Desmostylia (marine mammals)

The Desmostylia are the only completely extinct order of ) marine mammals. They were hippopotamus-like amphibious herbivores (Fig. 1) that were confined to the North Pacific Ocean and are known only as fossils of Oligocene and Miocene age. [For comprehensive references to the published literature on desmostylians, see Domning (1996).]

I. Desmostylian Relationships, Origins, and Distribution

Desmostylians have undergone more of a taxonomic odyssey than perhaps any other mammals. Although they were placed initially (and afterwards, most commonly) among either the Sirenia or the Proboscidea, various authors later assigned them to die Monotremata, Marsupialia, Multituberculata, or an order of their own. The latter view prevailed only in 1953. Today they are classified in the supraordinal group Tethytheria, together with the living Sirenia and Proboscidea; the Proboscidea are probably their sister group. The teeth of the most primitive desmostylians closely resemble those of anthracobunids, early tethytheres, which are sometimes considered true proboscideans.

Because the most primitive known desmostylians, as well as other early tethytheres, are found in Asia, this continent, bordering the eastern part of the ancient Tethys Sea, is likely to have been the area where this order arose, probably during the Paleocene or Eocene. However, desmostylians do not appear in the fossil record until the Late Oligocene, less than 30 million years ago. By that time they had already spread to the North American shore of the Pacific; and from then to their extinction some 20 million years later, they inhabited the North Pacific littoral from Japan to Baja California.

II. Anatomy and Mode of Life

The desmostylian skull (Fig. 2) features a more or less long, narrow, and litde-defiected rostrum; dorsally protruding orbits; a stout paroccipital process; an external auditory meatus nearly enclosed ventrally by contact of the posttympanic and postglenoid processes; and a large epitympanic sinus opening into the temporal fossa. The dental formula is primitively 3.1.4.3, with procumbent, transversely aligned incisors and canines; the lower canine is especially enlarged. The fourth lower deciduous premolar is primitively trilobed. The body (Fig. 1) is stout and compact, with a relatively short neck, a deep thorax, a broad sternum with paired plate-like sternebrae, a strongly arched lumbar spine, and a very reduced tail. The limbs are robust, with considerable torsion in the tibia and an ankle joint that is oblique to the tibial shaft (conditions also seen to varying degrees in many noncursorial land mammals). The metacarpals are longer than the metatarsals. The bones show some osteosclerosis (increased density) but no pachyostosis (increased volume).

Figure 1 Skeleton of Paleoparadoxia tabatai, a Miocene, desmostylian, in terrestrial pose. Total length is about 2.2 to. Note hyperextension and anterolateral direction of front toes, anterior direction of hind toes, and strong abduction of knees.

Controversy over desmostylians’ structure and posture has even surpassed the disagreements over their classification. Once complete skeletons were discovered, showing that they possessed four stout limbs and were capable of some sort of locomotion on land, paleontologists and artists created a startling variety of reconstructions, including ones resembling hippopotami, tapirs, sea lions, crocodiles, and creatures unlike anything else (see Inuzuka, 1982). The interpretations presented most recently and defended in the most detail have portrayed thein as hauling out in the manner of sea lions (Repenning), as walking with the limbs sprawled in a “herpetiform” stance (Inuzuka), or as keeping the legs under the body in a more conventional land-mammal fashion, with resemblances to ground sloths (Domning, 2001; Fig. 1).

In contrast, their aquatic behavior has occasioned little argument. Their style of swimming is generally agreed to have been like that of polar bears; i.e., alternate paddling with the foreliinbs while the hindlimbs were used for steering. Because desmostylian fossils are found exclusively in marine deposits, there has also never been any doubt expressed that they were strictly marine mammals, despite the lack of clearly aquatic specializations in their skeletons.

The peculiar, heavily enameled tooth structure of the more highly derived desmostylians has occasionally led to suggestions that they fed on molluscs or other shelled prey. However, these teeth tend to be high crowned, like those of grazing ungulates, rather than low, broad, and pavement-like as seen in animals that do crush shellfish. Moreover, the teeth of earlier desmostylians closely resemble those of undoubted herbivores. Hence a diet of marine plants for all desmostylians is now generally conceded.

III. Diversity

Only about six genera and fewer than a dozen species are currently recognized in this order (Table I). They could be grouped into at least two families, but conflicting schemes of family-level classification have not been completely reconciled. All these taxa, so far as is known, had broadly similar postcra-nial skeletons, and they are distinguished mainly by details of skull and dentition. The cladistic relationships of the desmostylian genera are generally agreed to be as follows; (Be-hemotops (Paleoparadoxia (Cornwallius (Desmostylus, Kronokotherium, and Vanderhoofius)))). The relationships among the latter three genera have yet to be clarified.

The most primitive form named so far is the Late Oligocene Behemotops, an animal nearly the size of a Nile hippopotamus and with low-crowned, anthracobunid-like teeth. The Miocene Paleoparadoxia (Fig. 1) is similar, but has a more retracted nasal opening, fewer premolars, and a long postcanine diastema. Cornwallius, another Late Oligocene genus, shows a tendency for the molar cusps to become more columnar. This trend continued in the Miocene with the genera Kronokotherium, Vanderhoofius, and Desmostylus; the latter name, meaning “bundle of columns,” expresses the appearance of a molar in these most highly derived and characteristic members of the group (Fig. 2). The procumbent incisors and canines are variously reduced, sometimes leaving only the large lower canines as digging organs, and the adult cheek dentition comprises only molars.

The diversity of desmostylians was constrained by their limited geographic range and dietary opportunities. Apparently they were adapted to cooler climates than the tropical sireni-ans, but they seem to have succumbed to competition from these more fully aquatic herbivores as soon as hydrodamaline sirenians evolved the ability to spread into the cool home waters of the desmostylians.

TABLE I Genera of Desmostylia and Their Temporal Range

Figure 2 Skull and mandible of Desmostylus hesperus, a Miocene desmostylian, in dorsal, lateral, and ventral views (immature specimen). The left mandibtdar angle is broken and the large right dental capstde is visible. Note the columnar cusps of the molars. A, nasal aperture; B, basisphenoid; Ch, choana; Cn, nuchal crest; Cnh, hypoglossal canal; Coc, occipital condyle; Cs, supraorbital canal; F, frontal; Fasq, anterior squamosal foramen; Fio, infraorbital foramen; Fj, jugular foramen; Fl, foramen lacerum; Fmd, mandibular fossa; Fmg, foramen magnum; Fov, foramen ovale; Fpa, parietal foramen; Fptna, greater palatine foramen; Fpmi, lesser palatine foramen; Fpz, postzygomatic foramen; Fst, stylomastoid foramen; Lt, temporal crest; M, maxilla; Ml, M2, upper first and second molars; N, nasal; O, occipital; Or, orbit; P4, upper fourth premolar; Pae, external auditory meatus; Pal, palatine; Par, parietal; Pm, premaxilla; Pp, paroccip-ital process; Sd, dental capsule; Sep, epitympanic sinus; T, squamosal (temporal); Ta, articular tubercle; Tm, muscular tubercle; Tp, pharyngeal tubercle; V, vomer; Z, zygomatic (Jugal). From Inuzuka (1988), reproduced with permission of the Geological Survey of fapan.