How teeth, tusks and horny pads evolved together in sea cows

Nearly three centuries ago, the shipwrecked crew of the St.Peter were tired, hungry and unprepared for a new species that greeted them: a giant herbivorous marine mammal.Georg Steller, the ship's naturalist, carefully documented this sirenian species that quietly inhabited the shores of what would become known as Bering Island, identifying its basic anatomy, some of its life history and, befitting his crew's dire situation at the far western end of the Aleutian Islands, how to kill and eat them [1].The expedition leader, Vitus Bering, died on the island, but Steller and his notes returned to Russia, only to fall into obscurity for over a century.In the interim, fur traders used knowledge of Bering Island and its plentiful sea cows as a stepping stone for exploitation farther east, towards Russian America [2], consuming the only extant population of this species in the process.By 1768, less than three decades after its discovery, Steller's sea cow (Hydrodamalis gigas) was hunted to extinction [1,2].Beyond a shortlist of dubious soft tissue fragments, most of what we can now understand about this extinct species derives from studies of its bones [3].Sea cows-extinct and extant alike, all belonging to the broader clade Sirenia-remain interesting to biologists today for a variety of reasons.Like whales, this group of marine mammals has terrestrial ancestors, but from an entirely different part of the mammalian family tree: sirenians belong to Afrotheria, a group of mammals whose living relatives include elephants, rock hyraxes, aardvarks, tenrecs, elephant shrews and others with African endemicity or origins [4].Like whales, sirenians underwent an ecological transition to obligate aquatic life during the middle to late Eocene (approximately 47-34 million years ago), following a sequence of locomotory transformations that parallel early whales: sirenians evolved paddle-like forelimbs and lost weight-bearing hindlimbs, concurrent with tail-propelled movement underwater using a tail fluke [5].Eocene-age sirenians have been uncovered from India to Africa to Europe, North America and the Caribbean, complicating simple biogeographical scenarios for this coastal-dwelling clade [6,7].Consequently, each new fossil sirenian has the potential to clarify or confound our understanding of their evolutionary history [4].Whereas whales were always carnivorous, herbivory has constrained sirenian evolution: sea cows never succeeded in adopting a pelagic lifestyle, and they remained tied to the coastal environments where their primary food-marine angiosperms, seagrass rhizomes or kelp-are abundant [6].Thickened ribs ensure that sea cows carry a form of permanent ballast for keeping their body horizontal to feed, yet sirenians show their afrothere connections most clearly in their mouths: like elephants, sirenians harvest plant matter, manipulated carefully by a well muscled muzzle, coupled with either use of paired tusks (as with dugongs) or ever-replacing molars (as with manatees); but unlike elephants, sirenians have keratinized (or horny) pads in the front of their mouths for processing plant material.And Steller's sea cow?Hydrodamalis belonged to a lineage of cold-water-adapted dugong relatives in the North Pacific that lost teeth entirely over several million years, and likely cropped kelp solely using a keratinized dental pad [8].Again, our only evidence comes from Steller's reporting and from subsequent observations by a few of his contemporaries [1,8].