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 epoch, about 170 mya. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees. 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, 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. One study has recovered both genera to be closely related and as stem-
eutherians outside modern placental mammals, but others have recovered
Protungulatum as a
pan-euungulate based on phylogenetic analysis and inner ear anatomy different from non-placentals. 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 clocks. The lineages leading to Xenarthra and Afrotheria probably originated around 90 mya, and Boreoeutheria underwent an initial diversification around 70-80 mya, that took place as mammals quickly evolved to take advantage of ecological
niches that were left open when most dinosaurs and other animals disappeared following the
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
synapsids). 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. In Africa, the
Afrotheria underwent a major adaptive radiation, which led to elephants,
elephant shrews,
tenrecs,
golden moles,
aardvarks, and
manatees. In South America a similar event occurred, with radiation of the Xenarthra, which led to modern
sloths,
anteaters, and
armadillos, as well as the extinct
ground sloths and
glyptodonts. Expansion in Laurasia was dominated by Boreoeutheria, which includes primates and rodents,
insectivores, carnivores,
perissodactyls and
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
Paleocene, while
multituberculate mammals diversified; afterwards, multituberculates decline and placentals explode in diversity. == Notes ==