The story of the extinction of Steller's sea cows is sad, but fascinating. What would our world look like if these huge sirenians were still living?
http://www.wired.com/wiredscience/2014/01/absurd-creature-of-the-week-stellers-sea-cow/
See the ADW account on Steller's sea cows:
http://animaldiversity.ummz.umich.edu/accounts/Hydrodamalis_gigas/