Fossil of 165‑Million‑Year‑Old Semiaquatic Mammal Reveals Early Mammalian Diversity

A well‑preserved fossil of Megacauda sungei, found in Inner Mongolia’s lake‑sediment rocks, shows a 20‑inch, 1‑2‑kg mammal that swam like a platypus and hunted fish and salamanders. The specimen retains fur impressions and a throat musculature that allows chewing and swallowing food one small lump at a time, a feature unique to modern mammals. Megacauda’s flattened, scaled tail and paddle‑like limbs indicate a semiaquatic lifestyle, while its carnivorous teeth reveal a diet of small vertebrates. As a docodont, it lived 165 million years ago, long before mammals diversified after the dinosaur extinction, illustrating that even in dinosaur‑dominated ecosystems mammals occupied varied ecological niches.