Fossil specimen of Eomaia scansoria, an early eutherian
Placentalia is a term used in some taxonomic classification schemes for one of the three major branches of mammals that includes extant members, the others being Monotremata (montremes) and Marsupialia (marsupials). With its members known as placental mammals or placentals,, extant members are characterized by live birth of offspring and having the fetus nourished during gestation via a well-developed placenta. Marsupials also have a extra-embryonic, temporary vascular organ that forms in the uterus of females that is known as a placenta, but it is of different structure.
In some taxonomic schemes, Placentalia is placed within the clade Eutheria within the class Mammalia. In this case, extant eutherians, their last common ancestor, and all extinct descendants of that ancestor are placentals. Some schemes use Placentalia in place of Eutheria, thus including those extinct mammals that share a more recent common ancestor with placentals than they do with Metatheria (marsupials and their nearest ancestors).
Fossils of known placentals have been dated to 65 million years ago in the Paleogene. Placentals became greatly diversified and dominate after the great Cretaceous-Tertiary extinction event killed about 76 percent of species on Earth, including most of the dinosaurs ,with the placentals diversifying into the ecological niches previous occupied by dinosaurs and prehistoric reptiles.
Mammals (class Mammalia) are a clade of endothermic amniotes distinguished from the reptiles and the birds by the possession of hair, three middle ear bones, mammary glands in females, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs.
In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones. The placentals constitute the crown group of the Eutheria.
While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.
As with all other mammals, placentals are endotherms and amniote vertebrates, and have at most two generations of teeth, unlike reptiles in which there can be many generations of teeth. In mammals, the articular and quadrate, which composes the jaw joint in reptiles, is incorporated into the middle ear as the outermost two ear ossicles (malleus and incus; reptiles have only one ear ossicle, thte columnella). While in birds and reptiles, the large aorta leaving the heart bends to the right, in mammals it bends to the left (Archibald 2001).
The features of Placentalia (and Eutheria) that distinguish them from metatherians, a group that includes modern marsupials, are:
- an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones (Ji et al. 2002).
- the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and middle cuneiform bones—in metatherians these joints are level with each other (Ji et al. 2002).
- various features of jaws and teeth (Ji et al. 2002).
Placental mammals are distinguished from other eutherians by:
- the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones (Ji et al. 2002).
- a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula (Ji et al. 2002).
- a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials and nonplacental eutherians have a narrower opening that allows only small, immature offspring to pass through (Weil 2002).
- the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals—nonplacental eutherians, marsupials, monotremes, and earlier mammaliaforms&mdas;has well as in other cynodonts that are closest to mammals. Their function is to stiffen the body during locomotion (Reilly and White 2003). This stiffening would be harmful in pregnant placentals, whose abdomens need to expand (Novacek et al. 1997).
Classification and origin of Placentalia
An example of a classification scheme that recognizes Placentalia is that of McKenna and Bell (1997). The following is a portion of the McKenna/Bell classification showing the placement of Prototeria, Theria, Marsupialia, and Placentalia:
There are different classification schemes for the ordering of groups within Placentalia. Molecular studies based on DNA analysis have revised the understanding of relationships among placental groups during the 21st century (Kriegs et al. 2006). Classification systems based on molecular studies reveal three major groups or lineages of placental mammals: Afrotheria, Xenarthra, and Boreoeutheria, all of which diverged from common ancestors in the Cretaceous (Archibald et al. 2001):
- Boreoeutheria, e.g. badgers, rabbits, guinea pigs, dogs
- Xenarthra, e.g. armadillos, anteaters
- Afrotheria, e.g. elephants, hyraxes, manatees
The exact relationships between these three lineages is currently a subject of debate, and three different hypotheses have been proposed with respect to which group is basal, or diverged first from other placentals. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), and Exafroplacentalia (basal Afrotheria) (Nishihara et al. 2009). Boreoeutheria in turn contains two major lineages: Euarchontoglires and Laurasiatheria.
Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago, depending on type of DNA (e.g. nuclear or mitochondrial) (Springer et al. 2003) and varying interpretations of paleogeographic data (Nishihara et al. 2009).
The earliest known placental mammal species (including Protungulatum donnae, Purgatorius coracis, and Leptacodon proserpinae) are all dated to the early Paleogene period, about 65 million years ago. O'Leary et al. (2013) place the oldest fossil placental, Protungulatum donnae, at 64.85 million years ago.
The Paleogene, which begins at the end of the Cretaceous, spans from roughly from 65 to 23 million years ago (mya) and is most notable as the time in which mammals, including placentals, became very diversified and dominant after the great Cretaceous-Tertiary extinction event killed roughly 50 percent of the plant and animal families on Earth, and about 76 percent of the species (Barnosky et al. 2011; dos Reis et al. 2014), including most of the dinosaurs. True placental mammals appeared shortly after the K-Pg extinction event and quickly diversified into ecological niches previously occupied by various groups of dinosaurs and other prehistoric reptiles (O'Leary et al. 2013). True placental mammals (the crown group of all modern placentals) arose from stem-group members of the clade Eutheria, which had existed since at least the mid-Jurassic period (Ji et al. 2002).
Dos Reis et al. (2014) claim that neither paleontological data nor molecular studies support a Paleogene origin of placental mammals and that Placentalia originated in the Cretaceous. This contradicts the view of those, like O'Leary et al. (2013), that posit the ancestor of placentals post-dated the Cretaceous-Paleogene boundary.
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