Placental mammals (infraclassPlacentalia/plæsənˈteɪliə/) are one of the three extant subdivisions of the classMammalia, the other two beingMonotremata andMarsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that thefetus is carried in theuterus of its mother to a relatively late stage of development. The name is something of a misnomer, considering that marsupials also nourish their fetuses via aplacenta,[9] though for a relatively briefer period, giving birth to less-developed young, which are then nurtured for a period inside the mother'spouch. Placentalia represents the only living group withinEutheria, which contains all mammals that are more closely related to placentals than they are to marsupials.
Placental mammals are anatomically distinguished from other mammals by:
a sufficiently wide opening at the bottom of thepelvis to allow the birth of a large baby relative to the size of the mother.[10]
the absence ofepipubic bones extending forward from the pelvis, which are found in all other mammals.[11] (Their function in non-placental mammals is to stiffen the body during locomotion,[11] but in placentals they would inhibit the expansion of the abdomen during pregnancy.)[12]
the rearmost bones of the foot fit into a socket formed by the ends of thetibia andfibula, forming a completemortise and tenon upper ankle joint.[13]
the presence of amalleolus at the bottom of the fibula.[13]
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.[16] "Jumping genes"-typeretroposon presence/absence patterns have provided corroboration of phylogenetic relationships inferred from molecular sequences.[17] It is now widely accepted that there are three major subdivisions or lineages of placental mammals:Boreoeutheria,Xenarthra, andAfrotheria. All of these diverged from common ancestors.
2022 studies of Bertrand, O. C. and Sarah L. Shelley have identifiedpalaeoryctids andtaeniodonts as basal placental mammal clades.[2][3]
The 19 living orders of Placentalia in the three groups are:[18]
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 isbasal or diverged first from other placentals. These hypotheses areAtlantogenata (basal Boreoeutheria),Epitheria (basal Xenarthra),Exafroplacentalia (basal Afrotheria) and a hypothesis supporting a near simultaneous divergence.[19] Estimates for the divergence times among these three placental groups mostly range from 105 to 120 million years ago (MYA), depending on the type of DNA, whether it is translated, and the phylogenetic method (e.g.nuclear ormitochondrial),[20][21] and varying interpretations ofpaleogeographic data.[19] In addition, a strict molecular clock does not hold, so it is necessary to assume models of how evolutionary rates change along lineages. These assumptions alone can make substantial differences to the relative ages of different mammal groups estimated with genomic data.[22]
Cladogram and classification based on Amrine-Madsen, H.et al. (2003)[23] and Asher, R. J.et al. (2009)[24] Compare with Waddell, Hasegawa and Okada (1999)[16] and Waddell et al. (2001).[20]
As of 2020[update], thegenome has been sequenced for at least one species in each extant placental order and in 83% of families (105 of 127 extant placental families).[25]
True placental mammals (thecrown group including all modern placentals) arose from stem-group members of the cladeEutheria, which had existed since at least theMiddle Jurassic period, about 170 mya. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees.[13]
True placentals may have originated in theLate Cretaceous around 90 mya, but the earliest undisputed fossils are dated to theCretaceous–Paleogene boundary (K-Pg boundary). The genusProtungulatum is sometimes placed as a stem-ungulate,[26] with probably the earliest known speciesP. coombsi from the strata within theHell Creek Formation specifically dated to at least 300,000 years before the K-Pg boundary.[1] The genusPurgatorius, sometimes considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary.[27] One study has recovered both genera to be closely related and as stem-eutherians outside modern placental mammals,[28] but others have recoveredProtungulatum as apan-euungulate based on phylogenetic analysis and inner ear anatomy different from non-placentals.[29][30] The rapid appearance of placentals after the mass extinction at the end of theCretaceous suggests that the group had already originated and undergone an initial diversification in the Late Cretaceous, as suggested bymolecular clocks.[31] The lineages leading to Xenarthra and Afrotheria probably originated around 90 mya, and Boreoeutheria underwent an initial diversification around 70-80 mya,[31] producing the lineages that eventually would lead to modern primates, rodents,insectivores,artiodactyls, andcarnivorans.
