Say hello to Castorocauda lutrasimilis, a primitive mammalioform from the middle Jurassic—164 million years ago. Despite its great age, it has evidence of fur and guard hairs still preserved in the fossil, and was rather large for its time. It’s estimated to have weighed about 500g (about a pound) and was over 400mm (over a foot) long in life, and as you can see from the reconstruction, shows signs of being aquatic. In size and lifestyle, it probably resembled the modern platypus.
Ji Q, Luo Z-X, Yuan C-X, Tabrum AR (2006) A Swimming Mammaliaform from the Middle Jurassic and Ecomorphological Diversification of Early Mammals. Science 311(5764):1123-1127.
Since people were asking about the phylogenetic relationships of this animal to modern forms, here’s a cladogram. The modern platypus is Ornithorhynchus, about the middle of the list; Didelphis is the opossum.