Mesozoic mammals have been thought to have been small, nocturnal, and confined to a few niches on land until the demise of the dinosaurs 65 million years ago. Most are recorded by isolated jaw fragments or teeth. Ji et al. (p. 1123; see the cover and the Perspective by Martin) now describe a Jurassic mammal from China that breaks this mold. The fossil is well preserved, and impressions of fur can be seen on its body and scales on a broad tail (similar to a beaver overall). The animal was fairly large, approaching not quite half a meter in length, and the shape of its limbs suggest that it was adapted for swimming and burrowing. The combination of both primitive and derived features in this early mammal, and the demonstration that mammals had occupied aquatic habitats by this time, expands the evolutionary innovations of early mammals.
WASHINGTON (AP) — For years the mammals living in the era of dinosaurs have been thought of as tiny shrew-like creatures scurrying through the underbrush. Now the discovery of a furry aquatic creature with seal-like teeth and a flat tail like a beaver has demolished that image.
Some 164 million years ago the newly discovered mammal was swimming in lakes in what is now northern China, eating fish and living with dinosaurs.
“Its lifestyle was probably very similar to the modern day platypus,” Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh, said in a statement. “It probably lived along river or lake banks. It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nest.”
Luo was part of a team led by Qiang Ji of the Chinese Academy of Geological Sciences in Beijing that discovered the remains in the Inner Mongolia region of China. They report their findings in Friday’s issue of the journal Science.
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.
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.
It’s the first evidence that some ancient mammals were semi-aquatic, indicating a greater diversification than previously thought, according to the researchers.
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 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 animal had fur, a broad scaly tail with vertebra similar to those in a beaver or otter, swimmer’s limbs and seal-like teeth for eating fish, they said.
The researchers found imprints of the fur, both guard hairs and short, dense under fur that would have kept water from the skin. Scales were also apparent on the tail as well as a suggestion of soft tissues. There was also the skeleton including teeth.
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.
Newfound remains of a beaver-like creature suggest that mammals swam with dinosaurs.
This critter was a “giant among midgets,” researchers said, dwarfing the other pint-sized mammals scurrying around during the Jurassic period.
It also holds the title of the first-known aquatic mammal, arriving on the scene nearly 100 million years before the previous record holder.
The animal, called Castorocauda lutrasimilis, was discovered in dried-up lake bed within the Inner Mongolia Region in China and has been dated to the Middle Jurassic period, about 164 million years ago.
Preservation of specialized, bi-layer furÃ¢â‚¬â€one to keep them warm and dry, the other for protectionÃ¢â‚¬â€is the first of its kind in mammals. Beavers and otters, which also have this type of fur, didn’t show up until about 55 million and 25 million years ago. Full-time aquatic mammals such as whales and manatees first appear during that period as well.
Like a beaver or platypus, the newfound creature’s hind feet were webbed for easy paddling while its front feet were better suited for digging and burrowing. It had a flattened, scaly tail like a beaver, and it sported sharp teeth, like an otter.
“We discovered the world’s first mammalian swimmer,” said study author Zhe-Xi Luo of the Carnegie Museum of Natural History. “This mode of life of fish eating, swimming, and digging combined is very different than the traditional stereotype that Jurassic mammals were very small, limited to eating insects, and lived mainly on the ground.”
The finding is detailed in the Feb. 24 issue of the journal Science.
Big mammal on the block
From snout to tail, Castorocauda, measures about 17 inches, roughly the same size as a modern-day female platypus. Indeed, at a time when most mammals weighed in just around one tenth of a pound, Castorocauda was a relative behemoth.
“This thing is close to 800 grams (1.75 pounds), so it’s a giant among the midgets,” Luo told LiveScience. “It got so much larger because it was capable of doing different things than the small-body mammals.”
As Castorocauda evolved larger and larger, Luo said, one of those different things was to make the move to the water, where its fish-snaring teeth likely helped it grow even more.
Although Castorocauda bears many similarities to beavers, otters, and platypuses, it’s not an ancestor of any of them. It is what scientists sometimes call a “dead-end species.”
Modern mammals fall into three categoriesÃ¢â‚¬â€the live-birthing placentals, the egg-laying monotremes, and the pouch-rearing marsupials. After comparing telltale jaw and skull characteristics of each of these groups to Castorocauda, researchers determined that it was something else entirely.
“This creature falls outside the common ancestor of both monotremes and beavers, although it is a near relative,” Luo said.
That means that mammalian adaptations for swimming must have evolved a second time. This is an example of convergent evolution, in which different species in similar environments adaptively evolve structures that look and function similarly, such as the wings of both birds and insects.
Castorocauda was most likely an egg layer, like platypuses, Luo said. Live birthing is a more recent adaptation, and this animal most likely went the primitive route.
Abruptly the origin of aquatic mammals is pushed back 100 million years. It’s interesting that the story-telling has already begun:
“As Castorocauda evolved larger and larger, Luo said, one of those different things was to make the move to the water, where its fish-snaring teeth likely helped it grow even more.”
It’s also interesting that soft-tissue features, including webbing between toes, carbonized underfur and fur impressions have conserved for 164 million years. See here for a hypothesis on how this preservation could occur:
Then to top it off the historical narrative isn’t adjusted but instead it’s yet another case of convergent evolution. Amazing how often this occurs.