A newly discovered, semi-aquatic mammal – which would have lived alongside the dinosaurs – had the streamlined appearance of a water rodent, feet almost identical to a platypus and teeth that grew like a reptile’s.
The researchers who studied the near-complete, 120-million-year-old fossil, named Dongoconodon platycauda, say its disparate anatomy offers a window into early mammal evolution.
D. platycauda was found in 2018 in Liaoning, China. at Yunnan University, China, and his colleagues studied the fossil after the collector who found it took it to the Mongolia Museum of Natural History. When it was alive, the mammal would have weighed up to 190 grams and measured about 40 centimetres long.
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In its overall appearance, it may have looked somewhat like a small nutria, also known as the coypu (Myocastor coypus), say the researchers. Like this modern aquatic mammal, D. platycauda had a streamlined body and a flattened tail adapted for swimming.
D. platycauda also had broad hands and feet “remarkably like” those of the modern platypus (Ornithorhynchus anatinus), says Bi. Also, several of its finger and toe bones preserve specialised flanges, indicating that the digits supported extensive webbing.
It would have been a strong swimmer, says Bi.
“The broad, webbed forefeet generated much of the propulsive force, while its flattened, tapered tail functioned primarily as a rudder for steering and stability,” he says.
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But what makes the ancient mammal most interesting is its teeth. For more than a century, he says, biology textbooks have identified that mammals replace some or all of their teeth just once during a lifetime – a condition called diphyodonty.
“Dongoconodon challenges this long-standing view,” says Bi.
The fossil preserves evidence that incisors, canines, premolar-like teeth and even molar-like teeth were replaced multiple times during the animal’s life – a condition called polyphyodonty.
“It is the only known mammal to exhibit polyphyodonty across all four tooth classes, a pattern previously associated with reptiles rather than mammals,” he says.
The specimen also has what the researchers say is one of the most complete preserved middle-ear regions known from an Early Cretaceous mammal. This includes its Meckel’s cartilage, a transient embryonic structure that temporarily connects the developing middle ear to the lower jaw in all mammals.
D. platycauda preserves an intermediate stage in which Meckel’s cartilage maintained only slight contact with the jaw while remaining connected to the tiny ear bones.
“The discovery changes our picture of early mammalian evolution,” says Bi.
Rather than changes to the teeth and ear bones evolving together as a tightly linked suite of characteristics, he says, these important mammalian features evolved independently, producing a remarkable diversity of evolutionary experiments during the age of dinosaurs.
“Our discovery shows that early mammalian evolution was considerably more flexible than the traditional textbook picture suggests,” he says.
at the University of Bristol in the UK says the team makes a good case for at least three sets of teeth and an intermediate ear anatomy.
“This is an interesting, quite late-occurring mammal that for some reason has retained hints of the reptilian ancestry more so than many other Mesozoic mammals that had apparently made those two transitions by the end of the Triassic [about 201 million years ago],” says Benton.
Nature Communications
