This 2018 video is about Castorocauda lutrasimilis.
From the Los Angeles Times:
China: swimming mammal from age of dinosaurs found
Date: 2/22/06 at 10:49PM
Fossil Overturns Ideas of Jurassic Mammals
By RANDOLPH E. SCHMID, AP Science Writer
The find shows that the ecological role of mammals in the time of dinosaurs was far greater than previously thought, said Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh.
The animal is the earliest swimming mammal to have been found and was the most primitive mammal to be preserved with fur, which is important to helping keep a constant body temperature, Luo said in a telephone interview.
For over a century, the stereotype of mammals living in that era has been of tiny, shrew-like creatures scurrying about in the underbrush trying to avoid the giant creatures that dominated the planet, Luo commented.
Now, a research team that included Luo has found that 164 million years ago, the newly discovered mammal with a flat, scaly tail like a beaver, vertebra like an otter and teeth like a seal was swimming in lakes and eating fish.
The team, led by Qiang Ji of the Chinese Academy of Geological Sciences in Beijing, discovered the remains in the Inner Mongolia region of China.
They report their findings in Friday’s issue of the journal Science.
Matthew Carrano, curator of dinosaurs at the Smithsonian’s National Museum of Natural History, called the find “a big deal.”
An important factor is how specialized the creature was, said Carrano, who was not part of the research group.
“It gives a hint that early mammals were not just these shadowy creatures at the time of dinosaurs” but were having their own evolution.
There have been hints of such animals in the past but nothing equal to the remains found by Luo and colleagues, he said.
Thomas Martin of the Research Institute Senckenberg in Frankfurt, Germany, said the discovery pushes back the mammal conquest of the waters by more than 100 million years.
“This exciting fossil is a further jigsaw puzzle piece in a series of recent discoveries,” commented Martin, who was not part of Luo’s team.
It’s the first evidence that some ancient mammals were semi-aquatic, indicating a greater diversification than previously thought, the researchers said.
Modern semi-aquatic mammals such as beavers and otters and aquatic mammals like whales did not appear until between 55 million years ago and 25 million years ago, according to the researchers.
The new animal is not related to modern beavers or otters but has features similar to them.
Thus the researchers named it Castorocauda lutrasimilis.
Castoro from the Latin for beaver, cauda for tail, lutra for river otter and similis meaning similar.
The researchers found imprints of the fur, both guard hairs and short, dense under fur that would have kept water from the skin.
Weighing in at between 1.1 and 1.7 pounds, about the size of a small female platypus, Castorocauda is also the largest known Jurassic early mammal.
The research was funded by the U.S. National Science Foundation, National Natural Science Foundation of China, Ministry of Science and Technology of China, Chinese Ministry of Land Resources, National Geographic Society and Carnegie Museum.
On the Net: here.
Repenomamus, mammal from dinosaur age: here.
Early mammals: here.