Modern semi-aquatic placental mammals (such as beavers and otters), and fully-aquatic placental mammals (such as whales and manatees) did not appear until Eocene to Oligocene (55 - 25 million years ago). By comparison, Castorocauda is at least 164 million year old. So it indicates that primitive docodont mammaliaforms evolved the semi-aquatic swimming independently in the Mesozoic, almost 100 millions years earlier than the Cenozoic placental mammals (beavers, otters, whales and manatees).
Other interesting features of Castorocauda include its teeth and size. Castorocauda developed molars specialized for feeding on small fish and aquatic invertebrates, similar to modern seals or the river otters.
It is also the largest known Jurassic mammaliaform (including mammals).
Most Mesozoic mammals are small (less than 50 grams) and generalized ground-living (terrestrial) mammals. Limited by their small size, and living in the shadow of much larger dinosaurs, most Mesozoic mammals are insectivorous. However, Castorocauda is a significant exception and very different from the typically small and terrestrial Mesozoic mammals. Castorocauda is at least 42.5 cm in body length and more than 6 cm in skull length. Scientists estimate that it weighed about 500 to 800 grams.
"So far, it is the only semi-aquatic mammal from the Jurassic," said Dr. Luo, "and it is also the largest-known Jurassic mammal. Partly because of its larger size, it was possible for Castorocauda to develop fish-eating and swimming adaptations."
The research team was led by Dr. Qiang Ji of Chinese Academy of Geological Sciences (Beijing, China) and Dr. Zhe-Xi Luo of Carnegie Museum of Natural History (Pittsburgh, USA). The art for the Science Magazine cover was created by Mark A. Klingler, scientific illustrator at Carnegie Museum of Natural History.
Their research was supported by funding from the National Science Foundation (USA), National Natural Science Foundation (China), Ministry of Science and Technology (China) (973 project), Ministry of Land Resources (China), National Geographic Society (USA) and Carnegie Museum of Natural History (USA).
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Adapted from materials provided by Carnegie Museum Of Natural History.
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MLA Carnegie Museum Of Natural History (2006, February 24). Scientists Discover First Swimming Mammal From The Jurassic. ScienceDaily. Retrieved January 13, 2009, from
http://www.sciencedaily.com /releases/2006/02/060224195600.htm
