Prorastomidae

Abstract: Sea cows (manatees, dugongs) are the only living marine mammals to feed solely on aquatic plants. Unlike whales or dolphins (Cetacea), the earliest evolutionary history of sirenians is poorly documented, and limited to a few fossils including skulls and skeletons of two genera composing the stem family of Prorastomidae (Prorastomus and Pezosiren). Surprisingly, these fossils come from the Eocene of Jamaica, while stem Hyracoidea and Proboscidea - the putative sister-groups to Sirenia - are recorded in Africa as early as the Late Paleocene. So far, the historical biogeography of early Sirenia has remained obscure given this paradox between phylogeny and fossil record. Here we use X-ray microtomography to investigate a newly discovered sirenian petrosal from the Eocene of Tunisia. This fossil represents the oldest occurrence of sirenians in Africa. The morphology of this petrosal is more primitive than the Jamaican prorastomids’ one, which emphasizes the basal position of this new African taxon within the Sirenia clade. This discovery testifies to the great antiquity of Sirenia in Africa, and therefore supports their African origin. While isotopic analyses previously suggested sirenians had adapted directly to the marine environment, new paleoenvironmental evidence suggests that basal-most sea cows were likely restricted to fresh waters.

Abstract: Sirenia are an order of large, aquatic, plant-eating, mostly tropical placental mammals that includes the modern seacows (dugongs and manatees) and their extinct relatives. Their nearest living relatives are the Proboscidea (elephants). Together with some extinct orders (including Desmostylia and Embrithopoda), Sirenia and Proboscidea make up a group known as Tethytheria, after their likely origin along the shores of the ancient Tethys Seaway in the Old World. Sirenians first appeared in the fossil record in the Eocene. They comprise four families, two extinct (Prorastomidae and Protosirenidae, both Eocene) and two extant (Trichechidae and Dugongidae). For most of the Tertiary, they formed multispecies communities in marine environments, partitioning seagrass resources in ways not completely understood. However, global cooling after the middle Miocene, and human predation in the North Pacific, diminished their diversity to only two living genera and four species. Key Concepts: Sirenians are herbivorous, completely aquatic mammals that evolved from land-dwelling ancestors. The closest living relatives of the Sirenia are the Proboscidea (elephants). Sirenians have a fossil record dating from the Eocene. Most sirenians of the past lived in salt water, and fed on seagrasses (marine flowering plants). The Family Dugongidae was the most diverse and successful family of sirenians. Several lineages of sirenians apparently specialised on eating large seagrass rhizomes, using bladelike, self-sharpening upper tusks. These specialised sirenians could act as ‘keystone species’, allowing both seagrasses and other kinds of sirenians to become locally more diverse. Unlike today, sirenians of the past formed communities of several coexisting species, with a different taxonomic makeup in each ocean basin. Diversity of sirenians diminished after the middle Miocene, due to global cooling. Manatees in the southeastern U.S. and Caribbean had a complicated history during the Pleistocene Ice Ages, shifting their range south and north with the alternating glacial and interglacial cycles. Keywords: dugongs; ecology; evolution; manatees; seacows