Yes David, the Steller's sea cow is extinct. It lived in the Bering sea and was hunted remorselessly by the russians during the 17th and 18th century I believe. I also believe Alaska was then still a russian possession. In the end there were so few Steller's sea cows left that they reached the extinction point.

Mineralized bones and ribs still wash ashore in Alaska and are collected and sold on the international market. I bought mine from a Danish supplier.