Sirenia (Dugongs and Manatees)


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

Figure 1.

Simplified phylogeny of the sirenians.

Figure 2.

The author with the composite skeleton of Pezosiren portelli, an amphibious Eocene sirenian.



Clementz MT, Sorbi S and Domning DP (2009) Evidence of Cenozoic environmental and ecological change from stable isotope analysis of sirenian remains from the Tethys‐Mediterranean region. Geology 37(4): 307–310.

Domning DP (2001b) Sirenians, seagrasses, and Cenozoic ecological change in the Caribbean. Palaeogeography, Palaeoclimatology, Palaeoecology 166(1–2): 27–50.

Domning DP (2005) Fossil sirenia of the West Atlantic and Caribbean region. VII. Pleistocene Trichechus manatus Linnaeus, 1758. Journal of Vertebrate Paleontology 25(3): 685–701.

Domning DP and Beatty BL (2007) Use of tusks in feeding by dugongid sirenians: observations and tests of hypotheses. Anatomical Record 290(6): 523–538.

Velez‐Juarbe J, Domning DP and Pyenson ND (2012) The present is not always the key to the past: iterative evolution of sympatric seacow (Dugongidae, Sirenia) assemblages in the past ∼26 million years. PLoS One 7(2): e31294.

Further Reading

Domning DP (1996) Bibliography and index of the Sirenia and Desmostylia. Smithsonian Contributions to Paleobiology 80: iii+611.

Domning DP (1999) Fossils explained 24: Sirenians (seacows). Geology Today 15: 75–79.

Domning DP (2001a) The earliest known fully quadrupedal sirenian. Nature 413(6856): 625–627.

Domning DP (2010) Bibliography and Index of the Sirenia and Desmostylia,

Gheerbrant E, Domning DP and Tassy P (2005) Paenungulata (Sirenia, Proboscidea, Hyracoidea, and relatives). In: Rose KD and Archibald JD (eds) The Rise of Placental Mammals: Origin and Relationships of the Major Extant Clades, pp. 84–105. Baltimore, MD: Johns Hopkins University Press.

Marsh H, O'Shea TJ and Reynolds JE III (2011) Ecology and Conservation of the Sirenia: Dugongs and Manatees. Cambridge, UK: Cambridge University Press.

Reynolds JE III and Odell DK (1991) Manatees and Dugongs. New York: Facts on File.

Reynolds JE III and Rommel SA (eds) (1999) Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press.

Twiss JR Jr and Reeves RR (eds) (1999) Conservation and Management of Marine Mammals. Washington, DC: Smithsonian Institution Press.

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How to Cite close
Domning, Daryl P(Mar 2012) Sirenia (Dugongs and Manatees). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001576.pub3]