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Placentalia

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"Placental" redirects here. For the organ interfacing between a placental mammalian mother and a fetus, seePlacenta.

Placentals
Temporal range:Paleocene-Holocene66.043–0MaPossibleLate Cretaceousrecord
Common vampire batEastern gray squirrelPlains zebraAardvarkHumpback whaleBlack and rufous elephant shrewHumanGround pangolinSunda flying lemurWest Indian manateeEuropean hedgehogNine-banded armadilloSouthern elephant sealAsian elephantReindeerGiant anteaterGiant pandaAmerican pika
Placentals from different orders.
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Eutheria
Infraclass: Placentalia
Owen,1837
Subgroups

For extinct groups, see text

Placental mammals(infraclassPlacentalia/plæsənˈteɪliə/) are one of the three extant subdivisions of the classMammalia,the other two beingMonotremataandMarsupialia.Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that thefetusis carried in theuterusof 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,[1]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 more closely related to placentals than to marsupials.

Anatomical features

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Placental mammals are anatomically distinguished from other mammals by:

  • a sufficiently wide opening at the bottom of thepelvisto allow the birth of a large baby relative to the size of the mother.[2]
  • the absence ofepipubic bonesextending 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 thetibiaandfibula,forming a completemortise and tenonupper ankle joint.[5]
  • the presence of amalleolusat the bottom of the fibula.[5]

Subdivisions

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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" -typeretroposonpresence/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,andAfrotheria.All of these diverged from common ancestors.

2022 studies of Bertrand, O. C. and Sarah L. Shelley have identifiedpalaeoryctidsandtaeniodontsas basal placental mammal clades.[8][9]

The living orders of placental mammals in the three groups are:[10]

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 isbasalor diverged first from other placentals. These hypotheses areAtlantogenata(basal Boreoeutheria),Epitheria(basal Xenarthra),Exafroplacentalia(basal Afrotheria) and a hypothesis supporting a near simultaneous divergence.[11]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.nuclearormitochondrial),[12][13]and varying interpretations ofpaleogeographicdata.[11]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.[14]

Placentalia

Cladogramand 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

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As of 2020,thegenomehas been sequenced for at least one species in each extant placental order and in 83% of families (105 of 127 extant placental families).[17]

Seelist of sequenced animal genomes.

Evolutionary history

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True placental mammals (thecrown groupincluding all modern placentals) arose from stem-group members of the cladeEutheria,which had existed since at least theMiddle Jurassicperiod, about 170 mya. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees.[5]

True placentals may have originated in theLate Cretaceousaround 90 mya, but the earliest undisputed fossils are from the earlyPaleocene,66 mya, following the Cretaceous–Paleogene extinction event. The speciesProtungulatum donnaeis sometimes placed as a stem-ungulate[18]known 1 meter above theCretaceous-Paleogene boundaryin the geological stratum that marks the Cretaceous–Paleogene extinction event[19]andPurgatorius,sometimes considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary;[20]both species, however, are sometimes placed outside the crown placental group, but many newer studies place them back ineutherians[further explanation needed].[21]The rapid appearance of placentals after the mass extinction at the end of theCretaceoussuggests that the group had already originated and undergone an initial diversification in the Late Cretaceous, as suggested bymolecular clocks.[22]The lineages leading to Xenarthra and Afrotheria probably originated around 90 mya, and Boreoeutheria underwent an initial diversification around 70-80 mya,[22]producing the lineages that eventually would lead to modern primates, rodents,insectivores,artiodactyls,andcarnivorans.

However, modern members of the placental orders originated in thePaleogenearound 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[23]that took place as mammals quickly evolved to take advantage of ecologicalnichesthat 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[24]). Mammals also exploited niches that the non-avian dinosaurs had never touched: for example,batsevolved 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 leftLaurasiaand colonized Africa and then South America viarafting.

In Africa, theAfrotheriaunderwent 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 slothsandglyptodonts.Expansion in Laurasia was dominated by Boreoeutheria, which includes primates and rodents,insectivores,carnivores,perissodactylsandartiodactyls.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,whilemultituberculatemammals diversified; afterwards, multituberculates decline and placentals explode in diversity.[24]

