Editing Placentalia (section)

== 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.<ref name="JiLuoYuan2002EarliestEutherian">{{cite journal |author=Ji, Q., Luo, Z-X., Yuan, C-X., Wible, J. R., Zhang, J-P. and Georgi, J. A. |title=The earliest known eutherian mammal |journal=Nature |volume=416 |pages=816–822 |date=April 2002 |doi=10.1038/416816a |pmid=11976675 |issue=6883|bibcode=2002Natur.416..816J |s2cid=4330626 }}</ref>

True placentals may have originated in the [[Late Cretaceous]] around 90 mya, but the earliest undisputed fossils are dated to the [[Cretaceous–Paleogene boundary]] (K-Pg boundary). The genus ''[[Protungulatum]]'' is sometimes placed as a stem-ungulate,<ref name="SCI-20130208">{{cite journal |last1=O'Leary |first1=Maureen A. |last2=Bloch |first2=Jonathan I. |last3=Flynn |first3=John J. |last4=Gaudin |first4=Timothy J. |last5=Giallombardo |first5=Andres |last6=Giannini |first6=Norberto P. |last7=Goldberg |first7=Suzann L. |last8=Kraatz |first8=Brian P. |last9=Luo |first9=Zhe-Xi |last10=Meng |first10=Jin |last11=Ni |first11=Michael J. |last12=Novacek |first12=Fernando A. |last13=Perini |first13=Zachary S. |last14=Randall |first14=Guillermo |last15=Rougier |first15=Eric J. |last16=Sargis |first16=Mary T. |last17=Silcox |first17=Nancy b. |last18=Simmons |first18=Micelle |last19=Spaulding |first19=Paul M. |last20=Velazco |first20=Marcelo |last21=Weksler |first21=John r. |last22=Wible |first22=Andrea L. |title=The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals |date=8 February 2013 |journal=[[Science (journal)|Science]] |volume=339 |pages=662–667 |doi=10.1126/science.1229237 |pmid=23393258 |issue=6120 |last23=Cirranello |first23=A. L. |bibcode=2013Sci...339..662O |hdl=11336/7302 |s2cid=206544776 |hdl-access=free }}</ref> with probably the earliest known species ''P. coombsi'' from the strata within the [[Hell Creek Formation]] specifically dated to at least 300,000 years before the K-Pg boundary.<ref name=Archibald2011/> The genus ''[[Purgatorius]]'', sometimes considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary.<ref>{{cite journal |last1=Fox |first1=R. C. |last2=Scott |first2=C. S. |year=2011 |title=A new, early Puercan (earliest Paleocene) species of Purgatorius (Plesiadapiformes, Primates) from Saskatchewan, Canada |journal=Journal of Paleontology |volume=85 |issue=3 |pages=537–548 |doi=10.1666/10-059.1|bibcode=2011JPal...85..537F |s2cid=131519722 }}</ref> One study has recovered both genera to be closely related and as stem-[[eutherian]]s outside modern placental mammals,<ref>{{cite journal |doi=10.1111/brv.12242 |pmid=28075073 |pmc=6849585 |title=Resolving the relationships of Paleocene placental mammals |year=2015 |journal=Biological Reviews |volume=92 |issue=1 |pages=521–550 |last1=Halliday |first1=Thomas J. D. }}</ref> but others have recovered ''Protungulatum'' as a [[Pan-Euungulata|pan-euungulate]] based on phylogenetic analysis and inner ear anatomy different from non-placentals.<ref>{{Cite journal| doi = 10.5252/g2015n4a1| issn = 1280-9659| volume = 37| issue = 4| pages = 397| last1 = de Muizon| first1 = Christian| last2 = Billet| first2 = Guillaume| last3 = Argot| first3 = Christine| last4 = Ladevèze| first4 = Sandrine| last5 = Goussard| first5 = Florent| title = Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology| journal = Geodiversitas| year = 2015 | s2cid = 131556174| url = https://bioone.org/journals/geodiversitas/volume-37/issue-4/g2015n4a1/Alcidedorbignya-inopinata-a-basal-pantodont-Placentalia-Mammalia-from-the-early/10.5252/g2015n4a1.full}}</ref><ref>{{Cite journal |last1=Orliac |first1=M. J. |last2=o'Leary |first2=M. A. |title=The inner ear of ''Protungulatum'' (Pan-Euungulata, Mammalia) |url=https://link.springer.com/article/10.1007/s10914-016-9327-z |year=2016 |journal=Journal of Mammalian Evolution |volume=23 |issue=4 |pages=337–352 |doi=10.1007/s10914-016-9327-z|s2cid=133815599 }}</ref> The rapid appearance of placentals after the mass extinction at the end of the [[Cretaceous]] suggests that the group had already originated and undergone an initial diversification in the Late Cretaceous, as suggested by [[molecular clock]]s.<ref name="Reis, M. 2012">{{cite journal |last1=dos Reis |first1=M. |last2=Inoue |first2=J. |last3=Hasegawa |first3=M. |last4=Asher |first4=R. J. |last5=Donoghue |first5=P. C. J. |last6=Yang |first6=Z. |year=2012 |title=Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny |journal=Proceedings of the Royal Society B |volume=279 |issue=1742 |pages=3491–3500 |doi=10.1098/rspb.2012.0683 |pmid=22628470 |pmc=3396900}}</ref> The lineages leading to Xenarthra and Afrotheria probably originated around 90 mya, and Boreoeutheria underwent an initial diversification around 70-80 mya,<ref name="Reis, M. 2012"/> producing the lineages that eventually would lead to modern primates, rodents, [[insectivores]], [[artiodactyls]], and [[carnivorans]].

