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Eotheroides

Eotheroides

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Eotheroides is an extinct genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 sirenian. It is an early member of the family Dugongidae
Dugongidae
Dugongidae is a family in the order of Sirenia.The family has one surviving species, the Dugong , one recently extinct species, the Steller's Sea Cow , and a number of extinct genera known from the fossil record....

, which includes the extant dugong
Dugong
The dugong is a large marine mammal which, together with the manatees, is one of four living species of the order Sirenia. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow , was hunted to extinction in the 18th century...

. Fossils have been found from Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, and Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...

. Eotheroides was first described by Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...

 in 1875 under the name Eotherium, which was replaced by the current name in 1899.

Based on endocranial casts, Eotheroiodes had a smaller endocranial volume than other contemporaneous sirenians such as Protosiren
Protosiren
Protosiren is an extinct early genus of the order Sirenia. Protosiren existed throughout the Lutetian and Bartonian stages of the Middle Eocene. Its geographic distribution was intercontinental: fossils have been found in the United States , Egypt, France, Hungary, India, and Pakistan...

. Unlike extant sirenians, Eotheroides possesses a tentorium cerebelli
Tentorium cerebelli
The tentorium cerebelli or cerebellar tentorium is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes.-Anatomy:...

, which appears as a distinct transverse groove on the skull.

Species


The type species, E. aegyptiacum, is known from the Lutetian
Lutetian
The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between and . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the Middle Eocene subepoch...

 Mokattam Limestone of Cairo
Cairo
Cairo , is the capital of Egypt and the largest city in the Arab world and Africa, and the 16th largest metropolitan area in the world. Nicknamed "The City of a Thousand Minarets" for its preponderance of Islamic architecture, Cairo has long been a centre of the region's political and cultural life...

, Egypt. Another species, E. lambondrano, was recently named on the basis of material found from Middle Eocene nearshore marine deposits in the Mahajanga Basin
Mahajanga Province
Mahajanga is a former province of Madagascar with an area of 150,023 km². It had a population of 1,896,000 . Its capital was Mahajanga.Except for Fianarantsoa, Mahajanga Province bordered all of the country's other provinces:...

 of Madagascar. The species was named after the Malagasy
Malagasy language
Malagasy is the national language of Madagascar, a member of the Austronesian family of languages. Most people in Madagascar speak it as a first language as do some people of Malagasy descent elsewhere.-History:...

 word for dugong, which translates as "water bushpig". It is known from a nearly complete skull and fragments of pachyosteosclerotic (thick) ribs. Based on its age and morphology, E. lambondrano may be ancestral to E. aegyptiacum. Described in 2009, E. lambondrano is the first pre-Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 mammal named from Madagascar, being known from an 80-million-year gap in the island's fossil record.

Several other species have been named, including E. babiae from India, E. majus from Egypt, and E. waghapadarensis, also from India. E. majus has been considered a senior subjective synonym of E. aegyptiacum.

Paleobiology


The teeth of Eotheroides were relatively unspecialized compared to those of extant sirenians, which are reduced as an adaptation for feeding on sea grass. The upper molars of E. lambondrano are considerably longer and wider than those of E. aegyptiacum, suggesting that they were less specialized. Eotheroides is likely to have been one of the first fully aquatic sea cows, although much of the skeleton, including the limbs, are unknown. Eotheroides lambondrano was found in association with the remains of other sea cows, crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

ns, and sea turtles, which suggests that the locality is representative of a coastal or estuarine environment.

Further reading


  • Marine Mammals: Evolutionary Biology by Annalisa Berta, James L. Sumich, and Kit M. Kovacs
  • The Beginning of the Age of Mammals by Kenneth D. Rose
  • Classification of Mammals by Malcolm C. McKenna and Susan K. Bell


External links