Complex sentence: Steller's sea cow was the largest of all sirenians, and had a much thicker layer of blubber than other sirenians; this was an adaptation to the cold waters of its environment.
Split into simpler sentences: Steller's sea cow was the largest of all sirenians, and had a thicker layer of blubber than the other members of the order. This adaptation was due to the cold waters of its environment.