Tritylodontidae

from Wikipedia, the free encyclopedia
Tritylodontidae
Skull of Bienotherium yunnanense

Skull of Bienotherium yunnanense

Temporal occurrence
Upper Triassic to Lower Cretaceous
203.6 to 99.6 million years
Locations
  • Worldwide
Systematics
Synapsids (Synapsida)
Therapsids (Therapsida)
Theriodontia
Cynodontia
Eucynodontia
Tritylodontidae
Scientific name
Tritylodontidae
Cope , 1884

The Tritylodontidae were a group of herbivorous cynodonts . They probably lived from the Upper Triassic to the Lower Cretaceous , or at least as far as the Middle Jurassic . This makes them the longest surviving Therapsids that are not mammals (if one disregards a relic occurrence of the Dicynodontia in Australia, as far as the Lower Cretaceous).

The first tritylodontid teeth were made in layers of the Upper Keuper of Württemberg in 1847 and 1866 and interpreted as the oldest fossils of mammals. The finds were described under the names Microlestes and Triglyphus . They are probably identical to tritylodon , the teeth and fragments of the skull were found in the South African Karoo and described by Richard Owen in 1884 .

Fossils of various other genera have been found in southern Africa, Europe and North America. Fossils of uncertain allocation come from South America, China and Mount Kirkpatrick in Antarctica. Isolated tritylodontid teeth from Russia come from the Lower Cretaceous .

features

The skull of the Tritylodontidae is very similar to a mammal. The skull windows are large and, due to the lack of bone brace (postorbital brace), merge directly into the bony eye socket (orbit).

The teeth are superficially similar to those of rodents and the multituberculata and are specialized for a vegetable diet. The similarity, however , arose convergent through a likely similar diet and is not based on closer relationship. Canines are missing and have been functionally replaced with a pair of enlarged incisors . Between the incisors and the complex, molar-like molars, there is a clear diastema ( Greek for “space”) in the upper and lower jaw . The molars have two roots . In the upper jaw they have three, in the lower jaw two longitudinal rows of cusps, which reach into the rows of the upper jaw when the mouth is closed. Tritylodontidae had two dentitions (diphyodontia).

Primitive features are the mandibular joint, which is not formed by Dentale and Squamosum , as in mammals , but by quadratum and Articulare as in reptiles . The ossicles are still simply built. On the shoulder girdle there is also an intermediate collarbone (interclavicula) and a raven bone (coracoid).

The skulls are five ( Bocatherium ) to 25 centimeters ( Kayentatherium and Tritylodon ) long. The rest of the skeleton, known mainly from oligocyphus , is also very similar to a mammal.

Paleobiology

A find published in 2018 from the Kayenta Formation in the US state of Arizona most likely belongs to a female from Kayentatherium . It dates back to the Lower Jurassic at an absolute age of around 184 million years. The find contains the remains of a litter consisting of a total of 38 young animals, which is a high number compared to the mammals . The skulls of the young correspond in their structure to smaller versions of those of the later adult animals. A shift towards smaller facial skulls and larger brain skulls, as is common in mammals, is not discernible. According to this, the representatives of the Tritylodontidae still had a reproductive strategy that can be viewed as originally within the amniotes . It involves large litters of young that are proportionally smaller versions of the adult individuals. Mammals, on the other hand, usually give birth to relatively small litters, and the offspring are also raised in a time-intensive manner and with higher energy costs. The relatively small skulls of the young of Kayentatherium also indicate that the Tritylodontidae were not involved in the rapid growth of the brain in the line of the parent mammals. Both the reduction in litter size and the increase in brain volume must have occurred shortly after the Tritylodontidae appeared.

Systematics

In the 19th century the Tritylodontidae were counted among the mammals and even today the view is held that of all therapsids they are the closest to mammals. Because of their specialization of the teeth (loss of the canine, cusp structure), however, they are not considered direct ancestors of mammals. Some scientists argue that they are their sister group .

literature

  • Thomas S. Kemp: The Origin & Evolution of Mammals. Oxford University Press, Oxford 2005. ISBN 0198507615 .
  • Robert L. Carroll: Paleontology and Evolution of the Vertebrates. Thieme, Stuttgart (1993), ISBN 3-13774-401-6
  • Oskar Kuhn: The mammal-like reptiles . A. Ziemsen Verlag, 2003, ISBN 3-89432-797-9

Individual evidence

  1. ^ Eva A. Hoffman and Timothy B. Rowe: Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth. Nature, 2018 doi: 10.1038 / s41586-018-0441-3

Web links

Commons : Tritylodontidae  - Collection of images, videos, and audio files