Therapsida

Inostrancevia, a Gorgonopsier from the Upper Permian (skeletal reconstruction)

Eurasia, Africa, South America, Australia, Antarctica

The therapsids ( Therapsida ) are a kinship group ( taxon ) amniotic land vertebrates ( Amniota ). They form together with primitive representatives who were formerly grouped under " Pelycosauria ," the group Synapsida ( " mammal -like reptiles "). In the modern, kladistisch and thus consistent phylogenetic systematics dominated, include the mammals to the therapsids and thus are living today ( recent ) representatives of this group of animals.

Although they thus form a common clade, a strictly linguistic separation between mammals and "non- mammalian therapsids " is for pragmatic reasons, sometimes (of English: non- mammalian therapsids ) maintained, since the non- mammalen Therapsid can be used as a specific stage of development ( english: grade ) are considered within the synapsids, distinguishing between the basalsten synapsids, the " Pelycosauria " ( also now considered as a grade ), and today's most advanced and synapsids, mammals stand. However, some non- mammale therapsids are related to the mammalian closer than others ( → paraphyly ). The exact designation "non- mammal" or " non-mammalian " is equally applicable to the more exclusive clade of therapsids ( Eutherapsida, Theriodontia, Cynodontia ) applied.

The therapsids were probably in the Permian, about 270 million years ago, from pelycosaurier -like synapsids forth. Today, more than 1000 kinds of non- mammaler therapsids are known, spread over more than 400 genera. In Perm and in the Lower Triassic, before the rise of the dinosaurs, they were the ecologically dominant amniotes.

History of Research

The first non- mammalen therapsids were found in 1838 in Russia and 1845 in South Africa. Her first explorers were Richard Owen (1804-1892) and Harry Govier Seeley ( 1839-1909 ). The most important of the early therapsids researchers may have been the South African Robert Broom (1866-1951), who coined the name of this taxon.

General characteristics and trends

Mammals are the modern, living today therapsids. Their morphology therefore shows both numerous similarities with the more original blueprint representatives as well as numerous differences particularly the most primitive therapsids. However, these differences did not occur suddenly, but through several modifications in a gradual development that begins at relatively reptilian representatives and ends in mammals. All these morphological changes are documented in the fossil record of therapsids.

Skull and jaw

Numerous transformations of the skull associated with an increasingly intensive processing of food in the mouth, that is, the food was not swallowed in the more modern therapsids as a whole, as do reptiles, but pre-shredded, which is a better and faster utilization of food allowed. This was an important prerequisite for the development of an active, mammals typical lifestyle.

• In the mandible, the therapsids won the dentary, the tooth-bearing bones in the course of evolution increasingly important, while the other lower jaw bones (post dental bone ), Angular, Supraangulare and articular, have been steadily smaller. Particularly striking here is the formation of the so-called Coronoidfortsatzes ( coronoid process ), an upward extension in the rear of the dentary. The Cynodontia, the most advanced therapsids, the mandible consists almost exclusively of the dental and Coronoidfortsatz is very large and has a shallow groove for the main chewing muscle, the masseter (the hollow ie in accordance with masseteric fossa ). The masseter muscle itself is also a feature of the cynodonts. It is the result of the strong magnification and division of the original pine sphincter ( musculus adductor mandibularis ) basalerer therapsids.

• Articular and quadrate ( a skull bone ) forming the temporomandibular joint in all non- mammalen tetrapods, ultimately resolved in mammals from the lower jaw and skull, and now formed the ossicles anvil ( incus ) and hammer ( malleus ). The temporomandibular joint is ( a skull bone ) formed in mammals by the Dental, the only remaining bone of the lower jaw, together with the squamosal. Thus, since there is no longer the original, primary jaw joint of tetrapods, one speaks of the secondary jaw joint of mammals. In more modern non- mammalen Cynodontiern, such as Diarthrognathus, articular and quadrate are not yet transformed into ossicles. Instead, these forms have a temporomandibular joint, involving both the bones of the primary and the secondary of the temporomandibular joint.

• The bone ridge which forms the lower boundary of the temporal window of the Pelycosaurier and basalsten therapsids and consists of a rearward extension of the Jugale and a front-facing extension of the Squamosums is formed already at basal Eutherapsiden as a laterally cantilevered arch. He is still in the form of the zygoma ( zygomatic arch ) in mammals exist. The temporal window itself, however, was noticeably smaller and is closed at the Cynodontiern. Through these modifications space and points of attachment for the jaw muscles were created.

• The mandibular symphysis, that is, the seam at which the left and right ramus meet is ossified in Cynodontiern, which the lower jaw is much stiffer. In contrast, basal synapsids have relatively mobile, and many other vertebrates partly even highly mobile lower jaw ( prime example: snakes).

