|Huxley , 1880|
The higher mammals or placental animals (Eutheria; Placentalia mostly only if the crown group is meant) form, like the egg-laying monotons (Protheria) and the marsupials (Metatheria), a subclass of mammals (Mammalia). The higher mammals are the most species-rich group - around 94 percent of the recent species belong to this taxon - and also the most diverse of mammals in terms of physique and habitats (1135 of 1229 genera).
The higher mammals have the typical mammalian features one such coat dress in hair , the three ossicles , mammary glands , the diaphragm and other under physique of mammals are described. The most noticeable differences are to be found in the structure of the sexual apparatus and in the mode of reproduction. As a result of the numerous adaptations to the most diverse ways of life and habitats, there are only a few exclusive features that distinguish this group from the other mammals.
Head and teeth
Like all mammals are the Higher mammals typically by a heterodontes characterized bite with four different types of teeth, the incisors ( incisors ), canines ( Canini ) and two types of back teeth ( premolars and molars ). Compared to the marsupials , which often have between 40 and 50 teeth, the higher mammals usually have fewer teeth. The original tooth formula was 3 / 3-1 / 1-4 / 4-3 / 3, i.e. three incisors, one canine, four premolars and three molars per half of the jaw, for a total of 44 teeth. This original tooth formula can still be found in some species, for example wild boar ; in most cases the number of teeth has decreased as a result of specialized nutrition. A few taxa, for example the anteater or the pangolin , have become completely toothless. The opposite case, an evolution-related increase in the number of teeth, has only occurred in a few cases: the giant armadillo ( Priodontes maximus ) has up to 100 pin-like teeth in its tubular snout, the highest number of all land mammals. The toothed whales represent a special case, the teeth of which have become uniform again ( homodont ); some species of dolphins have up to 260 teeth. In contrast to the marsupials, which create most of the teeth only once, the teeth - with the exception of the molars - are first created as deciduous teeth and then replaced by the permanent set of teeth and expanded by the molars.
There are also some differences in the arrangement of the cranial bones between the higher and the rest of the mammals, including the structure of the sphenoid and petrous bone . In the structure of the brain , placentas are characterized by the presence of the corpus callosum or bar - a large, transverse connection between the two cerebral hemispheres of the cerebrum .
The recent higher mammals differ from the cloacal and marsupial animals by the absence of the sac bones ( ossa epubica ), two bones protruding from the pubic bone of the pelvis . However, these are still present in the primitive representatives of this group and are likely to represent an original mammalian feature. Since these bones are present in the other mammals in both sexes, they should not originally have had anything to do with reproduction, but rather served the muscle attachment for the movement of the hind limbs.
In the structure of the urinary and sexual apparatus , the female higher mammals are characterized by a simple vagina ( monodelphia ) - marsupials have two. The structure of the womb ( uterus ) is different, but there is generally a tendency for the two Müller ducts to merge . Some groups have a pair of uterus ( uterus duplex ), for example rabbits and many rodents , in others the fusion occurs only partially, resulting in a "two-horned uterus" ( bicornuate uterus ). The fusion takes place furthest in most primates and some bats and secondary articulated animals : a simple uterus ( uterus simplex ) is created. In the male placenta, the scrotum (scrotum) - if present - lies behind the penis (in the marsupial it is in front of it). However, there are also groups without testicular descent ; among them the testes remain in the abdomen (about the elephants or elephant shrews ) or testicular descent is only partially completed (as with the whales ).
Diversity in physique
In general, there are hardly any specializations in the external physique that were reserved for cloacal or marsupials and that are not found in convergent form in the higher mammals. On the other hand, there are many forms within this group that have no counterpart in the other two mammalian taxa. For example, marine mammals ( whales , manatees and seals ) have only developed within the placenta . The only mammals capable of active flight, the bats , also belong to this group - in addition to some placentas ( e.g. giant gliders and gliding squirrels ), passive gliding has also developed in some marsupials (such as glider animals ).
