role as false digits and used them for support as they changed their foot posture.

Tracing the origin of the panda’s thumb

“…Although additional data on the radial sesamoid morphology of extinct ursids and other carnivorans (not available at the moment due to the scarce fossil radial sesamoid record) are required to fully test this scenario, the currently available evidence from the fossil record strongly suggests that the false thumb of lesser and giant pandas is best interpreted as a case of exaptive convergence, supporting the view that this process (Poe et al 2007;Legreneur et al 2011;Hutchinson et al 2011) might be more widespread than previously recognized. …”

“…However, the radial sesamoid can be considered a special kind of sesamoid, with a completely different role and therefore subjected to different anatomical strictures. In most instances, this bone is of similar size to other sesamoids, or even vestigial, but in some mammals, such as talpids (Krause and Jenkins 1983;Sánchez-Villagra and Menke 2005), many tenrecids (Salton and Sargis 2008) and elephants (Hutchinson et al 2011), it constitutes a digit-like element that is variously called 'os falciforme', 'prepollex' or 'predigit'. Furthermore, a truly hypertrophied radial sesamoid, constituting a functional 'false thumb', is considered to be present in the giant panda, Ailuropoda melanoleuca (Lankester and Lydekker 1901;Wood-Jones 1939a, b;Davis 1964;Gould 1978;Chorn and Hoffmann 1978;Endo et al 1996Endo et al , 1999aEndo et al , b, 2001aAntón, et al 2006;Salesa et al 2006a, b) and, to a lesser extent, in the red panda, Ailurus fulgens (Roberts and Gittleman 1984;Endo et al 2001bEndo et al , 2007Antón et al 2006;Salesa et al 2006b).…”

“…In the giant panda lineage, the evolutionary origin of the false thumb is less well understood, and thus it is uncertain whether it should be considered an exaptation regarding its current role in bamboo feeding (Hutchinson et al 2011), as in lesser pandas (Antón et al 2006;Salesa et al 2006a, b), or rather as an adaptation to food manipulation, as customarily thought (Wood-Jones 1939a, b;Davis 1964;Endo et al 1999b;Antón et al 2006;Salesa et al 2006b). The interpretation of the giant panda's false thumb as an adaptation to manipulation is supported by the small (ursine-like) radial sesamoid previously reported for the Late Miocene ailuropodine genus Indarctos (Roussiakis 2001).…”

Morphology, function and evolution of the pseudothumb in the Otton frog

“…Extra digit-like structures are also known in other taxa, including the 'sixth toes' of elephants, which have evolved to support weight (Hutchinson et al, 2011), and the mole's thumb, which lacks any apparent functional role (Sánchez-Villagra & Menke, 2005). Examination of the function of these unique characters has shed light on the developmental evolution of hands.…”

Camouflage Variations on a Theme of the Nymphalid Ground Plan

“…Although the concept of homology is still widely discussed (Patterson 1982;Roth 1988;Wagner 1989Wagner , 2007Brower and Schawaroch 1996;Hall 2000), Remane's criteria remain valuable consensuses that crystallize empirical facts through numerous careful observations of morphological structures. Currently, these criteria provide a powerful tool to decipher the homology of anatomical structures in a broad spectrum of animals and plants (for animals : Nagashima et al 2009;Hutchinson et al 2011;Luo 2011;Holland et al 2013;for plants: Sattler 1984;Buzgo et al 2004). The NGP is a scheme for describing homologous elements shared across species and thus should be evaluated within the logical framework of Remane's criteria.…”