A mammal is a vertebrate animal of the class Mammalia. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders.
Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers; and the simians, which include monkeys and apes. Primates arose 85–55 million years ago first from small terrestrial mammals, which adapted for life in tropical forests: many primate characteristics represent adaptations to the challenging environment among tree tops, including large brain sizes, binocular vision, color vision, vocalizations, shoulder girdles allowing a large degree of movement in the upper limbs, and opposable thumbs that enable better grasping and dexterity. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g (1 oz), to the eastern gorilla, weighing over 200 kg (440 lb). There are 376–524 species of living primates, depending on which classification is used. New primate species continue to be discovered: over 25 species were described in the 2000s, 36 in the 2010s, and six in the 2020s.
The treeshrews are small mammals native to the tropical forests of South and Southeast Asia. They make up the entire order Scandentia, which split into two families: the Tupaiidae, and the Ptilocercidae.
Placental mammals are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the 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, 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.
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups. The cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds, pterosaurs, and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird, bat, and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions.
Afrotheria is a superorder of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, elephant shrews, otter shrews, tenrecs, aardvarks, hyraxes, elephants, sea cows, and several extinct clades. Most groups of afrotheres share little or no superficial resemblance, and their similarities have only become known in recent times because of genetics and molecular studies. Many afrothere groups are found mostly or exclusively in Africa, reflecting the fact that Africa was an island continent from the Cretaceous until the early Miocene around 20 million years ago, when Afro-Arabia collided with Eurasia.
Euarchontoglires, synonymous with Supraprimates, is a clade and a superorder of mammals, the living members of which belong to one of the five following groups: rodents, lagomorphs, treeshrews, primates, and colugos.
The Euarchonta are a proposed grandorder of mammals: the order Scandentia (treeshrews), and its sister Primatomorpha mirorder, containing the Dermoptera or colugos and the primates.
Laurasiatheria is a superorder of placental mammals that groups together true insectivores (eulipotyphlans), bats (chiropterans), carnivorans, pangolins (pholidotes), even-toed ungulates (artiodactyls), odd-toed ungulates (perissodactyls), and all their extinct relatives. From systematics and phylogenetic perspectives, it is subdivided into order Eulipotyphla and clade Scrotifera. It is a sister group to Euarchontoglires with which it forms the magnorder Boreoeutheria. Laurasiatheria was discovered on the basis of the similar gene sequences shared by the mammals belonging to it; no anatomical features have yet been found that unite the group, although a few have been suggested such as a small coracoid process, a simplified hindgut and allantoic vessels that are large to moderate in size. The Laurasiatheria clade is based on DNA sequence analyses and retrotransposon presence/absence data. The superorder originated on the northern supercontinent of Laurasia, after it split from Gondwana when Pangaea broke up. Its last common ancestor is supposed to have lived between ca. 76 to 90 million years ago.
Epitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupials, monotremes, and xenarthrans. They are also characterized by having a shorter fibula relative to the tibia.
Neognathae is an infraclass of birds, called neognaths, within the class Aves of the clade Archosauria. Neognathae includes the majority of living birds; the exceptions being the tinamous and the flightless ratites, which belong instead to the sister taxon Palaeognathae. There are nearly 10,000 living species of neognaths.
Boreoeutheria is a magnorder of placental mammals that groups together superorders Euarchontoglires and Laurasiatheria. With a few exceptions, male boreoeutherians have a scrotum, an ancestral feature of the clade. The sub-clade Scrotifera was named after this feature.
The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.
In evolutionary biology, the flying primate hypothesis is that megabats, a subgroup of Chiroptera, form an evolutionary sister group of primates. The hypothesis began with Carl Linnaeus in 1758, and was again advanced by J.D. Smith in 1980. It was proposed in its modern form by Australian neuroscientist Jack Pettigrew in 1986 after he discovered that the connections between the retina and the superior colliculus in the megabat Pteropus were organized in the same way found in primates, and purportedly different from all other mammals. This was followed up by a longer study published in 1989, in which this was supported by the analysis of many other brain and body characteristics. Pettigrew suggested that flying foxes, colugos, and primates were all descendants of the same group of early arboreal mammals. The megabat flight and the colugo gliding could be both seen as locomotory adaptations to a life high above the ground.
Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components.
Leptictida is a possibly paraphyletic extinct order of eutherian mammals. Their classification is contentious: according to cladistic studies, they may be (distantly) related to Euarchontoglires, although they are more recently regarded as the first branch to split from basal eutherians. One recent large-scale cladistic analysis of eutherian mammals favored lepictidans as close to the placental crown-clade; and several other recent analyses that included data from Cretaceous non-eutherian mammals found Leptictis to belong to the superorder Afrotheria.
Oceanic dispersal is a type of biological dispersal that occurs when terrestrial organisms transfer from one land mass to another by way of a sea crossing. Island hopping is the crossing of an ocean by a series of shorter journeys between islands, as opposed to a single journey directly to the destination. Often this occurs via large rafts of floating vegetation such as are sometimes seen floating down major rivers in the tropics and washing out to sea, occasionally with animals trapped on them. Dispersal via such a raft is sometimes referred to as a rafting event. Colonization of land masses by plants can also occur via long-distance oceanic dispersal of floating seeds.
Primatomorpha is a proposed mirorder of mammals containing the orders Dermoptera and Primates. Primatomorpha is sister to Scandentia, together forming the Euarchonta.
Stephen Blair Hedges is Laura H. Carnell Professor of Science and director of the Center for Biodiversity at Temple University where he researches the tree of life and leads conservation efforts in Haiti and elsewhere. He co-founded Haiti National Trust.
Emma Caroline Teeling is an Irish zoologist, geneticist and genomicist, who specialises in the phylogenetics and genomics of bats. Her work includes understanding of the bat genome and study of how insights from other mammals such as bats might contribute to better understanding and management of ageing and a number of conditions, including deafness and blindness, in humans. She is the co-founder of the Bat1K project to map the genomes of all species of bat. She is also concerned with understanding of the places of bats in the environment and how to conserve their ecosystem.
