Complex sentence: The sea mink was the most aquatic member of Musteloidea except for the otters, and it was unusual in having rapidly evolved toward a marine habitat in the late Cenozoic.
Split into simpler sentences: The sea mink was the most aquatic member of Musteloidea. Except for the otters, it was almost certainly the most aquatic musteloid and was unusual in having rapidly evolved toward a marine habitat in the late Cenozoic.