Metaxytherium is an extinct genus of dugong that lived from the Oligocene until the end of the Pliocene. Fossil remains have been found in Africa, Europe, North America and South America. Generally marine seagrass specialists, they inhabited the warm and shallow waters of the Paratethys, Mediterranean, Caribbean Sea and Pacific coastline. American species of Metaxytherium are considered to be ancestral to the North Pacific family Hydrodamalinae, which includes the giant Steller's sea cow.
Discovery and naming The first remains of Metaxytherium were described in 1822 by Anselme-Gaëtan Demarest as a species of Hippo, H. medius before the genus name Metaxytherium was coined in 1840 by De Christol. Although the type species was initially designated to be M. cuvieri, later publications argued that the two species are synonymous and M. medium thus holds precedence. The grammatical changes of the species name were made to match the rules of the International Code of Zoological Nomenclature (ICZN). M. serresii was introduced by Gervais in 1847 to encompass the Metaxytherium finds made in Montpellier, France from the Early Pliocene. Although he later described and figured multiple specimens, no type specimen was ever introduced. M. floridanum was named in 1922 by Oliver P. Hay based on a piece of a right maxilla discovered 11 years prior near Mulberry, Florida. The genus Thalattosiren, described by Sickenberg in 1928, was erected from remains previously named M. pettersi, however, further research concluded that it should not only be synonymous with Metaxytherium on a genus level, but is also indistinguishable from M. medium as a species. M. arctodites was named in 1994 by Aranda-Manteca, Domning and Barnes based on three specimens found in California and Baja California, constituting the first valid record of this genus from the north-eastern Pacific. The most recently described species is Metaxytherium albifontanum, described in 2014 by Jorge Vélez-Juarbe and Daryl P. Domning. The name Metaxytherium means "in-between beast" or "intermediate beast" from the Greek μεταξύ (metaxy) and θηρίον (therion). The name derives from the original interpretation of it being an intermediate form between dugongs and manatees. There are several genera that are now synonymous with Metaxytherium, including Thalattosiren, Halianassa, Felsinoitherium, Cheirotherium and Hesperosiren. Several species previously considered distinct have likewise been synonymized with already established Metaxytherium species.
Species M. albifontanum Described in 2014, this species inhabited what is now Florida and South Carolina during the late Oligocene. The species name "albifontanum" is a composite of the Latin "albus" (=white) and "fontanus" (=of a spring), a name chosen due to the holotype being discovered near White Springs, Florida. More specifically, M. albifontanum was discovered in the Porters Landing Member of the Parachucla Formation and the Chandler Bridge Formation of South Carolina (25.7-23.6 Ma). It is the stratigraphically oldest species of Metaxytherium and relatively small, with a body size of 2 to 3 meters. M. arctodites M. arctodites is found in mid Miocene (15 - 14 Ma) layers of California and Baja California, specifically the Topanga Formation and Rosarito Beach Formation. The name is a combination of the Greek "arktos" and "hodites", in combination translating to "north traveler", an allusion to the genus dispersal northward as evidenced by M. arctodites. The authors further note that the genus name Metaxytherium (intermediate beast) coincidentally also works as an allusion to it being a bridge between the Halitheriinae and Hydrodamalinae. This species may have been less shorebound than other Metaxytherium, found in true marine sediments alongside mysticete whales, toothed whales and pelagic sharks. M. crataegense A West-Atlantic species found in the early to middle Miocene Montera Formation of Peru as well as the Hawthorne and Calvert formations of the United States, extending its range from western South America to the north-east of the United States. M. crataegense existed from the approximately late Hemingfordian (ca. 20.44 Ma) to the early Barstovian (ca. 13.6 Ma). The two species M. calvertense and M. riveroi from Cuba have both been sunk into this species as well. M. floridanum Named after the state of Florida, M. floridanum is found in the middle to late Miocene rock layers of northern and central Florida with many remains being found in the phosphate mines of Central Florida, belonging to the Lower Bone Valley Formation. M. floridanum remains specifically date to the Late Barstovian to Early Hemphillian age (14-7 Ma). It reached a length of approximately 3 meters. M. krahuletzi M. krahuletzi is the oldest species of Metaxytherium known from Europe, inhabiting the shallow waters of the Central Paratethys as well as nearby waters. The vast majority of M. krahuletzi fossils have been collected from the Burgschleinitz Formation of Eggenburg Bay, Austria. Fossils date to the Eggenburgian Age (20.8 - 18.3 Ma) to the Ottnangian (18.3 - 17 Ma). A specimen from France tentatively referred to M. cf. krahuletzi dates to the Karpatian Age, roughly 17.5-16.5 Ma. The Metaxytherium bonebeds of Eggenburg Bay have been suggested to represent mass death sites, possibly the result of reduced food supply following heavy storm events known to have occurred in this part of the Paratethys. The species was named in honor of Austrian geologist Johann Krahuletz.
M. medium The type species, M. medium is known from the Middle Miocene (Langhian) of Western and Central Europe (France, Netherlands, Austria), Greece and presumably Northern Africa. The most recent records of M. medium stem from late Miocene (Tortonian) Tuscany (10.5 to 8.14 Ma). M. serresii A species originally described to include Pliocene (Early Zanclean) specimen from Montpellier, France, later discoveries extended its range to late Miocene (latest Tortonian) Italy as well as Sahabi, Libya. M. serresii is notably smaller than any of the European species before or after it across its range, while the size of the tusks had relatively increased. It was named by Paul Gervais after Marcel de Serres.
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