Ameridelphia
The division of living beings into systematics is a continuous subject of research. Different systematic classifications exist side by side and one after the other. The taxon treated here has become obsolete due to new research or is not part of the group systematics presented in the German-language Wikipedia.
The Ameridelphia form one of the two large groups of marsupial mammals (Metatheria). With the exception of the Chiloé opossum ( Dromiciops gliroides ), which is part of the Australidelphia , all marsupials native to South , Central and North America are included . The extinct Sparassodonta ("pouch hyenas") are also included in this group.
It is not yet certain that the Ameridelphia form a monophyletic group, that is, include all descendants of their common ancestor. It is possible that the Mausopossums (Paucituberculata) are more closely related to the Australidelphia than to the opossum rats (Didelphimorphia) with which they are united here.
The delimitation of the group from the other marsupials is based primarily on molecular biological data, but also on the presence of a crest and a long rostrum, a trunk-like structure. Studies have also uncovered a connection, which only occurs in the Ameridelphia, of two sperm cells to form a pair of sperm cells, which is considered to be one of the strongest arguments for the belonging together of the animals.
The original tooth formula was probably 5/5, 1/1, 3/3, 4/4, that is, the ancestral species of the Ameridelphia probably had five incisors, one canine, three premolar and four molar teeth in both the upper and lower jaw .
distribution and habitat
Ameridelphia live exclusively on the American continent, almost all of them in South or Central America, only a few like the Virginia or Northern Opossum ( Didelphis virginiana ) also in North America. Their habitat is very diverse and ranges from tropical forests to lush grasslands to arid areas; there is even a form adapted to life in water through webbed feet. Some species can be found up to three kilometers above sea level, while others live at sea level.
convergence
Above all, the Mausopossums (Paucituberculata) have developed parallel to the shrews , whose ecological role they also play.
In the extinct Ameridelphia there are further examples of convergent evolution : In addition to species that resemble the insectivores (Insectivora) of the Laurasiatheria mammal group , there are also forms that are more similar to desert rats . Particularly striking is the convergent development of large carnivores , among which one can even find marsupial versions of the saber-toothed cats with the species of the genus Thylacosmilus .
Tribal history
The time of separation of the Ameridelphia from its sister group, the Australidelphia, varies between 128 million and 61.7 million years, depending on the scientific source. At present, a split between the two lines of development about 75 million years ago and thus in the late Cretaceous period is considered to be most likely.
South America only separated from Antarctica about 35 million years ago, and all the marsupial mammals living on the latter continent then died out due to increasing icing. Until the emergence of the Panamanian land bridge to North America, the South American animals were a successful group in many ecological niches, but afterwards they clearly lost shape diversity in competition with the higher mammals that are dominant today and today only play a subordinate role in comparison with these. With the northern opossum ( Didelphis virginiana ) only one species managed to expand its habitat significantly to the north.
Systematics
Today around 90 species in 22 genera are counted among the Ameridelphia , which are divided into two recent orders :
- The opossum or possums (Didelphimorphia) are the larger group and include approximately 80 species. As the name suggests, they have a real pouch that the youngsters grow up in. Your physique is considered to be quite original; the weight can be up to five kilograms. The majority of the opossum rats are nocturnal loners.
- Only 5 species are counted among the Mausopossums (Paucituberculata). Despite belonging to the bag mammals, they do not have a real pouch, but only a fold of skin. Only a few inches tall, they mainly live on insects and worms; however, the killing and subsequent consumption of young rodents has also been reported.
- Another order was formed by the Sparassodonta ("pouch hyenas") , which died out 2 to 3 million years ago .
- The extinct family Stagodontidae with representatives like Didelphodon is also sometimes placed near the Ameridelphia.
Both fossil groups are alternatively viewed as more original Metatheria and then stand outside the Ameridelphia.
literature
- TS Kemp: The Origin & Evolution of Mammals. Oxford University Press, Oxford 2005, ISBN 0-19-850761-5 .
