Each of the major clades of marine mammals stems from different terrestrial origins—cetaceans sharing a com-< » mon ancestry with ungulates, pinnipeds with carnivo-rans, and sirenians and desmostylians with paenungulates (elephants and kin). The degree to which these groups have become specialized for an aquatic existence varies dramatically, with some modem forms spending a considerable portion of their lives on land (pinnipeds), and others none (cetaceans and sirenians); their ability to locomote terrestrially varies accordingly. For those clades whose extant members are exclusively aquatic, the fossil record provides insights into the terrestrial locomotory capabilities of early transitional forms.
I. Pinnipedia and Kin
Being the geologically youngest group of marine mammals, pinnipeds evince the most obvious signs of their terrestrial ancestry. The limbs of pinnipeds are substantially shortened, reducing the terrestrial agility of these animals relative to their carnivoran allies. Even so, pinnipeds retain considerable mobility on land, but the way in which that mobility is achieved varies remarkably with the group. Pinnipeds can be categorized simple mindedly as “wrigglers” or “walkers.” In phocids, during progression on land, the hind flippers remain outstretched posteriorly above the ground, with forward motion being achieved through lurching. Weight is borne not by the flippers but alternately by the abdomen and pelvic region. After the latter is drawn forward and planted, earthworm-like extension of the torso pitches the remainder of the animal forward (Tarasoff et al., 1972). By contrast, in otariids and the walrus (Odobenus rosmarus), the hind legs are capable of being turned forward, with the soles and the palms contacting the ground in a more typically mammalian fashion. These drastically different modes of terrestrial locomotion influenced the now-refuted notion of multiple pinniped origins and were long argued to substantiate an exclusive grouping of otariids and walruses. Because use of the limbs in terrestrial locomotion is obviously a primitive feature, however, it bears neither on the question of a close otariid walrus relationship nor on the relationship of phocids to other pinnipeds.
Although typically playing little role in phocid terrestrial locomotion, the fore flippers are sometimes employed to a minor extent. In phocines the fore flippers can be used to help drag the body forward, whereas in elephant seals they provide balance. The crabeater seal (Lobodon carcinophaga) uses alternate strokes of its fore flippers and a sinuous flapping of its hindquarters to move rapidly on ice (O’Gorman, 1963).
In otariids, walking is accomplished with the abdomen held clear of the ground, with weight being borne equally by all four flippers (Beentjes, 1990). The elongated digits of the anterior limb point posterolaterally, with a sharp bend between the manus and the forearm occurring between the rows of carpals. The hindlimbs are bent forward at the ankle, with the toes pointing anterolaterally. As in all pinnipeds, the left and right knees are incorporated within the body wall, severely limiting the forward-backward excursion of the limbs (much as long, tight skirts do in humans). During a slow walk the fore flippers are moved alternately, whereas the hind flippers move in unison in some species and alternately in others. A gallop moves otariids more quickly, with the fore flippers being moved forward in unison and then the hind flippers. Walruses locomote in a similar manner, but most of the weight is carried by the abdomen (which rests on the substrate), with forceful lunges propelling the animals forward (Gordon, 1981).
The early pinnipedimorph Enaliarctos (approximately 23 million years in age) is known from a beautifully preserved skeleton, including flipper-footed limbs (Berta et al, 1989). Nothing of its postcranial anatomy suggests that Enaliarctos moved any differently on land than do modern otariids. Similarly, the extinct desmatophocids and allodesmids probably walked in the manner of otariids and the walrus.
II. Cetacea and Kin
Apart from sliding onto decks at marine theme parks, and snatching the occasional penguin or pinniped from water’s edge, modern cetaceans are completely inept on land. Such was not always the case, however, as spectacular fossil finds have revealed whale progenitors retaining four fully formed appendages. While diese early fossils—often termed “archaeocetes”—are nearly universally termed cetaceans, the latter name is not yet defined formally. Cetacea is probably best used, however, to designate the least inclusive clade encompassing mysticetes and odontocetes, in which case “archaeocetes” are not cetaceans by definition (despite being obviously closely related to cetaceans). The recendy described Ambulocetus (meaning walking whale) from roughly 52-million-year-old river deposits in Pakistan is the most remarkable of these transitional animals.
In contrast to cetaceans, the elbow, wrist, and digital joints remained fully functional in Ambulocetus. The hands sprawled laterally during terrestrial locomotion; movements of the forelimb were probably reminiscent of those seen in otariids. Contrasting again with modern fonns, hindlimbs remained fully developed in Ambulocetus. On land, the short, stout femur was apparently rotated laterally, causing the enormous feet to be directed laterally. Hindlimb motion likely mimicked that of otariids, with propulsion stemming from extension of the lower back.
III. Desmostylia
Desmostylians inhabited coastal waters of mainly the northern Pacific during Oligocene and Miocene time. Inasmuch as desmostylians retain four stout but otherwise normally proportioned limbs, which are not modified into paddles, and their remains are found in near-shore marine sediments, the group is generally considered to have been amphibious, and therefore capable of locomoting terrestrially. The stance of these animals is highly peculiar, however, with strongly inturned fore and hind feet. Fusion of the radius and ulna severely curtailed rotation of the forelimb; details of foot posture in group remain somewhat controversial.
IV. Sirenia
Although sirenians do not venture onto land, there is no doubt that their forebears did. Eocene outgroups to Sirenia show limited but persuasive evidence that these animals retained functional hindlimbs, particularly the hip structure in Protosiren (Domning et al, 1994). This stage in the ancestry of Sirenia thus records the quadrupedal (and likely amphibious) antecedents to fully aquatic members of the clade.
