Complex sentence: Meanwhile, the first true mammals evolve, remaining relatively small sized but otherwise expanding ecologically; the Jurassic "Castorocauda", for example, had adaptations for swimming, digging and catching fish.
Split into simpler sentences: Meanwhile, the first true mammals evolve, but remained relatively small sized. The Jurassic Castorocauda, for example, had adaptations for swimming, digging and catching fish.