Webbed-footed Cretaceous mammal from Liaoning replaced every tooth type more than once

Bi sets the teeth against a long-standing textbook view. For more than a century, he said, biology textbooks have described mammals as replacing some or all of their teeth just once in a lifetime, a condition called diphyodonty ^[6]. "Dongoconodon challenges this long-standing view," Bi said ^[15]. The fossil preserves evidence that its incisors, canines, premolar-like teeth and molar-like teeth were all replaced multiple times, the condition called polyphyodonty ^[7]. "It is the only known mammal to exhibit polyphyodonty across all four tooth classes, a pattern previously associated with reptiles rather than mammals," he said ^[8].

The body that carried those teeth was built for water ^[3]. Bi said its broad hands and feet are "remarkably like" those of the modern platypus, and flanges on several finger and toe bones indicate the digits supported extensive webbing ^[4]. "The broad, webbed forefeet generated much of the propulsive force, while its flattened, tapered tail functioned primarily as a rudder for steering and stability," Bi said ^[5].

Bi's wider argument covers the ear as well as the teeth. In his account, changes to the teeth and to the ear bones did not evolve together as a tightly linked suite. They evolved independently, producing a diversity of evolutionary experiments during the age of dinosaurs ^[11]. "Our discovery shows that early mammalian evolution was considerably more flexible than the traditional textbook picture suggests," he said ^[12].

The anatomy gives that argument something concrete to rest on. The ear is at an intermediate stage ^[10], while the tooth replacement follows the pattern Bi associates with reptiles ^[8]. That mismatch inside one skull is the most direct support in the report for Bi's case.

Benton accepts the anatomy but puts the animal in a different place in time. "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]," he said ^[14]. On those dates, D. platycauda lived about 81 million years after the end of the Triassic ^[1]. Benton chose the word "retained". On his reading, this is a late lineage keeping features that many other Mesozoic mammals had apparently left behind long before.

The difference matters for Bi's larger claim. An animal that late can show that tooth replacement and ear separation were able to sit at different stages in one mammal. It is weaker evidence about the order in which those traits first came together in earlier mammals. I think the fossil supports the first point well and the second only modestly.

The thing this doesn't tell you is how common the combination was. The work rests on a single near-complete fossil ^[1], and answering that question needs more specimens from the same group.