Therapsids
| Therapsids | ||||||||||||
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Inostrancevia , a Gorgonopsier from the Upper Permian of Russia (skeleton reconstruction ) |
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| Temporal occurrence | ||||||||||||
| Middle Permian until today (non-mammal Therapsiden to Lower Cretaceous ) | ||||||||||||
| 270 to 0 (or 100) million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Therapsida | ||||||||||||
| Broom , 1905 |
The Therapsiden (Therapsida) are an extensive family group ( taxon ) of amniotic land vertebrates (Amniota), which also includes mammals . They went probably in the late Permian , before about 275 million years ago, from pelycosaur -like synapsids forth. Today well over 1000 species of non-mammal therapsids are known, which are distributed over more than 400 genera . Especially in the Permian but also in the early Triassic , i.e. before the rise of the dinosaurs , they were the ecologically dominant amniotes. The modern therapsids, represented by mammals, began their triumphant advance only after the dinosaurs became extinct.
Research history
The first non-mammal therapsids were found in Russia in 1838 and in South Africa in 1845. Richard Owen (1804-1892) and Harry Govier Seeley (1839-1909) were among her first explorers . The most important of the early therapsid researchers is likely to have been the South African Robert Broom (1866–1951), who also coined the name of this taxon.
General characteristics and development tendencies
Mammals are the modern therapsids living today. Their morphology therefore shows numerous similarities with the blueprint of original representatives as well as numerous differences to especially the most original Therapsids. However, these differences did not arise suddenly, but rather through several modifications in a gradual development that begins with representatives that are still relatively reptilian and ends with mammals (see → evolution of mammals ). All of these morphological changes are documented in the Therapsid fossil record .
Skull and jaw
Numerous reshuffles of the skull are associated with an increasingly intensive processing of the food in the oral cavity, i. H. In the more modern Therapsids, the food was no longer swallowed whole, as reptiles do, but pre-chopped, which enabled better and faster food utilization. This was an important prerequisite for the development of an active, mammalian lifestyle.
- In the lower jaw of the Therapsids, the dental , the tooth-bearing bone, became increasingly important in the course of evolution, while the other lower jaw bones ( postdental bones ), angular , supraangular and articular , became steadily smaller. Particularly noticeable here is the formation of the so-called coronoid process ( processus coronoideus ), an upwardly directed process in the back of the dentary. In the cynodontia, the most modern therapsids, the lower jaw consists almost exclusively of the dental and the coronoid process is very large and has a shallow hollow for the most important masseter muscle, the masseter (the hollow is called fossa masseterica ). The masseter muscle itself is also a characteristic of the cynodontics. It is the result of the strong enlargement and division of the original jaw sphincter ( musculus adductor mandibularis ) of basal therapsids.
- Articulare and quadratum (a skull bone), which form the temporomandibular joint in all non-mammal tetrapods, ultimately separated from the lower jaw or skull in mammals and now formed the ossicles anvil (incus) and hammer (malleus). In mammals, the temporomandibular joint is formed by the dental, the only remaining bone of the lower jaw, together with the squamosum (a skull bone). Since the original, primary temporomandibular joint of the tetrapods is no longer present, one speaks of the secondary temporomandibular joint in mammals . In more modern non-mammal cynodontists, such as B. Diarthrognathus , the articular and quadratum are not yet transformed into ossicles. Instead, these forms have a temporomandibular joint that involves both the bones of the primary and secondary TMJ.
- The bone web, the lower boundary of the temporal window forms the pelycosaur and basalsten Therapsid and a rearwardly directed extension of the Jugale and a forwardly directed extension of the Squamosums there is, due to a narrow become dorsal cranial vault already in basal Eutherapsiden the only element of the rear Side wall of the skull, resulting in the typical skull morphology of the "higher" Therapsids. This bone bar is still present in mammals in the form of the zygomatic arch ( arcus zygomaticus ). The temporal window itself is in the cynodonti by a secondary side wall of the skull consisting of a downwardly directed outgrowth of the parietal, an upwardly directed outgrowth of the epipterygoid or alisphenoid (a bone of the roof of the mouth) and a forwardly directed but further "inside" (medial to the zygomatic arch) lying outgrowth of the squamosum closed again. These modifications created attachment points for the complex jaw muscles, while at the same time widening the brain skull.
- The mandibular symphysis, d. That is, the suture at which the left and right branches of the lower jaw meet is ossified in cynodontics, which makes the lower jaw considerably stiffer. In contrast, basal synapsids have relatively motile, and many other vertebrates e.g. Sometimes even highly mobile lower jaws (prime example: snakes ).
