Dongoconodon platycauda, a small eutriconodontan mammal that lived alongside dinosaurs in what is now northeastern China paddled through the water with webbed, platypus-like hands and feet, and replaced its teeth multiple times.
An artist’s reconstruction of Dongoconodon platycauda. Image credit: Shi Yukun.
Dongoconodon platycauda roamed our planet during the Early Cretaceous epoch, some 120 million years ago.
The animal was roughly the size of a water vole and weighed an estimated 115 to 190 grams.
It was a member of the Eutriconodonta, an order of early mammals that lived during the Jurassic and Cretaceous periods.
“Mammals originated through the progressive acquisition of key morphological and developmental innovations that underlie their evolutionary success across diverse ecological niches,” said Yunnan University paleontologist Shundong Bi and colleagues.
“Among these, diphyodonty (presence of two generations of teeth) and a fully detached middle ear are widely regarded as two key diagnostic features of living mammals and represent fundamental transitions in mammalian evolution.”
Dongoconodon platycauda is represented by a nearly complete articulated skeleton with a well-preserved skull and jaws.
The fossil was collected in 2016 from the Changzigou locality of the Jiufotang Formation in China’s Liaoning province.
CT scans showed teeth in several stages of eruption and development, including replacement teeth beneath other replacements.
According to the authors, the animal appears to have had three generations of teeth at some positions, across all four tooth types: incisors, canines, premolar-like teeth and molar-like teeth.
Its close eutriconodontan relative, Gobiconodon borissiaki, has been reported to replace some of its back teeth more than once.
The researchers propose that this return to repeated tooth replacement is linked to a meat-eating lifestyle.
Holotype specimen of Dongoconodon platycauda. Image credit: Bi et al., doi: 10.1038/s41467-026-77339-w.
“Dongoconodon has 11 tooth positions in the upper jaw and 10 in the mandible,” the researchers said.
“Teeth are in different stages of eruption, and the replacement teeth are in different states of formation.”
“The most telling quadrant of the dentition for interpreting the replacement pattern in Donogoconodon is the right lower mandible.”
“Anterior to the erupted canine are three unerupted teeth within the bone, a first incisor with its replacement followed by a second incisor.”
“In the postcanine dentition, the second and third molariforms are fully erupted.”
“To interpret this arrangement as a diphyodont dentition requires that these molariforms fully erupted before what would be two deciduous incisors, given that the first incisor has what would be a permanent replacement.”
“Such a pattern does not occur in any living or extinct mammals.”
Dongoconodon platycauda was also apparently a good swimmer.
Its hands and feet resemble the platypus’, with flattened, elongated bones.
Small flanges on the finger bones match the places where webbing attaches in the platypus.
Its tail vertebrae are flattened and box-like, similar to those of the nutria, a semiaquatic South American rodent with a lightly haired tail.
The scientists conclude that the tail was used for steering and stability, while the forelimbs provided most of the propulsion.
To test the swimming interpretation, they compared 13 limb-bone measurements of the fossil against 124 living species with known lifestyles. The fossil fell within the range of the semiaquatic species.
“The new fossil shows adaptations for semiaquatic life, with hands and feet like the platypus that were similarly webbed,” the authors concluded.
“Its unusual pattern of dorsoventrally flattened tail bones resembles the living nutria, a rodent native to South America.”
“Reappearance of polyphyodonty and molariform replacement in the new fossil and related gobiconodontids is probably associated with their ecomorphological niche and carnivorous diet.”
The discovery of Dongoconodon platycauda is described in a paper published this week in the journal Nature Communications.
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S. Bi et al. 2026. A polyphyodont, semiaquatic eutriconodontan mammal from the Early Cretaceous of China. Nat Commun 17, 10032; doi: 10.1038/s41467-026-77339-w