However, modern members of the placental orders originated in the [[Paleogene]] 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<ref>{{cite journal |last1=Alroy |first1=J |year=1999 |title=The fossil record of North American Mammals: evidence for a Palaeocene evolutionary radiation |journal=Systematic Biology |volume=48 |issue=1 |pages=107–118 |doi=10.1080/106351599260472 |pmid=12078635 |doi-access=free}}</ref> that took place as mammals quickly evolved to take advantage of ecological [[Ecological niche|niches]] that were left open when most dinosaurs and other animals disappeared following the [[Chicxulub impactor|Chicxulub 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 competing [[synapsid]]s<ref name="Brocklehurst et al 2021">{{cite journal |last1=Brocklehurst |first1=Neil |last2=Panciroli |first2=Elsa |last3=Benevento |first3=Gemma Louise |last4=Benson |first4=Roger B.J. |title=Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals |journal=Current Biology |date=July 2021 |volume=31 |issue=13 |pages=2955–2963.e4 |doi=10.1016/j.cub.2021.04.044 |pmid=34004143 |s2cid=234782605 |url=https://ora.ox.ac.uk/objects/uuid:bda82407-db76-4c15-b061-ceb9ae271dd5 |doi-access=free |bibcode=2021CBio...31E2955B }}</ref>). 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 left [[Laurasia]] and colonized Africa and then South America via [[Rafting event|rafting]].

In Africa, the [[Afrotheria]] underwent a major adaptive radiation, which led to elephants, [[elephant shrew]]s, [[tenrec]]s, [[golden mole]]s, [[aardvark]]s, and [[manatee]]s. In South America a similar event occurred, with radiation of the Xenarthra, which led to modern [[sloth]]s, [[anteater]]s, and [[armadillo]]s, as well as the extinct [[ground sloths]] and [[glyptodonts]]. Expansion in Laurasia was dominated by Boreoeutheria, which includes primates and rodents, [[insectivore]]s, carnivores, [[Perissodactyla|perissodactyls]] and [[artiodactyl]]s. 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 [[Paleocene]], while [[Multituberculata|multituberculate]] mammals diversified; afterwards, multituberculates decline and placentals explode in diversity.<ref name="Brocklehurst et al 2021"/>