- An increasing differentiation of teeth can also be observed in the course of the Therapsid evolution. In the case of basal synapsids, there was only a slight morphological differentiation of the dentition into “precanine”, “caniniform” and “postcanine” teeth. The tooth shape was essentially the same and the canine-shaped teeth differed from the other teeth only in their size. However, cynodontals, including mammals, often have a highly differentiated set of teeth in which postcanine teeth (now referred to as molars or cheek teeth ) were or are provided with jagged or bumpy chewing surfaces. A one-time change of teeth , the so-called diphyodontia , on the other hand, only demonstrably occurred in the more modern cynodontists. In all more basal forms, as is common in non-mammal vertebrates, the teeth on each tooth position were changed several times in life ( polyphyodontics ).
An enlargement of the brain skull, however, only takes place at the end of the Therapsid evolution, within the "higher" cynodontia. Even in the more basal cynodontics, the brain skull and thus also the brain have a maximum size of 20 percent of the mean brain skull size of a modern mammal, which is at the upper end of the size spectrum of modern reptiles.
Trunk and limbs
Therapsids developed a parasagittal gait ("Stemmgang"), in which the legs, instead of spread apart as in lizards , are placed under the body, which is an advantage for breathing. In more basal forms, the forelegs were more clearly spread apart than the hind legs, such as in dicynodontics. Today's mammals have a fully parasagittal gait (front and rear legs placed under the body). With the development of the parasagittal gait, the formation of symmetrical feet with the finger and toe phalanx formula (2-3-3-3-3) typical of mammals was also associated.
In the cynodontics, the regression of the ribs on the cervical and lumbar vertebrae resulted in the mammalian chest . In addition, the non-mammalian cynodontics, like almost all modern mammals, already had 7 cervical vertebrae.
Soft tissues
Due to the lack of soft tissue preservation in fossils, it is unclear from when or where on the developmental line to mammals, such as fur, outer ear (i.e. auricle), cartilaginous nose with external nasal mucous membranes , mammary glands, or endothermia arose.
The hair coat was probably not initially created for the purpose of thermal insulation , but for sensory reasons. The vibrissae still perform this function today. Probably only later, and probably in the course of endothermia, therapsids developed a thicker, insulating fur. Hair development may be linked to skin glands, such as sweat glands . Skin impressions from Estemmenosuchus show that basal therapsids already had flaky, glandular skin. Presumably the mammary glands emerged from apocrine sweat glands. It is assumed that early cynodontics, which were already very close to mammals, were not only egg-laying, but also pouch-bearing ( pouch bones are also found in these), and secreted sweat should strengthen the hatched young or the lime-free synapsid eggs, which represented a selection advantage. These glands subsequently also developed secretions containing lime , fat or protein , from which the milk of mammals developed.
Variety of shapes
Non-mammalian therapsids occupied a large number of ecological niches in the Permian and Triassic . Titanosuchia were some of the first great land herbivores . They had short, massive bodies and small, high-set skulls. These skulls show an overgrown, nodular surface, which suggests that they were covered by hardened skin during their lifetime and were used as weapons in intraspecific battles, similar to today's marine iguanas . Similarly large, geologically younger herbivores were the anomodontia . These include the dicynodontia, whose teeth are characterized by two tusk-like tusks. The well-documented genus Lystrosaurus , which is widely distributed across Pangea , belongs to this group . Most of them were about the size of a bear. However, some taxa, such as Diictodon , were about the size of a cat and probably built earthworks.
The Anteosauria, which form the taxon Dinocephalia with the Titanosuchiern , were usually large carnivores with elongated jaws, which probably hunted prey of the same size. Many basal representatives of the Theriodontia were also predators, such as the large Gorgonopsia . The cynodonts, in which all modern mammals also belong, belong to the smaller representatives of the Theriodontic animals. Some basal cynodontics may already be furry and endothermic.
Systematics
Classic system
In the classical system, therapsids form a taxon of the rank of an order , which exclusively includes extinct representatives. Together with the order Pelycosauria , the therapsids are grouped into the subclass Synapsida (Theromorpha, "mammal-like reptiles"). The Synapsida, in turn, are classified as subtaxons of the class Reptilia in the classical system and are therefore relatively clearly differentiated from mammals, although they are more closely related to mammals than to today's reptiles.
