title: 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.
authors.affiliation: Department of Veterinary Basic Sciences and Structure and Motion Laboratory, The Royal Veterinary College, Hatfield, UK.
authors.forename: John R
authors.initials: JR
authors.lastname: Hutchinson
authors.forename: Cyrille
authors.initials: C
authors.lastname: Delmer
authors.forename: Charlotte E
authors.initials: CE
authors.lastname: Miller
authors.forename: Thomas
authors.initials: T
authors.lastname: Hildebrandt
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Pitsillides
authors.forename: Alan
authors.initials: A
authors.lastname: Boyde
mesh_terms: D000273:Adipose Tissue / Q000033:anatomy & histology / Q000502:physiology; D000818:Animals; D005075:Biological Evolution; D001696:Biomechanical Phenomena; D002356:Cartilage / Q000033:anatomy & histology / Q000254:growth & development; D004606:Elephants / Q000033:anatomy & histology* / Q000254:growth & development / Q000502:physiology; D005260:Female; D005528:Foot / Q000033:anatomy & histology* / Q000254:growth & development / Q000502:physiology; D033023:Foot Joints / Q000033:anatomy & histology / Q000502:physiology; D005552:Forelimb; D005580:Fossils; D006614:Hindlimb; D008124:Locomotion; D008855:Microscopy, Electron, Scanning; D009068:Movement; D010012:Osteogenesis; D012716:Sesamoid Bones / Q000033:anatomy & histology* / Q000254:growth & development; D014034:Toes / Q000033:anatomy & histology* / Q000254:growth & development / Q000502:physiology; D014057:Tomography, X-Ray Computed