Castorocauda

Castorocauda was a genus of small, semi-aquatic proto-mammals which lived in the Jurassic period, about 154 million years ago.

Castorocauda Temporal range: Upper Jurassic
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Synapsida
(unranked):	Mammaliaformes
Order:	Docodonta
Family:	Docodontidae
Genus:	Castorocauda
Species
Castorocauda lutrasimilis

Its fossil was found in lakebed sediments of the Daohugou Beds of Inner Mongolia. Castorocauda was highly specialized, with adaptations like those of modern semi-aquatic mammals such as beavers, otters, and the platypus (convergent evolution). It is the earliest animal found with fur.[1][2]

Castorocauda is one of the near-mammals in a group called the Docodonta.[2]

Although Castorocauda was not a crown group mammal, it is important in the study of how mammals evolved. It was the first find with an almost complete skeleton. Also it is not a "small nocturnal insectivore".[2]

• It was larger than most Mesozoic mammaliaform fossils – about 17 in (43 cm) from its nose to the tip of its 5-inch (130 mm) tail, and may have weighed 500–800 g (18–28 oz).
• It is the earliest absolutely certain evidence of hair and fur. Previously the earliest was Eomaia, a crown group mammal from about 125M years ago.
• It had aquatic adaptations including flattened tail bones and remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. Previously the earliest known semi-aquatic mammaliaforms were from the Eocene, about 110 million years later.
• Castorocauda's powerful forelimbs look adapted for digging. This feature and the spurs on its ankles make it resemble the platypus, which also swims and digs.
• Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding. These molars are curved backwards, which helps in grasping slippery prey.