Modern system
In the modern, cladistic and consequently phylogenetic systematics , Synapsida and Therapsida are defined as clades , whereby the Therapsida form a more exclusive clade within the Synapsida. Both clades also include the mammals . The mammals are thus still living ( recent ) representatives of these two clades.
The following cladogram gives a current hypothesis on the therapsid family tree (Chinsamy-Turan, 2008).
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" Clades & Grades "
Although they form a common clade, for pragmatic reasons a purely linguistic separation between mammals and "non-mammalian therapsids" (from English : non-mammalian therapsids ) is maintained, because the non-mammalian therapsids can be classified as a certain stage of development (English : grade ) within the synapsids, where they stand between the most basal synapsids, the "Pelycosauria" (also considered as straight today ), and the modern and most modern synapsids, the mammals, and some non-mammal therapsids are more closely related to the mammals are than others (→ paraphyly ). The more detailed designation "non-mammal" or "non-mammal" is applied in the same way to the more exclusive clades of the therapsids (Eutherapsida, Theriodontia , Cynodontia ).
Systematic definition of mammals
The evolutionary-historical boundary between non-mammal cynodontics and “real” mammals is vague and difficult to define. Already basal cynodontals may have had a very mammal-like appearance. Often the secondary TMJ has been cited as the defining mammalian characteristic. Another possibility is the definition of the taxon Mammalia as a crown group (ger .: crown group ), d. that is, the group that includes all mammals living today and their closest extinct relatives. However, this meant the exclusion of cynodontal genera that already had a secondary temporomandibular joint and are generally already regarded as "real" mammals (e.g. Morganucodon ). These cynodontals, which are very closely related to the crown group mammals, but stand outside the crown group, are instead grouped together with the crown mammals under the name Mammaliaformes ("mammalian").
literature
- Robert L. Carroll : Paleontology and Evolution of the Vertebrates. Thieme-Verlag, Stuttgart 1993, ISBN 3-13-774401-6 .
- Anusuya Chinsamy-Turan (Ed.): Forerunners of Mammals: Radiation, Histology, Biology. Indiana University Press, Bloomington 2008, 352 pp., ISBN 0-25335-697-0 .
- Thomas S. Kemp: The Origin & Evolution of Mammals. Oxford University Press, Oxford 2005, ISBN 0-19-850761-5 .
- Oskar Kuhn : The mammal-like reptiles . 2nd unchanged edition. A. Ziemsen Verlag, Wittenberg 2003 (unchanged reprint of the 1st edition from 1970), ISBN 3-89432-797-9 .
- Bruce S. Rubidge, Christian A. Sidor: Evolutionary patterns among permo-triassic Therapsids. Annual Review of Ecology and Systematics Vol. 32, 2001, pp. 449-480, PDF (1.9 MB)
Web links
- Palaeos.com: Therapsida (Eng.)
- Tree of Life: Therapsida (Engl.)
Individual evidence
- ↑ M. Takechi, S. Kuratani: History of studies on mammalian middle ear evolution: a comparative morphological and developmental biology perspective. Journal of Experimental Zoology Part B (Molecular and Developmental Evolution), Vol. 314, No. 6, 2010, pp. 417-433 DOI: 10.1002 / jez. B.21347
- ^ Luo Zhe-Xi, Zofia Kielan-Jaworowska, Richard L. Cifelli: Evolution of Dental Replacement in Mammals. Bulletin of the Carnegie Museum of Natural History. Vol. 36, 2004, pp. 159–175, doi : 10.2992 / 0145-9058 (2004) 36 [159: EODRIM] 2.0.CO; 2 (alternative full text access ResearchGate )
- ^ Albert F. Bennett, John A. Ruben: The Metabolic and Thermoregulatory Status of Therapsids. In N. Hotton III, PD Maclean, JJ Roth, EC Roth (Eds.): The Ecology and Biology of Mammal-Like Reptiles. Smithsonian Institution Press, Washington, DC, 1986, pp. 207-218, ISBN 0-87474-524-1
- ↑ TS Kemp: The origin and early radiation of the therapsid mammal-like reptiles: a palaeobiological hypothesis. Journal of Evolutionary Biology, Vol. 19, No. 4, 2006, pp. 1231-1247 DOI: 10.1111 / j.1420-9101.2005.01076.x
- ↑ Olav Oftedal: The mammary gland and its origin during synapsid evolution. Journal of Mammary Gland Biology and Neoplasia. Vol. 7, No. 3, 2002, pp. 225-252, doi : 10.1023 / A: 1022896515287 .