A team of international researchers have discovered a new species of primitive mammal capable of swimming in the Middle Jurassic lake beds of China.
In a cover article published in Science, the team of researchers from Carnegie Museum of Natural History, Nanjing University, and Chinese Academy of Geological Sciences describe a fossilized skeleton of Castorocauda lutrasimilis ([Castoro] - Latin for beaver, [cauda] - Latin for tail, [lutra] -Latin river otter] and [similis] - Latin for similarity). Castorocauda had a beaver-like tail, strong arms for digging, and sharp teeth specialized for aquatic feeding, similar to the modern river otter.
Castorocauda is a new taxon of docodonts, an extinct mammal group that existed from the Middle Jurassic to Late Cretaceous. But this Mesozoic group has no modern descendants, and is not directly related to modern placental mammals.
Uncovered from the Middle Jurassic Jiulongshan Formation of the Inner Mongolia Region, dated approximately 164 million years ago, Castorocauda is the earliest-known mammal that had specialized skeletal and soft-tissue features for swimming and teeth for eating fish. This significant fossil offers the first evidence that some Mesozoic mammals occupied the semi-aquatic niche and that Mesozoic mammals as a whole had a much great ecological diversification than previously thought.
Castorocauda is preserved with a pelt (guard hairs and under furs), making it the most primitive-known mammal to be preserved with hairs. Carbonized in the fossil, the short and dense under-furs were to keep water from the skin; the longer guard hairs are preserved as impressions on the fossil slab. Fossilized furs of this animal provide fresh evidence on phylogenetic evolution of mammalian fur βοΏ½" this kind of specialized pelt developed well before the rise of modern mammals. All previously discovered fossils with fur belong to the more derived taxa within the Mammalia or mammalian crown group.
"Its lifestyle was probably very similar to the modern day platypus," said Dr. Zhe-Xi Luo, curator of Vertebrate Paleontology at Carnegie Museum of Natural History. "It probably lived along river or lake banks. It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nest."
Dr. Luo pointed out that, perfectly shaped for aquatic life, Castorocauda had a broad and scaly tail that propelled it through water just like the modern beaver. Its tail vertebrae are also similar to those of beavers and otters. Because Castorocauda is not related to modern placentals, its adaptation for swimming is a convergent evolution to the modern beaver and modern river otter, both of which are placentals.
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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