References

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  1. ^Renfree, M. B. (March 2010). "Review: Marsupials: placental mammals with a difference".Placenta.31 Supplement: S21–6.doi:10.1016/j.placenta.2009.12.023.PMID20079531.
  2. ^Weil, A. (April 2002). "Mammalian evolution: Upwards and onwards".Nature.416(6883): 798–799.Bibcode:2002Natur.416..798W.doi:10.1038/416798a.PMID11976661.S2CID4332049.
  3. ^abReilly, S. M. & White, T. D. (January 2003). "Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals".Science.299(5605): 400–402.Bibcode:2003Sci...299..400R.doi:10.1126/science.1074905.PMID12532019.S2CID41470665.
  4. ^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)
  5. ^abcJi, Q., Luo, Z-X., Yuan, C-X., Wible, J. R., Zhang, J-P. and Georgi, J. A. (April 2002). "The earliest known eutherian mammal".Nature.416(6883): 816–822.Bibcode:2002Natur.416..816J.doi:10.1038/416816a.PMID11976675.S2CID4330626.{{cite journal}}:CS1 maint: multiple names: authors list (link)
  6. ^abWaddell, P. J.; Okada, N.; Hasegawa, M. (1999). "Towards Resolving the Interordinal Relationships of Placental Mammals".Systematic Biology.48(1): 1–5.
  7. ^Kriegs, Jan Ole; Churakov, Gennady; Kiefmann, Martin; Jordan, Ursula; Brosius, Jürgen; Schmitz, Jürgen (2006)."Retroposed Elements as Archives for the Evolutionary History of Placental Mammals".PLOS Biology.4(4): e91.doi:10.1371/journal.pbio.0040091.PMC1395351.PMID16515367.
  8. ^Bertrand, O. C.; Shelley, S. L.; Williamson, T. E.; Wible, J. R.; Chester, S. G. B.; Flynn, J. J.; Holbrook, L. T.; Lyson, T. R.; Meng, J.; Miller, I. M.; Püschel, H. P.; Smith, T.; Spaulding, M.; Tseng, Z. J.; Brusatte, S. L. (2022)."Brawn before brains in placental mammals after the end-Cretaceous extinction".Science.376(6588): 80–85.Bibcode:2022Sci...376...80B.doi:10.1126/science.abl5584.hdl:20.500.11820/d7fb8c6e-886e-4c1d-9977-0cd6406fda20.
  9. ^Sarah L. Shelley (2022.) "The phylogeny of Paleocene mammals and the evolution of Placentalia", in"The Society of Vertebrate Paleontology 82nd annual meeting"
  10. ^Archibald JD, Averianov AO, Ekdale EG (November 2001)."Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals".Nature.414(6859): 62–5.Bibcode:2001Natur.414...62A.doi:10.1038/35102048.PMID11689942.
  11. ^abNishihara, H.; Maruyama, S.; Okada, N. (2009)."Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals".Proceedings of the National Academy of Sciences.106(13): 5235–5240.Bibcode:2009PNAS..106.5235N.doi:10.1073/pnas.0809297106.PMC2655268.PMID19286970.
  12. ^abWaddell, P. J.; Kishino, H.; Ota, R. (2001). "A phylogenetic foundation for comparative mammalian genomics".Genome Informatics Series.12:141–154.
  13. ^Springer, Mark S.; Murphy, William J.; Eizirik, Eduardo; O'Brien, Stephen J. (2003)."Placental mammal diversification and the Cretaceous–Tertiary boundary".Proceedings of the National Academy of Sciences.100(3): 1056–1061.Bibcode:2003PNAS..100.1056S.doi:10.1073/pnas.0334222100.PMC298725.PMID12552136.
  14. ^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.
  15. ^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.doi:10.1016/s1055-7903(03)00118-0.PMID12878460.
  16. ^Asher, R. J.; Bennett, N.; Lehmann, T. (2009)."The new framework for understanding placental mammal evolution".BioEssays.31(8): 853–864.doi:10.1002/bies.200900053.PMID19582725.
  17. ^Zoonomia Consortium (2020)A comparative genomics multitool for scientific discovery and conservation.Nature587, 240–245
  18. ^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.
  19. ^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.
  20. ^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.
  21. ^Halliday, Thomas J. D. (2015)."Resolving the relationships of Paleocene placental mammals".Biological Reviews.92(1): 521–550.doi:10.1111/brv.12242.PMC6849585.PMID28075073.
  22. ^abdos Reis, M.; Inoue, J.; Hasegawa, M.; Asher, R. J.; Donoghue, P. C. J.; Yang, Z. (2012)."Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny".Proceedings of the Royal Society B.279(1742): 3491–3500.doi:10.1098/rspb.2012.0683.PMC3396900.PMID22628470.
  23. ^Alroy, J (1999)."The fossil record of North American Mammals: evidence for a Palaeocene evolutionary radiation".Systematic Biology.48(1): 107–118.doi:10.1080/106351599260472.PMID12078635.
  24. ^abBrocklehurst, Neil; Panciroli, Elsa; Benevento, Gemma Louise; Benson, Roger B.J. (July 2021)."Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals".Current Biology.31(13): 2955–2963.e4.doi:10.1016/j.cub.2021.04.044.PMID34004143.S2CID234782605.
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