Castorocauda lutrasimilis

Castorocauda was an extinct docodont mammaliaform that lived during the Middle Jurassic period in what is now China. It was a small, furry animal that physically resembled a modern otter or beaver.

The species was officially described in 2006 based on a remarkably well-preserved fossil discovered in the Tiaojishan Formation of Inner Mongolia. The fossil provided the earliest direct evidence of fur in the mammalian fossil record.

The animal's preserved skeleton and fur impressions reveal adaptations for swimming and digging, including a broad, flattened tail similar to a beaver and robust limbs. This indicates it lived a largely aquatic lifestyle, hunting fish and aquatic invertebrates in prehistoric lakes.

It proved that early mammal-like creatures diversified into specialized ecological niches, like aquatic habitats, much earlier than previously thought. Prior to its discovery in the mid-2000s, scientists believed Mesozoic mammals were mostly tiny, land-dwelling insectivores.