In the Jiufotang Formation in northeastern China, about 120 million years ago, there lived a small creature that, by all appearances, could swim no worse than a modern platypus. Chinese paleontologists have described a new species, Dongoconodon platycauda — an early true mammal whose remains preserve a rare combination of features.
The skeleton, found in 2016, was studied using computed tomography. It turned out that the animal had broad paws with flattened finger bones, similar to the webbed limbs of a platypus, and an angular tail resembling that of a nutria. Apparently, the little beast rowed with its front paws and steered its direction of movement with its tail. A comparison of bone proportions with data on 124 modern species confirmed a semiaquatic lifestyle.
The main surprise was the dental system. Most mammals replace their teeth once — milk teeth with permanent ones. Here, however, beneath the already erupted and forming replacement teeth, an enamel cap of a third generation was discovered. Traces of three sets can be traced in all types of teeth: incisors, canines, premolars and molar-like teeth. Such a phenomenon is unknown in any living or extinct mammal.
In addition, the fossil preserved a transitional structure of the middle ear: an ossified Meckel's cartilage still connected the lower jaw to the bones of the auditory apparatus. In modern mammals, these elements are completely separated. The find illustrates how this separation occurred in evolution.
The discovery shows that the early evolution of mammals was more flexible than is commonly believed: key traits — tooth replacement, ear structure, adaptations to the environment — did not necessarily develop synchronously. A single fossil combined several "innovations" at once.
Such finds remind us how diverse the paths of adaptation to water could be long before the appearance of modern groups.
