Hyperphalangy is an increase in the number of phalanges beyond the plesiomorphic mammal condition of three phalanges-per-digit. Hyperphalangy was present among extinct marine reptiles -- ichthyosaurs, plesiosaurs, and mosasaurs -- but not other marine mammals, leaving whales as the only marine mammals to develop this characteristic. The evolutionary process continued over time, and a very derived form of hyperphalangy, with six or more phalanges per digit, evolved convergently in rorqual whales and oceanic dolphins, and was likely associated with another wave of signaling within the interdigital tissues. Other mammals
In ungulates (hoofed mammals) the forelimb is optimized for speed and endurance by a combination of length of stride and rapid step; the proximal forelimb segments are short with large muscles, while the distal segments are elongated with less musculature. In two of the major groups of ungulates, odd-toed and even-toed ungulates, what remain of the "hands" — the metacarpal and phalangeal bones — are elongated to the extent that they serve little use beyond locomotion. The giraffe, the largest even-toed ungulate, has large terminal phalanges and fused metacarpal bones able to absorb the stress from running.The sloth spends its life hanging upside-down from branches, and has highly specialized third and fourth digits for the purpose. They have short and squat proximal phalanges with much longer terminal phalanges. They have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joints.
The arboreal specialization of these terminal phalanges makes it impossible for the sloth to walk on the ground where the animal has to drag its body with its claws. Additional images
See also
Hand
Foot

References
External links

MedTerms.com Medical Dictionary