• Also be observed in the course of evolution therapsides increasing bit differentiation. In basal synapsids was merely a slight morphological differentiation of the dentition in " präcanine ", " caniniforme 'and' postcanine ' teeth. Cynodonts, including mammals, however, often have a highly differentiated set of teeth in the postcanine teeth (now as molars or molars ) having teeth or occlusal nodular provided were or are. A one-time change of teeth, the so-called Diphyodontie, however, proven to have occurred only in the more modern Cynodontiern. In all basal forms, the teeth were, as with non - mammalen vertebrates common practice changed on each tooth position several times in life ( Polyphyodontie ).

Less significant changes relate to the cranium. This, and thus the brain has, in more basal Cynodontiern a maximum size of 20 percent of the average skull size of today's mammal that moves at the upper end of the size spectrum of modern reptiles.

Trunk and limbs

Therapsids developed a parasaggitalen transition, in which the legs, rather than splayed out like lizards, are placed rather under the body, which is an advantage in breathing. For more basal forms the front legs were spread apart clearer than the hind legs, such as in Dicynodontiern. Today's mammals have a fully parasaggitale gait ( the front and hind legs under the body placed ). With the development of parasaggitalen Ganges and the development of symmetric feet was accompanied by the typical mammalian phalangeal formula ( 2-3-3-3-3 ).

The Cynodontiern was created by the regression of the ribs to the neck and lumbar vertebrae of the mammal typical chest. In addition, already had the non- mammalen cynodonts, like almost all of today's mammals, seven cervical vertebrae.

Soft parts

Due to the lack of soft tissue preservation in fossils, it is unclear as to when or where developed on the line of development of the mammalian features such as fur, the outer ear ( pinna that is ), cartilaginous external nose with nasal mucosa, mammary glands or endothermic.

The coat was probably not initially for the purpose of thermal insulation, but for sensory reasons. This function practice the vibrissae nowadays still pending. Probably later, and probably in the course of endothermy developed Therapsid a dense, insulating fur. The development of hair may be on the skin glands, such as sweat glands, knotted. Skin imprints of Estemmenosuchus show that already had basal therapsids a scaleless, glandular skin. Probably went from apocrine sweat glands produce the milk glands. It is believed that early, mammals already very closely related, cynodonts were not only oviparous, but also bag -supporting ( marsupial bones are found in these as well ), and should strengthen secreted sweat the hatched young or kalklosen Synapsideneier what a selective advantage represented. These glands subsequently developed and lime, fat, or protein-containing secretions, from which developed the milk of mammals.

Systematic definition of mammalian

The historical evolution boundary between non- mammalen Cynodontiern and " true" mammals is unclear and difficult to define. Already basal cynodonts may have had a very mammal -like appearance. Often the secondary jaw joint was cited as a defining mammalian feature. Another possibility is the definition of the taxon Mammalia as a crown group (English: crown group), that is, the group that includes all extant mammals and their closest extinct relatives. However, this meant the exclusion of cynodonts genera, which already had a secondary TMJ and are generally already considered as "real " mammal (eg Morganucodon ). This, with the crown group mammals very closely related cynodonts, we group with the former instead under the name Mammaliaformes ( "mammal -like " ) together.

Variety of forms

Non - mammalian therapsids occupied in the Permian and Triassic in a variety of ecological niches. Titanosuchia presented some of the first large land herbivores. They had a short, bulky body and small, high -seated skull. These skulls show a deformed nodular surface, which suggests that they were covered during the lifetime of hardened skin and served in intraspecific fighting as a weapon, similar to today's sea lizards. Similarly, large, geologically younger herbivores were the Anomodontia. These include the Dicynodontia whose dentition is characterized by two tusk -like tusks. This group includes the well-documented and well over Pangaea widespread genus Lystrosaurus. Mostly it was about bears large animals. Some taxa, such as Diictodon, however, were about cats big and put on probably Erdbaue.

The Anteosauria, which were formed with the Titanosuchiern the taxon Dinocephalia usually large carnivores, with its elongated jaws, which probably accounted for as large prey hunting. Many basal representatives of Theriodontia were robbers, such as the large breed also Gorgonopsia. Among the smaller representatives of Theridontier include the cynodonts, in which classify all modern mammals. Some basal cynodonts may have already contributed a fur and were endothermic.

System

The following cladogram is a current hypothesis for Therapsidenstammbaum ( Chinsamy - Turan, 2008) again.

Sphenacodontidae †

Biarmosuchidae †

Burnetiidae †

Anteosaurus †

Titanosuchidae †

Tapinocephalidae †

Styracocephalidae †

Venyukovioidea †

Dromasauroidea †

Dicynodontia †

Gorgonopsia †

Therocephalia †

Cynodontia including mammals