The ability to continuously replace teeth is a feature commonly found in crocodiles and sharks, but not in most mammals. However, a newly discovered Chinese fossil from the time of the dinosaurs challenges this traditional view. This ancient mammal, which had teeth similar to reptiles, could replace its teeth throughout its life—unlike most modern mammals, which stop at two sets. In October 2026, a team of palaeontologists led by Shundong Bi at Yunnan University described the discovery in Nature Communications. The fossil, named Dongoconodon platycauda, is about 120 million years old and was found in China. It was a semi-aquatic animal with a unique ability to regrow its teeth continuously, like lizards. This challenges the long-held belief that having only two sets of teeth was a defining trait of mammals. The nearly complete skeleton of Dongoconodon platycauda shows that it was well adapted to an aquatic lifestyle. Weighing about the same as a mouse and measuring around 40 centimeters in length, it likely swam in the lakes and rivers of ancient China during the Cretaceous period. Its webbed hands and feet, similar to those of a platypus, suggest it was a skilled swimmer. The fossil was found in the Liaoning deposits, known for preserving ancient feathers and early birds, and it is remarkably well-preserved. The animal's broad, flattened tail, like that of a beaver, would have helped it steer, while its front limbs provided the main propulsion. This combination of features indicates that Dongoconodon platycauda lived a fully aquatic life, which was rare among early mammals. However, the most intriguing aspect of the fossil is its teeth. X-ray scans revealed that the animal had teeth at various stages of growth, indicating that it continuously replaced its teeth throughout its life. Modern mammals typically have only two sets of teeth: milk teeth followed by permanent ones. This allows for precise alignment of the jaws, but Dongoconodon platycauda's continuous renewal of teeth suggests a diet that required sharp, constantly worn teeth, likely a carnivorous one. This challenges the idea that having only two sets of teeth was a defining feature of mammals, as this ancient animal shows that such a trait may have evolved later. The discovery of Dongoconodon platycauda suggests that the evolution of mammals was not a straightforward path but rather a series of experiments and adaptations. Some early mammals may have retained the ability to replace teeth continuously, a feature more commonly seen in reptiles. Scientists hope that other fossils from the same deposit will help them understand when and why certain mammal lineages changed their dental strategies.