Epitheria
Encyclopedia
Epitherians comprise all the placental mammals except the Xenarthra
. They are primarily characterized by having a stirrup
-shaped stapes in the middle ear
, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupial
s, monotreme
s, and xenarthra
ns. They are also characterized by having a shorter fibula relative to the tibia
.
Epitheres are though to have appeared in the early part of the Late Cretaceous
age. Before the end of the Mesozoic
Era, ancestral forms of most of the living orders (such as the ungulate
s and the insectivores
) had already appeared. After the extinction of non-avian
dinosaur
s (in the Early Paleocene
), the epitheres' diversity exploded, and by the end of the Eocene, all living orders of Epitheria had appeared. Epitheres are one of the most successful groups of animals.
Monophyly
of Epitheria has been challenged by molecular phylogenetic
studies. While preliminary analysis of a set of retroposon
s shared by both Afrotheria
, and Boreoeutheria
(presence/absence data
) supported the Epitheria clade, more extensive analysis of such transposable element insertions around the time of the divergence of Xenarthra, Afrotheria, and Boreoeutheria strongly support the hypothesis of a near-concomitant origin (trifurcation) of these three superorders of mammals.
and Boreoeutheria
, or Afrotheria and Exafroplacentalia
(Notolegia) at the base of the tree:
Xenarthra
The superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
. They are primarily characterized by having a stirrup
Stirrup
A stirrup is a light frame or ring that holds the foot of a rider, attached to the saddle by a strap, often called a stirrup leather. Stirrups are usually paired and are used to aid in mounting and as a support while using a riding animal...
-shaped stapes in the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s, monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s, and xenarthra
Xenarthra
The superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
ns. They are also characterized by having a shorter fibula relative to the tibia
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
.
Epitheres are though to have appeared in the early part of the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
age. Before the end of the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
Era, ancestral forms of most of the living orders (such as the ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
s and the insectivores
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...
) had already appeared. After the extinction of non-avian
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s (in the Early Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
), the epitheres' diversity exploded, and by the end of the Eocene, all living orders of Epitheria had appeared. Epitheres are one of the most successful groups of animals.
Monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of Epitheria has been challenged by molecular phylogenetic
Molecular phylogeny
Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
studies. While preliminary analysis of a set of retroposon
Retroposon
Retroposons are repetitive DNA fragments which are inserted into chromosomes after they had been reverse transcribed from any RNA molecule. In contrast to retrotransposons, they never encode Reverse Transcriptase . Therefore, they are non-autonomous elements with regard to transposition activity...
s shared by both Afrotheria
Afrotheria
Afrotheria is a clade of mammals, the living members of which belong to groups from Africa or of African origin: golden moles, sengis , tenrecs, aardvarks, hyraxes, elephants and sea cows. The common ancestry of these animals was not recognized until the late 1990s...
, and Boreoeutheria
Boreoeutheria
Boreoeutheria is a clade of placental mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires...
(presence/absence data
Retrotransposon Marker
Retrotransposon markers are retrotransposons that are used as cladistic markers.The analysis of SINEs – Short INterspersed Elements – LINEs – Long INterspersed Elements – or truncated LTRs – Long Terminal Repeats – as molecular cladistic markers represents a particularly interesting complement to...
) supported the Epitheria clade, more extensive analysis of such transposable element insertions around the time of the divergence of Xenarthra, Afrotheria, and Boreoeutheria strongly support the hypothesis of a near-concomitant origin (trifurcation) of these three superorders of mammals.
Alternative hypotheses
Alternative hypotheses place either AtlantogenataAtlantogenata
Atlantogenata is a proposed clade of mammals containing the cohorts or super-orders Afrotheria, Xenarthra and Meridiungulata. These groups originated and radiated in the South American and African continents, presumably in the Cretaceous. Together with Boreoeutheria it makes up Eutheria...
and Boreoeutheria
Boreoeutheria
Boreoeutheria is a clade of placental mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires...
, or Afrotheria and Exafroplacentalia
Exafroplacentalia
Exafroplacentalia or Notolegia is a subcohort selected in 2001 on the basis of molecular researches.-Exafroplacentalia:Exafroplacentalia places Xenarthra as a sister group to the magnorder Boreoeutheria , thus making Afrotheria a primitive group of placental mammals...
(Notolegia) at the base of the tree:
External links
- Summary of molecular support for Epitheria
- Waddell PJ, Kishino H, Ota R. 2001. A phylogenetic foundation for comparative mammalian genomics. Genome Inform Ser Workshop Genome Inform 12: 141–154
- Mark S. Springer, William J. Murphy, Eduardo Eizirik, and Stephen J. O'Brien (Edited by Morris Goodman). 2002 Placental mammal diversification and the Cretaceous-Tertiary boundary
- Wildman D.E.; Chen C.; Erez O.; Grossman L.I.; Goodman M.; Romero R. 2006. Evolution of the mammalian placenta revealed by phylogenetic analysis. PNAS 103 (9): 3203–3208
- Nikolaev, S., Montoya-Burgos, J.I., Margulies, E.H., Rougemont, J., Nyffeler, B., Antonarakis, S.E. 2007. Early history of mammals is elucidated with the ENCODE multiple species sequencing data. PLoS Genet. 3:e2, doi:10.1371/journal.pgen.0030002.
- Gennady Churakov, Jan Ole Kriegs, Robert Baertsch, Anja Zemann, Jürgen Brosius, Jürgen Schmitz. 2008. Mosaic retroposon insertion patterns in placental mammals