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.
Shundong Bi 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. Spectacular feathered dinosaur fossil shows bird-like adaptations 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. 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.
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. 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. 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. A stunningly intact marine reptile named Austronaga minuta that lived around 245 million years ago may have looked like a miniature version of more famous later ocean predators, like plesiosaurs “Our discovery shows that early mammalian evolution was considerably more flexible than the traditional textbook picture suggests,” he says.
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