Distribution and way of life
Higher mammals are common worldwide. They can be found on all continents , in all oceans and on most islands. Only in Australia were they not the dominant group of mammals. Before humans landed there and introduced numerous neozoa , there were relatively few species, namely bats and old world mice . The mammal fauna of remote islands was limited until the arrival of humans. On many islands, including New Zealand , bats were the only mammals.
Higher mammals have colonized almost all regions of the world and are found in most habitats. They can be found in deserts , steppes and forests as well as in high mountains and in the polar regions . The deep sea and the interior of the Antarctic continent are among the few regions in which (at least with the exception of temporary human residence) no higher mammals are found .
With regard to the way of life, it is difficult to make general statements. Only in terms of social behavior can it be observed that almost all mammal species that maintain complex group structures and a hierarchical ranking can be found in this group. However, this is not the preferred way of life of these animals, there are also solitary, paired or other social forms. The diet varies greatly, from which nothing special can be derived.
An exclusive feature of the higher mammals is the trophoblast , the outer cell layer of a fertilized egg cell. This creates an immune barrier and prevents the mother's immune system from responding to the embryo . This is linked to a powerful placenta , a chorioallantoic placenta , which consists of embryonic sheaths ( chorion and allantois ) and the uterine mucosa and supplies the embryo with nutrients. This ensures a longer gestation period than marsupials and a more advanced level of development at birth. The gestation period varies greatly, with some hamster species it is only 16 days, with African elephants it can last up to 25 months. The developmental state of the newborns is also different and also depends on the way of life, there are nest stools (for example predators and rodents ) as well as those who flee nests (such as cloven-hoofed animals and whales ).
The earliest known representatives of the higher mammals lived in the Mesozoic Era ; the oldest known representative is Juramaia sinensis from the Liaoning Province in northeastern China, which is 160 million years old ( Upper Jurassic ), (see also evolution of mammals ).
Compared to other mammalian taxa such as the marsupials or the Multituberculata , the fossil evidence from the Cretaceous period is relatively poor, so far only representatives of these animals have been found in East Asia and North America. The best-known genera of this era include Asioryctes , the Leptictida , which are possibly ancestors of the insectivores , the Zalambdalestidae (possible ancestors of the rodents ), the Zhelestidae (possible ancestors of the " ungulates ") and Cimolestes (possibly an ancestor of the predators ). In general, however, the assignment to today's taxa is controversial; mammals that are undoubtedly related to today's species did not appear until the Paleocene .
With the extinction of the dinosaurs , many ecological niches were vacated, which were occupied by a multitude of newly emerging groups of mammals. In the course of the Cenozoic the placenta developed into the dominant group of vertebrates on most continents, only in South America , the Antarctic (until it was iced over) and in Australia noteworthy marsupial faunas could develop. The history of the development of the placenta animals was by no means straight, but was shaped by evolutionary dead ends, displacement processes and once again completely extinct groups of mammals. In the Paleocene and Eocene in particular , there were a number of orders that were not related to today's groups. Most of today's mammal orders have been documented since the Eocene, including the ancestors of the most specialized groups, the bats and whales . The mammals reached their greatest biodiversity in the Miocene ; since then the climatic conditions have deteriorated continuously, right up to the ice ages of the Pleistocene . The climatic shifts, combined with human influences, have since led to a decline in biodiversity.
Eutheria or Placentalia?
There are two scientific names in use for this taxon, Eutheria and Placentalia. In some systematics, these terms are used synonymously, but differentiated in others: The placentalia are defined as today's representatives and all their fossil relatives ( crown group ), while Eutheria is broader and also includes the basic representatives such as Eomaia . However, since many primitive representatives only by sparse fossil discoveries are known and the Placentalia in this sense paraphyletisch could be, this separation is controversial.
The higher mammals and the marsupials are usually combined to form the group of Theria . This group is defined by a number of common derived characteristics ( autapomorphies ), including viviparity (giving birth to living offspring) and the presence of teats . Some researchers, however, reject the Theria, they consider the cesspool and marsupial to be more closely related and place them in a taxon Marsupionta . Certain genetic data are given for this taxon, but the overwhelming morphological and genetic findings speak against this thesis.
