Complex sentence: Bering island was the only known habitat of Steller's sea cow, an immense (over 4,000 kg) sirenian similar to the manatee, was hunted to extinction within forty years of its discovery in 1741.
Split into simpler sentences: Bering island was the only known habitat of Steller's sea cow, an immense (over 4,000 kg) sirenian similar to the manatee. The sea cow was hunted to extinction within forty years of its discovery in 1741.