Hadrocodium

from Wikipedia, the free encyclopedia
Hadrocodium
Temporal occurrence
Lower Jurassic ( Sinemurian )
195 million years
Locations
Systematics
Synapsids (Synapsida)
Therapsids (Therapsida)
Cynodontia
Eucynodontia
Mammaliaformes
Hadrocodium
Scientific name
Hadrocodium
Luo , Crompton & Sun , 2001

Hadrocodium wui is considered to be the oldest known representative of mammals (Mammalia). The animal's skull was just twelve millimeters long. It lived in the Jurassic Age, previously known only as the first heyday of the great dinosaurs . The fossil itself was found back in 1985 in the Lower Lufeng Formation in the People's Republic of China . In years of precision work, the animal's skull was exposed before it could be examined. For this reason, the special position of animals in the mammalian system was only presented to the public in 2001.

While the lower jaw of the mammals' reptile-like ancestors is connected to the skull via a primary jaw joint consisting of the quadratum and articular bones , a new joint occurs in their place in mammals. This is formed from the lower jaw bone (dental) and the scaly bone (squamosum). The former temporomandibular joint bones are given a completely new function: They become the ossicles of the hammer (malleus) and anvil (incus), which are still articulated. This condition, i.e. a secondary temporomandibular joint and three auditory ossicles, was also found in Hadrocodium wui , some 40 million years earlier than previously assumed.

A gradual separation of the middle ear bones from the jaw is believed. This is supported by other fossils such as the skulls of the Sinoconodon and Morganuconodon, also found in China . Although these already had a secondary temporomandibular joint, the hammer and anvil are still connected to it. The final step in the separation between the jaw and the middle ear is related to the enlargement of the brain , according to Zhe-Xi Luo of the Carnegie Museum of Natural History . The expansion of the skull likely pushed the middle ear away from the jaw.

literature

  • Zhe-Xi Luo, Alfred W. Crompton, Ai-Lin Sun: A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics. In: Science . 292, 2001, pp. 1535-1540.

Web links