Castorocauda
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Castorocauda , image of life |
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| Middle Jurassic | ||||||||||||
| 164 million years | ||||||||||||
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| Castorocauda | ||||||||||||
| Ji , Luo , Yuan , Tabrum , 2006 | ||||||||||||
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Castorocauda ("beaver tail") is an extinct genus of mammal-like animals ( Mammaliaformes ) from the docodonta group . It lived in the middle Jurassic about 164 million years ago and its physique showed clear adaptations to a semi- aquatic (partly in water) way of life. The only known species so far is Castorocauda lutrasimilis .
description
Castorocauda reached a length of over 42 centimeters (the skull was over 6 centimeters long), its weight is estimated at 500 to 800 grams. This made it larger than all other mammals and mammal-like animals of the Jura.
The animal had a broad, largely hairless and scaled tail, which probably resembled that of the beaver (Castoridae). The toes of the hind legs were webbed, the front legs were short and strong. Presumably, leaves his life with that of the platypus compare that floating in the water looking for food. The front legs could have been used for paddling or digging earthworks.
The remains of a fur can be clearly seen on the fossil , which was composed of outer hair and undercoat and which already resembled that of today's mammals. It is therefore the oldest known mammal-like animal in which a fur is clearly documented and a sign that the body hair - perhaps combined with thermoregulatory abilities - developed even before the actual mammals emerged .
The well-preserved lower jaw has four incisors , one canine , five premolars and six molars per half of the jaw . The structure of the molars in the broad crowns largely corresponds to that of the rest of the docodonta . The two foremost molars, which had five cusps arranged one behind the other, are a specialty. From the structure of the teeth it is concluded that Castorocauda ate fish and aquatic invertebrates.
Discovery and naming
Castorocauda is known by a nearly complete skeleton, that in the Jiulongshan -Formation in Chinese territory autonomous Inner Mongolia found and in 2006 by Qiang Ji Zhe-Xi Luo, Chong-Xi Yuan, and Alan R. Tabrum firstdescribed was. The generic name Castorocauda comes from the Latin "castor" for beaver and "cauda" for tail, the specific epithet lutrasimilis is derived from the words "lutra" ( otter ) and "similis" (similar).
Systematics
In its physique, Castorocauda shows similarities with today's aquatic mammals such as beavers , otters and platypus . These similarities are not based on kinship, but on convergent evolution , since the lineages of modern mammals did not develop until a much later point in time. Castorocauda is classified in the group of Docodonta , a taxon that was prevalent in Eurasia and North America in the Jurassic and the Early Cretaceous Period . They belong to a group of animals that are largely similar to today's mammals, but differ in details such as the structure of the auditory ossicles and the temporomandibular joint and therefore not yet classified as mammals in the narrower sense ( sensu stricto ), but as "mammal-like" animals ( Mammaliaformes ) or mammals in the broader sense ( sensu lato ). Even in the case of an earlier discovered representative of the Docodonta, Haldanodon , the stocky limbs had been interpreted as a possible adaptation to a digging or water-dwelling way of life.
Importance of the find
Castorocauda was the largest known mammal (or mammal-like animal) of the Jurassic and, through its adaptations to aquatic life, anticipated a development previously known only from Cenozoic mammals (over 100 million years later). Until recently, all mammals of the Mesozoic Era were thought to be small, relatively unspecialized animals that were mostly nocturnal insectivores. Castorocauda fits into a number of recently discovered Mesozoic mammals, which prove that there were already significant specializations in that era, such as the predatory genus Repenomamus , the anteater-like genus Fruitafossor or the volaticotherium with sliding membranes .
literature
- Qiang Ji, Zhe-Xi Luo, Chong-Xi Yuan, and Alan R. Tabrum: A Swimming Mammaliaform from the Middle Jurassic and Ecomorphological Diversification of Early Mammals. In: Science , 311, 2006: 5764 pp. 1123-1127. doi : 10.1126 / science.1123026