THE first fossils discovered in China shook the family trees of birds and dinosaurs. Now the latest find is shaking our own. The fossil is of a semi-aquatic cousin of modern mammals called Castorocauda lutrasimilis, which lived 164 million years ago.
Looking like a cross between a platypus, river otter and beaver, this ancient mammal would have been almost half a metre long from nose to tail and weighed about half a kilogram 鈥� about the size of a grey squirrel. If the date of the fossil is confirmed, it makes Castorocauda the largest 鈥渕ammaliaform鈥�, or mammal-like animal, ever found from the Jurassic period, which lasted from 200 to 145 million years ago. It also sports the oldest known fur.
Palaeontologists have long thought that the only mammals living under the feet of the dinosaurs were small shrew-like animals. Now they are beginning to understand just how much they have been missing. Last year, the discovery of the metre-long Repenomamus giganticus in China showed that mammals had reached the size of a badger by about 130 million years ago (快猫短视频, 15 January 2005, p 6). Castorocauda reveals that earlier mammals were not only bigger than thought, but far more diverse.
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Hans-Dieter Sues of the Smithsonian Institution鈥檚 National Museum of Natural History in Washington DC told 快猫短视频 that such discoveries 鈥渁re completely reconfiguring our understanding of Mesozoic mammals鈥�.
The new fossil comes from the Jiulongshan formation in Inner Mongolia, which last year yielded Pedopenna, or 鈥渇eather-foot鈥�, the oldest known feathered dinosaur (快猫短视频, 19 February 2005, p 14). Sediments settling to the bottom of an ancient lake flattened the fossil and preserved it in exquisite detail, like the famed feathered dinosaurs of China鈥檚 Yixian formation. Hair impressions surround the body, which includes a 20-centimetre, flat, beaver-like tail. The animal had a full mammalian pelt, with guard hairs and under fur, and scales on the tail like a modern beaver, says Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, whose team has reported the fossil鈥檚 discovery in Science (vol 311, p 1123).
鈥淭he discovery shows that fur, modern skin structures and warm-blooded metabolism evolved very early in mammals鈥�
The fossil isn鈥檛 only notable for its size. As well as its broad flat tail, Castorocauda had webbed feet and limbs adapted for swimming, and teeth specialised for catching fish, making it the earliest mammal known to have lived partly in the water. Another 100 million years would pass before ancestral whales and manatees turned to the water. Castorocauda probably lived like a modern platypus, says Luo, digging a tunnel to nest and lay eggs, and going from the tunnel into the water to feed.
The fossil鈥檚 tail resembles that of a modern beaver so strongly that it gave Castorocauda its genus name 鈥� castor for beaver and cauda for tail. The species name lutrasimilis means otter-like. But the relationships are distant. Castorocauda lies outside the line of descent to all modern mammals, so it is a mammaliaform rather than a true mammal. Its teeth show it belonged to a group called docodonts, which lived from the middle of the Jurassic to about the middle of the Cretaceous. The group has been known for over a century, but only from teeth.
The discovery shows that fur, modern skin structures and warm-blooded metabolism originated very early in mammals. 鈥淗air keeps us warm, and sweat glands help us to dissipate heat, so skin is part of the adaptation to constant body temperature,鈥� says Luo. Mammary glands evolved from sweat glands, but this fossil casts no light on their origins; it is preserved belly-down and appears to be male because it has an ankle spur like that of the male platypus.
Oddly, Castorocauda has overlapping rib bones, a trait found among reptiles called cynodonts that gave rise to mammals, but not in later mammals. The overlap is not found in the cynodonts considered closest to mammals, so Luo thinks it re-evolved rather than being a holdover from its distant ancestors.
