Placentalia
Placental mammals | |
---|---|
Placentals from different orders. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Clade: | Eutheria |
Infraclass: | Placentalia Owen, 1837 |
Subgroups | |
|
Placental mammals (
Anatomical features
Placental mammals are anatomically distinguished from other mammals by:
- a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother.[2]
- the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals.[3] (Their function in non-placental mammals is to stiffen the body during locomotion,[3] but in placentals they would inhibit the expansion of the abdomen during pregnancy.)[4]
- the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint.[5]
- the presence of a malleolus at the bottom of the fibula.[5]
Subdivisions
Analysis of molecular data led to rapid changes in assessments of the phylogeny of placental orders at the close of the 20th century. A novel phylogeny and classification of placental orders appeared with Waddell, Hasegawa and Okada in 1999.[6] "Jumping genes"-type retroposon presence/absence patterns have provided corroboration of phylogenetic relationships inferred from molecular sequences.[7] It is now widely accepted that there are three major subdivisions or lineages of placental mammals: Boreoeutheria, Xenarthra, and Afrotheria. All of these diverged from common ancestors.
2022 studies of Bertrand, O. C. and Sarah L. Shelley have identified palaeoryctids and taeniodonts as basal placental mammal clades.[8][9]
The living orders of placental mammals in the three groups are:[10]
- Magnorder Atlantogenata (armadillos, sloths, anteaters, aardvarks, elephant shrews, golden moles, otter shrews, tenrecs, hyraxes, elephants, and sirenians)
- Superorder sloths, and anteaters)
- Order armadillos)
- Order sloths and anteaters)
- Order
- Superorder manatees, and dugongs)
- Grandorder Afroinsectiphilia
- Order Tubulidentata (aardvarks)
- Mirorder tenrecs)
- Order golden moles)
- Order elephant shrews)
- Order
- Order
- Grandorder Paenungulata
- Order hyraxes)
- Mirorder manatees)
- Order elephants)
- Order manatees)
- Order
- Order
- Grandorder Afroinsectiphilia
- Superorder
- Magnorder Boreoeutheria
- Superorder hares, and rodents)
- Grandorder Gliriformes
- Mirorder Glires
- Order pikas)
- Order beavers, etc.)
- Order
- Mirorder Glires
- Grandorder Euarchonta
- Order Scandentia (treeshrews)
- Mirorder Primatomorpha
- Order colugos)
- Order lorises, etc.)
- Order
- Order
- Grandorder
- Superorder seals, and hoofed mammals)
- Order shrews, moles, and solenodons)
- Order bats)
- Grandorder Ferungulata
- Mirorder Euungulata
- Order whales, hippopotamuses, etc.)
- Order tapirs)
- Order
- Mirorder mongooses, etc.)
- Order pangolins)
- Order , etc.)
- Order
- Mirorder
- Order
- Superorder
The exact relationships among these three lineages is currently a subject of debate, and four different hypotheses have been proposed with respect to which group is
Placentalia |
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Cladogram and classification based on Amrine-Madsen, H. et al. (2003)[15] and Asher, R. J. et al. (2009)[16] Compare with Waddell, Hasegawa and Okada (1999)[6] and Waddell et al. (2001).[12]
Genomics
As of 2020[update], the genome has been sequenced for at least one species in each extant placental order and in 83% of families (105 of 127 extant placental families).[17]
See list of sequenced animal genomes.
Evolutionary history
True placental mammals (the crown group including all modern placentals) arose from stem-group members of the clade Eutheria, which had existed since at least the Middle Jurassic period, about 170 mya. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees.[5]
True placentals may have originated in the
However, modern members of the placental orders originated in the
The evolution of land placentals followed different pathways on different continents since they cannot easily cross large bodies of water. An exception is smaller placentals such as rodents and primates, who left
In Africa, the
A study on eutherian diversity suggests that placental diversity was constrained during the
References
- PMID 20079531.
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: CS1 maint: multiple names: authors list (link - ^ S2CID 4330626.)
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: CS1 maint: multiple names: authors list (link - ^ a b Waddell, P. J.; Okada, N.; Hasegawa, M. (1999). "Towards Resolving the Interordinal Relationships of Placental Mammals". Systematic Biology. 48 (1): 1–5.
- PMID 16515367.
- .
- ^ Sarah L. Shelley (2022.) "The phylogeny of Paleocene mammals and the evolution of Placentalia", in "The Society of Vertebrate Paleontology 82nd annual meeting"
- PMID 11689942.
- ^ PMID 19286970.
- ^ a b Waddell, P. J.; Kishino, H.; Ota, R. (2001). "A phylogenetic foundation for comparative mammalian genomics". Genome Informatics Series. 12: 141–154.
- PMID 12552136.
- ^ Kitazoe, Y.; Kishino, H.; Waddell, P. J.; Nakajima, T.; Okabayashi, T.; Watabe, T.; Okuhara, Y. (2007). "Robust time estimation reconciles views of the antiquity of placental mammals". PLoS ONE. 2 (e384): 1–11.
- PMID 12878460.
- PMID 19582725.
- ^ Zoonomia Consortium (2020) A comparative genomics multitool for scientific discovery and conservation. Nature 587, 240–245
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- ^ Archibald, J.D., 1982. A study of Mammalia and geology across the Cretaceous-Tertiary boundary in Garfield County, Montana. University of California Publications in Geological Sciences 122, 286.
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