About 165 million years ago, a furry critter that swam like a platypus, hunted like an otter and had a tail similar to a beaver’s frolicked in the lakes and rivers of what is now China pursuing fish and salamanders, the oldest-known example of a semiaquatic lifestyle in mammalian evolutionary history.
Researchers are describing a fossil unearthed in the Inner Mongolia region of a mammalian species named Megacauda sungei that lived during the Jurassic Period in the age of dinosaurs.
Megacauda was about 20 inches (50 cm) long and weighed 2.2 to 4.4 pounds (1 to 2 kg). While dwarfed by most dinosaurs, Megacauda was still twice as big as the next-largest member of the mammalian lineage in the Jurassic — most were tiny shrew-sized creatures at that time — and displayed some critical anatomic features shared with modern mammals.
The well-preserved fossil even retains skin and fur impressions and reveals a clear body contour. It shows Megacauda bore the same throat musculoskeletal structure that enables modern mammals, including people, to swallow chewed food one bit at a time, rather than in massive gobs, and that provides babies the ability to suckle mother’s milk.
“Modern mammals are unique among living vertebrates in their feeding function. Most mammals can and must chew, with complex teeth, the ingested food down to tiny bits and pieces, then swallow the chewed-up food, with saliva, one small lump at a time. The food lump is squeezed by a muscle tube of our throat passage, known as the pharynx, into the esophagus,” said University of Chicago paleontologist Zhe-Xi Luo, senior author of the research published this week in the journal Nature.
“This mammal-style of swallowing differs from the swallows of other vertebrates, most of which just gulp large chunks of food or swallow the prey whole. Crocodiles gulp down large birds. Pythons stretch the jaws apart to swallow a sizable mammal. Even a thin snake can easily swallow a whole rodent. They have teeth, but because their teeth and jaws cannot chew, they just make do by swallowing chunks,” Luo said.
Megacauda, like modern mammals, has the delicate bone structure underneath the skull and jointed hyoid throat bones that would work alongside muscles of the throat and tongue. Also like modern mammals, it could breathe and feed at the same time.
The first primitive, or basal, mammals appeared about 225 million years ago during the Triassic Period, which preceded the Jurassic. Mammals remained subordinate until an asteroid strike 66 million years ago toppled the dinosaurs, but managed to move into a variety of ecological niches.
ECOLOGICAL DIVERSITY
Megacauda was part of a mammalian group called docodonts that first appeared about 200 million years ago and flourished for about 90 million years but left no living descendants. Various docodonts burrowed, lived on the ground or in trees, or, as Megacauda illustrates, became semiaquatic.
“Megacauda adds to a growing body of evidence that mammals could be ecologically diverse, even in the dinosaur-dominated ecosystem,” Luo said.
Its fossil was unearthed in rock formed from lake sediment.
“It swam by using its front and hind legs for paddling and performing undulating movements with its tail,” University of Bonn paleontologist and study co-author Thomas Martin said. “It probably dug burrows in the river banks or lake shores.”
It had body proportions similar to a platypus, especially its torso and back legs, though without the famous platypus duckbill snout. Its broad tail was flattened, like that of a beaver or platypus, and was covered by small scales, as a beaver’s is. In fact, its scientific name means “big tail.”
A smaller semiaquatic Jurassic docodont named Castorocauda, also from China, lived a bit later than Megacauda.
Megacauda’s teeth and jaws were suited for a carnivorous diet, as those of today’s semiaquatic mammal predators like otters and minks are. Its specialized canine teeth had three roots, unusual for today’s mammals, and its shearing cheek teeth could cut and chew meat differently from modern carnivorous mammals. Its tooth cusps were curved backward to help catch slippery prey like fish.
Bits of bones were found in Megacauda’s stomach, showing it ate small vertebrates. Many salamander fossils were found in the same rocks.
Megacauda had to watch out for theropods, the meat-eating dinosaurs, and may have been nocturnal, or perhaps active in dawn or dusk hours, Luo said.
“The predatory threat against Megacauda would likely be from the feathered theropod dinosaurs,” Luo said.