Sirenia The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct ...

is the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

of placental mammals which comprises modern "sea cows" (

manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living speci ...

s and the

Dugong The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest m ...

) and their extinct relatives. They are the only extant

herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...

marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their ...

s and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-old

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

record (early

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

-recent). They attained modest diversity during the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

and

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

, but have since declined as a result of climatic cooling, oceanographic changes, and human interference. Two genera and four species are extant: ''

Trichechus Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...

'', which includes the three species of manatee that live along the

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

coasts and in rivers and coastlines of

the Americas The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World. Along with th ...

and western

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, and ''

'', which is found in the

Indian Indian or Indians may refer to: Peoples South Asia * Indian people, people of Indian nationality, or people who have an Indian ancestor ** Non-resident Indian, a citizen of India who has temporarily emigrated to another country * South Asia ...

and

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

oceans.

Origins

Sirenians, along with

Proboscidea The Proboscidea (; , ) are a taxonomic order of afrotherian mammals containing one living family ( Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives. Fr ...

(elephants), group together with the extinct

Desmostylia The Desmostylia (from Greek δεσμά ''desma'', "bundle", and στῦλος ''stylos'', "pillar") are an extinct order of aquatic mammals that existed from the early Oligocene (Rupelian) to the late Miocene ( Tortonian) (). Desmostylians are t ...

and likely the extinct

Embrithopoda Embrithopoda ("heavy-footed") is an order of extinct mammals known from Asia, Africa and Eastern Europe. Most of the embrithopod genera are known exclusively from jaws and teeth dated from the late Paleocene to the late Eocene; however, the orde ...

to form the

Tethytheria Tethytheria is a clade of mammals that includes the sirenians and proboscideans, as well as the extinct order Embrithopoda. Though there is strong anatomical and molecular support for the monophyly of Tethytheria, the interrelationships between ...

. Tethytheria is thought to have evolved from primitive hoofed mammals ("

condylarth Condylarthra is an informal group – previously considered an order – of extinct placental mammals, known primarily from the Paleocene and Eocene epochs. They are considered early, primitive ungulates. It is now largely considered to be a waste ...

s") along the shores of the ancient

Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...

. Tethytheria, combined with

Hyracoidea Hyraxes (), also called dassies, are small, thickset, herbivorous mammals in the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Typically, they measure between long and weigh between . They are superficially simila ...

(hyraxes), forms a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

called

Paenungulata Paenungulata (from Latin ''paene'' "almost" + ''ungulātus'' "having hoofs") is a clade of "sub- ungulates", which groups three extant mammal orders: Proboscidea (including elephants), Sirenia (sea cows, including dugongs and manatees), and H ...

. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian clades, with strong support from morphological and molecular interpretations. The ancestry of Sirenia is remote from that of

Cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...

and

Pinnipedia Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus ...

, although they are thought to have evolved an aquatic lifestyle around the same time.

Fossil history

The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian known to date, ''

Prorastomus ''Prorastomus sirenoides'' is an extinct species of primitive sirenian that lived during the Eocene Epoch 40 million years ago in Jamaica. Taxonomy The generic name ''Prorastomus'', a combination of Greek (''prōra''), prow, and (''stoma''), mo ...

'', was found in Jamaica, not the

Old World The "Old World" is a term for Afro-Eurasia that originated in Europe , after Europeans became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia, which were previously thought of by thei ...

; however more recently the contemporary ''

Sobrarbesiren ''Sobrarbesiren'' (meaning "siren from Sobrarbe") is a genus of extinct sirenian that lived in the Eocene, about 47 million years ago. The type and only species is ''S. cardieli'', known from a multitude of specimens from the Spanish Pyrenees. '' ...

'' has been recovered from

Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = '' Plus ultra'' (Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , ...

. The first known quadrupedal sirenian was ''

Pezosiren ''Pezosiren portelli'', also known as the "walking manatee", is a basal sirenian from the early Eocene of Jamaica, 50 million years ago. The type specimen is represented by a Jamaican fossil skeleton, described in 2001 by Daryl Domning, a marine ...

'' from the early Eocene. The earliest known sea cows, of the families

Prorastomidae Prorastomidae is a family of extinct sirenians from Jamaica, related to the extant manatees and dugong. The family includes the oldest known fossils of Sirenians, represented in two genera: *''Pezosiren'' ''Pezosiren'' comprises one known species ...

and

Protosirenidae Protosirenidae is an extinct primitive family of the order Sirenia. Protosirenids are thought to have been amphibious quadrupeds, meaning that they spent their time both on land and in the water and had four legs. See also *Dugongidae Dugong ...

, are both confined to the Eocene, and were about the size of a pig, four-legged amphibious creatures. By the time the Eocene drew to a close, came the appearance of the

Dugongidae Dugongidae is a family in the order of Sirenia. The family has one surviving species, the dugong (''Dugong dugon''), one recently extinct species, Steller's sea cow (''Hydrodamalis gigas''), and a number of extinct genera known from fossil reco ...

; sirenians had acquired their familiar fully aquatic streamlined body with flipper-like front legs with no hind limbs, powerful tail with horizontal caudal fin, with up and down movements which move them through the water, like cetaceans. The last of the sirenian families to appear,

Trichechidae Trichechidae is a family of sirenians that includes all living manatees and several extinct genera. Systematics TRICHECHIDAE *MiosireninaeM. Voss. 2014. On the invalidity of Halitherium schinzii Kaup, 1838 (Mammalia, Sirenia), with comments o ...

, are thought to have originated from early dugongids in the late Eocene or early Oligocene. The current fossil record documents some major stages in hindlimb and pelvic reduction (well developed hindlimbs attached to a sacrum (Prorastomidae), well developed hindlimbs without proper sacrum (Protosirenidae) and reduced innominate with hindlimbs reduced or absent (Dugongidae and Trichechidae).Domning, D.P. (2000). ”The readaptation of Eocene sirenians to life in water”. ''Historical Biology'' 14(1-2): p. 115-119. DO
10.1080/10292380009380559
/ref> Since sirenians first evolved, they have been herbivores, likely depending on

seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the ...

es and aquatic

angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...

s (flowering plants) for food. To the present, almost all have remained tropical (with the notable exception of

Steller's Sea Cow Steller's sea cow (''Hydrodamalis gigas'') is an extinct sirenian described by Georg Wilhelm Steller in 1741. At that time, it was found only around the Commander Islands in the Bering Sea between Alaska and Russia; its range extended across ...

), marine and angiosperm consumers. Sea cows are shallow divers with large lungs. They have heavy skeletons to help them stay submerged; the bones are pachyostotic (swollen) and

osteosclerotic Osteosclerosis is a disorder that is characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and ...

(dense), especially the ribs which are often found as fossils. Eocene sirenians, like

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

mammals but in contrast to other

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

ones, have five instead of four premolars, giving them a 3.1.5.3 dental formula. Whether this condition is truly a primitive retention in sirenians is still under debate. Although cheek teeth are relied on for identifying species in other mammals, they do not vary to a significant degree among sirenians in their morphology, but are almost always low-crowned ( brachyodont) with two rows of large, rounded cusps ( bunobilophodont). The most easily identifiable parts of sirenian skeletons are the skull and mandible, especially the frontal and other skull bones. With the exception of a pair of tusk-like first upper incisors present in most species, front teeth (incisors and canines) are lacking in all, except the earliest sirenians.

References

External links

Manatee and Elephant Fossils
{{Evolution Sirenians

Sirenians The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...

Origins

Fossil history

See also

References

Further reading

External links