The recent orders
- Tenrecus (Afrosoricida)
- Elephant jerk (Macroscelidea)
- Tubular teeth (Tubulidentata, with the only recent species aardvark ( Orycteropus afer ))
- Schliefer (Hyracoidea)
- Russell animals (Proboscidea, with the only extant family Elephants (Elephantidae))
- Manatees (Sirenia)
- Armored siderails (Cingulata, with the only group of armadillos still alive today (Dasypoda))
- Tooth arms (pilosa)
- Pointed Squirrel (Scandentia)
- Giant Glider (Dermoptera)
- Primates (Primates)
- Rodents (Rodentia)
- Hare-like (Lagomorpha)
- Insect eater (Eulipotyphla)
- Bats (chiroptera)
- Pangolins (Pholidota)
- Predators (Carnivora, including seals (Pinnipedia))
- Unpaired ungulate (Perissodactyla)
- Artiodactyla (Artiodactyla)
- Whales (Cetacea) - the whales evolved from the even-toed ungulates, so that both are grouped together as cetartiodactyla from a cladistic point of view .
The four parent orders of today
The relationships between these orders were controversial for a long time. Some superordinates were established mainly on the basis of morphological similarities that are now considered outdated or at least highly doubted. These include, for example, the " ungulates " (ungulata), the ungulates, odd- toed ungulates, hyrax and others. The insectivores also included more groups in the past than they do today, so the tenre karts and sometimes the elephants, shrews and giant gliders were also included. The last two were sometimes found in the archonta , which also included primates and bats. Rodents and rabbits were grouped together as glires ; Armadillos and tooth arms as secondary articulated animals (Xenarthra), and pangolins were sometimes included in this group. The systematic affiliation of some difficult to classify orders such as aardvark and giant glider was particularly controversial.
Since the entry of molecular biology into systematic research, the picture of the relationships within the higher mammals has changed. Some of the above-mentioned superorders turned out to be polyphyletic , that is, they summarized forms that were not closely related and only developed convergently . Today there is a slowly increasing consensus on four superordinate orders within the placenta animals. The assumed division into these superordinate orders in the Cretaceous period coincides with the plate-tectonic breakup of the continents Laurasia and Gondwana .
|Phylogenetic system of the higher mammals|
|according to Maureen A. O'Leary et al.|
- The secondary articulated animals (Xenarthra) are, due to the structure of the vertebrae, the only morphologically well-documented hierarchy. They summarize the armored secondary articulated animals (cingulata) and tooth arms (pilosa). The group is of South American origin and today all of its representatives are limited to the American continent. In 2017, the name Americatheria was proposed for the more comprehensive group, which also includes the ancestral forms of the collateral articulated animals that are still not fossilized.
- The superorder Afrotheria are a species-poor group that occurs mainly in Africa . This group includes the tenre-kitties , elephants , tube-teeth , hyrax , manatees and elephants , including the Bibymalagasia, which became extinct around 1000 years ago . Within the Afrotheria, manatees and proboscis form a taxon Tethytheria , Tethytheria and hyrax are grouped together as Paenungulata .
- The Laurasiatheria have their origin on the former continent Laurasia and are composed of insectivores , bats , cetartiodactyla ( even-toed ungulates and whales ), odd-toed ungulates , pangolins and predators . Within this group, the insectivores probably form the sister taxon of the other orders, which are summarized as Scrotifera . (Some researchers even suspect that the hedgehogs do not belong to the insectivores, but form the sister taxon of all other higher mammals, but this is highly controversial.) A close relationship between pangolin and predator (as the taxon Ferae ) is considered likely, the relationships the other orders are still under discussion. One possible taxon Zooamata includes ferae and unpaired ungulates. Zooamata and bats may form a common taxon, Pegasoferae ; This contradicts the theory of Fereuungulata , which are said to be composed of Ferae, odd-toed ungulates and cetartiodactyla.
- The Euarchontoglires probably also originated in today's Eurasia . This group includes two taxa, on the one hand the glires with rodents and hares - a group that is also well documented morphologically - and on the other hand the euarchonta , which includes primates, shrews and giant gliders.
