Placentalia

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Placental mammal
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Placental mammals
Temporal range:
Ma
Possible Late Cretaceous record
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 classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Eutheria
Infraclass: Placentalia
Owen, 1837
Subgroups

Placental mammals (

fetus is carried in the uterus 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 a placenta,[1] though for a relatively briefer period, giving birth to less developed young which are then nurtured for a period inside the mother's pouch. Placentalia represents the only living group within Eutheria
, which contains all mammals more closely related to placentals than to marsupials.

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]

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

paleogeographic data.[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
Atlantogenata

Xenarthra

Afrotheria

Boreoeutheria
Euarchontoglires

Glires

Euarchonta

Laurasiatheria

Eulipotyphla

Scrotifera

Chiroptera

Ferungulata
Ferae

Pholidota

Carnivora

Euungulata

Perissodactyla

Artiodactyla

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, 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

carnivorans
.

However, modern members of the placental orders originated in the

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 left

rafting
.

In Africa, the

artiodactyls
. 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 the

multituberculate mammals diversified; afterwards, multituberculates decline and placentals explode in diversity.[24]

References

  1. .
  2. .
  3. ^ .
  4. S2CID 205026882.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  5. ^
    S2CID 4330626.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  6. ^ a b Waddell, P. J.; Okada, N.; Hasegawa, M. (1999). "Towards Resolving the Interordinal Relationships of Placental Mammals". Systematic Biology. 48 (1): 1–5.
  7. PMID 16515367
    .
  8. .
  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. PMID 11689942
    .
  11. ^ .
  12. ^ a b Waddell, P. J.; Kishino, H.; Ota, R. (2001). "A phylogenetic foundation for comparative mammalian genomics". Genome Informatics Series. 12: 141–154.
  13. PMID 12552136
    .
  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. PMID 12878460
    .
  16. .
  17. ^ Zoonomia Consortium (2020) A comparative genomics multitool for scientific discovery and conservation. Nature 587, 240–245
  18. S2CID 206544776
    .
  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. S2CID 131519722
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  21. .
  22. ^ .
  23. .
  24. ^ .

External links