However, modern members of the placental orders originated in thePaleogene around 66 to 23 mya, following the Cretaceous–Paleogene extinction event. The evolution of crown orders such modern primates, rodents, and carnivores appears to be part of an adaptive radiation[32] that took place as mammals quickly evolved to take advantage of ecologicalniches that were left open when most dinosaurs and other animals disappeared following theChicxulub asteroid impact. As they occupied new niches, mammals rapidly increased in body size, and began to take over the large herbivore and large carnivore niches that had been left open by the decimation of the dinosaurs (and perhaps more relevantly competingsynapsids[33]). Mammals also exploited niches that the non-avian dinosaurs had never touched: for example,bats evolved flight and echolocation, allowing them to be highly effective nocturnal, aerial insectivores; and whales first occupied freshwater lakes and rivers and then moved into the oceans. Primates, meanwhile, acquired specialized grasping hands and feet which allowed them to grasp branches, and large eyes with keener vision which allowed them to forage in the dark.
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 leftLaurasia and colonized Africa and then South America viarafting.
In Africa, theAfrotheria underwent a major adaptive radiation, which led to elephants,elephant shrews,tenrecs,golden moles,aardvarks, andmanatees. In South America a similar event occurred, with radiation of the Xenarthra, which led to modernsloths,anteaters, andarmadillos, as well as the extinctground sloths andglyptodonts. Expansion in Laurasia was dominated by Boreoeutheria, which includes primates and rodents,insectivores, carnivores,perissodactyls andartiodactyls. These groups expanded beyond a single continent when land bridges formed linking Africa to Eurasia and South America to North America.
A study on eutherian diversity suggests that placental diversity was constrained during thePaleocene, whilemultituberculate mammals diversified; afterwards, multituberculates decline and placentals explode in diversity.[33]
^GeneraAlveugena,Ambilestes andProcerberus are the immediate outgroups to order Taeniodonta, with genusAlveugena classified as a sister taxon to this order.
^Turnbull, W. D. (1971.) "The trinity therians: Their bearing on evolution in marsupials and other therians." In: ed. A. A. Dahlberg"Dental morphology and evolution", Chicago: University of Chicago Press, pp. 151–179.
^Novacek, M. J., Rougier, G. W, Wible, J. R., McKenna, M. C, Dashzeveg, D. and Horovitz, I. (October 1997). "Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia".Nature.389 (6650):483–486.Bibcode:1997Natur.389..483N.doi:10.1038/39020.PMID9333234.S2CID205026882.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Velut, S; Destrieux, C; Kakou, M (May 1998). "[Morphologic anatomy of the corpus callosum]".Neuro-Chirurgie.44 (1 Suppl):17–30.PMID9757322.
^abWaddell, P. J.; Okada, N.; Hasegawa, M. (1999). "Towards Resolving the Interordinal Relationships of Placental Mammals".Systematic Biology.48 (1):1–5.doi:10.1093/sysbio/48.1.1.
^abWaddell, P. J.; Kishino, H.; Ota, R. (2001). "A phylogenetic foundation for comparative mammalian genomics".Genome Informatics Series.12:141–154.PMID11791233.
^Amrine-Madsen, H.; Koepfli, K. P.; Wayne, R. K.; Springer, M. S. (2003). "A new phylogenetic marker, apoliprotein B, provides compelling evidence for eutherian relationships".Molecular Phylogenetics and Evolution.28 (2):225–240.Bibcode:2003MolPE..28..225A.doi:10.1016/s1055-7903(03)00118-0.PMID12878460.
^O'Leary, Maureen A.; Bloch, Jonathan I.; Flynn, John J.; Gaudin, Timothy J.; Giallombardo, Andres; Giannini, Norberto P.; Goldberg, Suzann L.; Kraatz, Brian P.; Luo, Zhe-Xi; Meng, Jin; Ni, Michael J.; Novacek, Fernando A.; Perini, Zachary S.; Randall, Guillermo; Rougier, Eric J.; Sargis, Mary T.; Silcox, Nancy b.; Simmons, Micelle; Spaulding, Paul M.; Velazco, Marcelo; Weksler, John r.; Wible, Andrea L.; Cirranello, A. L. (8 February 2013). "The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals".Science.339 (6120):662–667.Bibcode:2013Sci...339..662O.doi:10.1126/science.1229237.hdl:11336/7302.PMID23393258.S2CID206544776.
^Fox, R. C.; Scott, C. S. (2011). "A new, early Puercan (earliest Paleocene) species of Purgatorius (Plesiadapiformes, Primates) from Saskatchewan, Canada".Journal of Paleontology.85 (3):537–548.Bibcode:2011JPal...85..537F.doi:10.1666/10-059.1.S2CID131519722.
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