Megacauda sungei is the largest-known mammal relative from the Jurassic, according to a new study, though the platypus-like animal still weighed only as much as a small dog.
The semiaquatic hunter lived between 168 million and 163 million years ago in what is now Inner Mongolia, China, where it patrolled freshwater lakes and rivers for prey. Researchers say it appears to be the earliest known mammal-type animal with a semiaquatic lifestyle.
Scientists uncovered an exceptionally preserved fossil in 2016 that indicates the animal weighed roughly 2.2 to 4.4 pounds, or 1 to 2 kilograms. The species name, Megacauda sungei, references its prominent tail, with Megacauda meaning big tail. Findings were published Wednesday, Oct. 7, in the journal Nature.
The specimen preserves a mosaic of traits seen in living species, including a broad, flattened tail suited for swimming and short hind limbs with relatively long feet. One of the study’s authors described it as chimera-like, noting the rear of the body and the limbs resembled those of a platypus, well adapted for swimming or burrowing along riverbanks.
Its teeth, however, were distinct from a platypus. The fossil shows large canines and slicing cheek teeth specialized for cutting and processing meat. Researchers characterized it as a predator with unusually robust canines, drawing comparisons to both otters and platypuses.
A revealing fossil of Megacauda sungei
Fragments of vertebrate bone preserved where the animal’s stomach would have been, along with associated fossils in the surrounding rock, suggest that Megacauda sungei preyed on salamanders. Based on the environment and body plan, researchers infer it likely sheltered in burrows on land and foraged in lakes and rivers for fish and invertebrates.
The fossil offers rare insight into throat anatomy. Modern mammals have hyoid bones arranged in a U-shaped configuration that enables them to swallow chewed food in discrete portions and allows infants to suckle milk. Study authors report that M. sungei possessed the structures needed for this function.
Researchers say this represents a key evolutionary development, one that allowed early mammals to diversify into carnivores, insectivores, herbivores, and omnivores by exploiting a wider range of diets. The presence of this apparatus in M. sungei indicates that the capacity for suckling and efficient swallowing had already evolved by the Middle Jurassic, between about 174 million and 164 million years ago.
That timing places the innovation before the proposed divergence of marsupials and placental mammals around 160 million years ago, following their descent from an egg-laying ancestor that lived roughly 180 million years ago. One co-author said the evidence pushes the origin of this trait to before the split between modern marsupials and eutherian, or placental, mammals.
Independent experts not involved in the study noted that neonatal suckling was previously considered a hallmark of more advanced mammals. The new fossil suggests the behavior arose earlier, in the common ancestor of docodontans and mammals at least 165 million years ago.
Megacauda sungei belongs to an extinct branch of docodonts, close relatives of mammals that lived from about 200 million to 120 million years ago during the Jurassic and Cretaceous. Docodonts show remarkable ecological variety. Examples include the tree-dwelling, shrew-like Microdocodon gracilis, the burrowing Docofossor brachydactylus with shovel-like forepaws, and the semiaquatic Castorocauda lutrasimilis, sometimes described as a Jurassic beaver, which lived around 164 million years ago.
Specialists say the new fossil underscores how often aquatic habits emerged among early mammal-like species. They add that the discovery highlights the wide geographic spread and ecological breadth of docodonts through the age of dinosaurs, bolstering evidence that early mammaliaforms occupied diverse niches long before the rise of living mammals.
Citation: Li, Y., Li, P., Neander, A., I., Zhang, H., Zhou, C., Martin, T., and Luo, Z. (2026a). A Jurassic mammaliaform swimmer and transformation of the mammalian pharynx. Nature. https://doi.org/10.1038/s41586-026-11107-0