The relationships of the four superordinates
The relationships between the four superordinate levels are still partly controversial. Most researchers assume a sister relationship between Laurasiatheria and Euarchontoglires, the common taxon is called Boreoeutheria and summarizes the taxa that originated in the northern continents.
Classically, the secondary articulated animals are considered a sister group of the other higher mammals, which are summarized as Epitheria . However, this theory is controversial, due to some molecular similarities it is sometimes assumed that the Afrotheria split off from the higher mammals before the secondary mammals (Boreoeutheria and secondary animals then form the taxon Exafroplacentalia ). Finally, a third theory sees a common taxon Atlantogenata in Afrotheria and secondary articulated animals , which would then be the sister taxon of Boreoeutheria. A cladogram from a recent publication (February 2013) in the US journal Science is shown above.
Since the inner systematics of the higher mammals has been redefined by molecular biology, the assignment of extinct orders has become more difficult. Since no genetic material for comparison can be taken from fossils, one has to rely on morphological comparisons; as mentioned above, these have often proven to be deceptive. That is why only the most important extinct groups can be listed here; a systematic classification is often difficult.
- As South American ungulates (Meridiungulata) five American taxa are summarized, which were characterized by hoofed feet and lived primarily in South America. These are the Litopterna , the Notoungulata , the Astrapotheria , the Pyrotheria and the Xenungulata . However, it is still uncertain whether they actually form a natural group.
- The Desmostylia were hippopotamus-like, water-dwelling animals that are mostly classified in the Paenungulata .
- The embrithopoda resembled outwardly rhinos, also they are counted among the paenungulata.
- The Hyaenodonta and Oxyaenodonta were carnivores, whether or not they were related to today's predators is not entirely certain.
- The Dinocerata were among the earliest giant mammals.
- The Leptictida are already known from the Cretaceous period, they may be related to the insectivores.
- The Plesiadapiformes are often placed in close relation to the primates. It is probably a paraphyletic group made up of primate tribal representatives.
- A number of groups are summarized as Cimolesta , including Tillodonta , Taeniodonta , Pantodonta and Apatotheria . It is unclear whether they form a natural group, sometimes they are classified in the Ferae.
- A number of extinct, hoofed animals are summarized as the primordial ungulates (condylarthra). Since the ungulates most likely do not form a natural group, the systematics of this group is also unclear; it is probably a para or polyphyletic group . In addition to some early representatives such as the Zhelestidae , the carnivorous Mesonychia are also included.
- Thomas S. Kemp: The Origin & Evolution of Mammals . Oxford University Press, Oxford 2005, 331 pages, ISBN 0-19-850761-5 .
- William J. Murphy, Eduardo Eizirik, Mark S. Springer et al. : Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics , Science, Vol 294, Issue 5550, pp. 2348-2351, December 14, 2001, doi: 10.1126 / science.1067179
- Jan Ole Kriegs, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Jürgen Brosius, Jürgen Schmitz. (2006): Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4 (4): e91, doi: 10.1371 / journal.pbio.0040091
- Wible JE et al. : Cretaceous eutherians and Laurasian origin of placental mammals near the K / T boundary , Nature (2007) 447, pp. 1003-1006
- Zhe-Xi Luo, Chong-Xi Yuan, Qing-Jin Meng, Qiang Ji .: A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature , 2011; 476 (7361): 442 doi: 10.1038 / nature10291
- Maureen A. O'Leary et al. : The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals. Science 8, February 2013, Volume 339, No. 6120 Pages 662–667, doi: 10.1126 / science.1229237
- Bruce J. Shockey: New Early Diverging Cingulate (Xenarthra: Peltephilidae) from the Late Oligocene of Bolivia and Considerations Regarding the Origin of Crown Xenarthra. Bulletin of the Peabody Museum of Natural History 58 (2), 2017, pp. 371-396
- Jonathan I. Bloch, Mary T. Silcox, Doug M. Boyer & Eric J. Sargis: New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates. PNAS , January 23, 2007, vol. 104, no.4 doi: 10.1073 / pnas.0610579104