Steller's Sea Cows
Hydrodamalis gigas (Moo?)
Common Ancestors Seen today:
The only differences that can actually be noticed by the naked eye are the tails. The dugong has a fluked tail (like a whale), while manatees have a round shaped tail.
Other than that, all three descendants are completely in-separable.
Why is it extinct?:
Its mass was supposed to be an average of 11,000 kilograms (24683 pounds)..