Evolution of hyperphalangy and digit reduction in the cetacean manus

Abstract

Cetaceans (whales, dolphins, and porpoises) have a soft tissue flipper that encases most of the forelimb, and elongated digits with an increased number of phalanges (hyperphalangy). In addition, some cetaceans exhibit a reduction in digit number. Although toothed cetaceans (odontocetes) are pentadactylous, most baleen whales (mysticetes) are tetradactylous and also lack a metacarpal. This study conducts a survey of cetacean metacarpal and phalangeal morphologies, traces the evolution of hyperphalangy in a phylogenetic context, optimizes characters onto previously published cetacean phylogenies, and tests various digit loss hypotheses. Dissections were performed on 16 cetacean flippers representing 10 species (8 mysticetes, 2 odontocetes). Phalangeal count data were derived from forelimb radiographs (36 odontocetes, 5 mysticetes), osteological specimens of articulated forelimbs (8 mysticetes), and were supplemented with published counts. Modal phalangeal counts were coded as ordered and unpolarized characters and optimized onto two known cetacean phylogenies. Results indicate that digital ray I is reduced in many cetaceans (except Globicephala) and all elements of digital ray I were lost in tetradactylous mysticetes. Fossil evidence indicates this ray may have been lost approximately 14 Ma. Most odontocetes also reduce the number of phalangeal elements in digit V, while mysticetes typically retain the plesiomorphic condition of three phalanges. Results from modal phalangeal counts show the greatest degree of hyperphalangy in digits II and III in odontocetes and digits III and IV in mysticetes. Fossil evidence indicates cetacean hyperphalangy evolved by at least 7-8 Ma. Digit loss and digit positioning may underlie disparate flipper shapes, with narrow, elongate flippers facilitating fast swimming and broad flippers aiding slow turns. Hyperphalangy may help distribute leading edge forces, and multiple interphalangeal joints may smooth leading edge flipper contour.