COOPER, Lisa N.; BERTA, Annalisa; DEMERE, Thomas; Biology Department, San Diego State University; Biology Department, San Diego State University; Paleontology Department, San Diego State University: Evolution of the Baleen Whale Forelimb

Mysticetes (baleen whales) are traditionally described as having immobile elbows, wrists and digits, but new evidence suggests some of these joints are mobile. By examining the gross anatomy and histology of extant mysticete forelimbs, we address the range of mobility of each joint and identify the mechanical constraint in any immobile joint(s). Studying forelimb morphology in a phylogenetic context allows estimation of the time when significant locomotor adaptations occurred in mysticetes. In extant taxa, a bony ridge on the distal humerus locks the radius and ulna in place, thereby immobilizing the elbow joint. The wrist joint is mobile since the carpo-metacarpal joints have the ability to flex and extend. The interphalangeal joints are immobile in balaenopterids (except the humpback whale) based on fusion of adjacent phalangeal epiphyses via a thin strip of fibrocartilage. This fusion limits the interphalangeal joints to a mobility range of zero to eight degrees. The elbow and interphalangeal joints lost mobility 30 mya in aetiocetids (fossil toothed mysticetes); whereas, the wrist has maintained a limited ability to flex and extend throughout the history of this group. We interpret this overall loss in forelimb mobility as an indication that the forelimb has evolved from a mobile limb in archaic whales into a rigid appendage that generates lift and aids in maintaining lateral stability. Furthermore, ossification in the manus of fossil and extant mysticetes represents a neotenic pattern with delayed endochondral ossification of the carpals and halted secondary ossification of the phalanges.