Summary
A nearly complete fossil from China reveals that the Early Cretaceous mammal Dongoconodon replaced teeth in multiple generations at some positions. Its skeleton also points to semiaquatic life and preserves a short connection between the middle-ear region and the jaw.
A nearly complete fossil from northeastern China documents an unusual combination in an Early Cretaceous mammal: multiple generations of teeth at some positions, a partly attached middle-ear structure and skeletal features associated with swimming. The newly named species, Dongoconodon platycauda, lived about 120 million years ago, according to a study in Nature Communications.
The fossil complicates a simple picture in which mammalian traits appeared together in a straight line. Its teeth show that at least some early mammals could retain repeated replacement at particular tooth positions, even as other parts of their anatomy were taking different evolutionary paths.
Tooth replacement was not uniform
Living mammals typically have two generations of teeth at some positions: an initial set and a replacement set. Reptiles commonly replace teeth repeatedly. The fossil record makes it difficult to determine how many generations an extinct mammal had, because fossils rarely preserve a sequence of growth stages.
The Dongoconodon specimen preserves teeth at different stages of eruption and formation. Computed tomography helped the researchers examine teeth still inside the jaw. In the lower jaw, an incisor has a replacement tooth, while other unerupted teeth appear to belong to a later generation. The authors interpret this arrangement as evidence of polyphyodonty—more than two generations—at some tooth positions.
The pattern is not the same across the whole tooth row. The researchers report three generations at some incisor and premolar positions, and evidence of a third generation beneath a replacement for one molariform. They use “molariform” for complex posterior teeth, because a tooth’s position and shape alone do not establish whether it was an unreplaced molar or a replaced premolar. On that basis, the animal had three premolariforms and four molariforms, with replacements for the two front molariforms.
The finding adds Dongoconodon to evidence that repeated replacement occurred in some Mesozoic mammal lineages. The authors suggest that this pattern in Dongoconodon and related gobiconodontids may be associated with their likely carnivorous specialisation. That dietary interpretation is based on the animal’s teeth and jaw shape, rather than direct evidence of its meals.
A mammal adapted to water
The skeleton also points to a semiaquatic way of life. Its broad hands and feet have proportions that allowed the digits to spread, increasing the surface area available during a swimming stroke. Some finger bones have flanges resembling those where webbing attaches in the platypus.
The tail’s vertebrae provide another clue. Its front section is flattened, and the researchers compare the overall pattern with that of the semiaquatic nutria, or coypu. They propose that the tail helped with steering and stabilisation, while the robust forefeet supplied much of the swimming propulsion.
To assess locomotion, the researchers compared 13 skeletal measurements from Dongoconodon with measurements from 124 living mammal species assigned to locomotor groups. The fossil fell within the semiaquatic group’s morphological range. The analysis classified 77.8% of the living semiaquatic species correctly overall; its 100% posterior probability for Dongoconodon is a model classification, not a direct observation of the fossil swimming.
A short-lived connection between jaw and ear
The fossil also preserves an ossified Meckel’s cartilage, a structure associated with the developmental connection between the lower jaw and the middle ear. In Dongoconodon, it has a small contact area with the mandible and connects at its other end to the middle-ear region. The authors interpret this short attachment as evidence for a stage in the evolutionary separation of the auditory bones from the jaw.
Together with other fossil forms showing longer attachments or none, this anatomy supports a sequence in which the connection between Meckel’s cartilage and the mandible was reduced before the middle-ear bones became fully detached. The paper notes that the details of this transition remain debated, and that dental replacement patterns are also poorly known for many Mesozoic mammals.
The specimen therefore presents a mosaic of traits rather than a single evolutionary package: repeated tooth replacement at some positions, a partly reduced jaw-ear connection and adaptations consistent with semiaquatic movement. It adds anatomical evidence for the diversity of early mammals and for the possibility that major features evolved along partly independent paths.