Evolution of sirenians

Evolution of sirenians

Sirenia is the order of placental mammals which comprise modern "sea cows" (manatees and the Dugong) and their extinct relatives. They are the only extant herbivorous marine mammals and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-old fossil record (early Eocene-recent). They attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference. Two genera and four species are extant: "Trichechus" which includes the three species of manatee that live along the Atlantic coasts and in rivers and coastlines of the Americas and western Africa, and "Dugong" which is found in the Indian and Pacific oceans.

Early origins

[
Amazonian Manatee.]
[


thumb|right|Dugong.]
[
West Indian Manatee.]
[
West African Manatee.]
[
Steller's Sea Cow (extinct).] Sirenians, along with Proboscidea (elephants), group together with the extinct Desmostylia and likely the extinct Embrithopoda to form the Tethytheria. Tethytheria is interpreted to have evolved from primitive hoofed mammals known as condylarths, along the shores of the ancient Tethys Ocean.

Tethytheria, combined with Hyracoidea (hyraxes) forms an inclusive group called Paenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian orders, with strong support from morphological and molecular interpretations. The ancestry of Sirenia is remote from that of Cetacea and Pinnipedia, though 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", was found in Jamaica, not the Old World.

The earliest known sea cows, of the families Prorastomidae and Protosirenidae, 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; 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 who made their appearance, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. The current fossil record documents all major stages in hindlimb and pelvic reduction to the extreme reduction in the modern manatee pelvis, providing an example of dramatic morphological change among fossil vertebrates.

Since sirenians first evolved, they have been herbivores, likely depending on seagrasses and aquatic angiosperms (flowering plants) for food. To the present, almost all have remained tropical, 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 (dense), especially the ribs which are often found as fossils.

Eocene sirenians, like Mesozoic mammals but in contrast to other Cenozoic 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.

See also

*Evolution of cetaceans

References

*Sirenian Evolution, Daryl P. Domning; Encyclopedia of Marine Mammals Perrin, Würsig and Thewissen
* [http://www.edwardtbabinski.us/manatee/sirenians.html Origin of Sirenians]

External links

* [http://darla.neoucom.edu/DEPTS/ANAT/Thewissen/fos_man_ele/index.html Manatee and Elephant Fossils]


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Evolution of cetaceans — The approximately 80 modern species in the order Cetacea. A phylogeny showing the …   Wikipedia

  • Evolution — This article is about evolution in biology. For other uses, see Evolution (disambiguation). For a generally accessible and less technical introduction to the topic, see Introduction to evolution. Part of a series on …   Wikipedia

  • Evolution of sexual reproduction — The evolution of sexual reproduction is currently described by several competing scientific hypotheses. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species[1]. Many protists reproduce… …   Wikipedia

  • Evolution of ageing — Enquiry into the evolution of ageing aims to explain why almost all living things weaken and die with age. There is not yet agreement in the scientific community on a single answer. The evolutionary origin of senescence remains a fundamental… …   Wikipedia

  • Evolution of mammals — Restoration of Thrinaxodon, a member of the cynodont group, which includes the ancestors of mammals Further information: Evolutionary history of life The evolution of mammals within the synapsid lineage (sometimes called mammal like reptiles )… …   Wikipedia

  • Evolution of mammalian auditory ossicles — The evolution of mammalian auditory ossicles is one of the most well documented[1] and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re purposing of existing… …   Wikipedia

  • Evolution of biological complexity — Part of the Biology series on Evolution Mechanisms and processes Adaptation Genetic drift Gene flow Mutation Selection Speciation …   Wikipedia

  • Evolution of the horse — This image shows a representative sequence but should not be construed to represent a straight line evolution of the horse. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected… …   Wikipedia

  • Evolution of dinosaurs — This article gives an outline and examples of dinosaur evolution. For a detailed list of interrelationships see Dinosaur classification. Further information: Evolution of reptiles Evolution of dinosaurs Dinosaurs evolved from the archosaurs 232… …   Wikipedia

  • Evolution of hair — A recent study by scientists from Medical University of Vienna traced the origins of hair to the common ancestor of mammals, birds and lizards that lived 310 million years ago. The study found chickens, lizards and humans all possessed a similar… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”