Steller's sea cow: the first historical extinction of a marine mammal at human hands
Their closest living relatives are the dugong and manatees, known collectively as the sirenians. But while all four surviving species of sirenian live in warm tropical waters, Steller's sea cow had become highly specialised to the sub-Arctic waters of the northern Pacific Ocean. This specialisation included growing to incredible sizes: adults could reach up to 10 metres in length while weighing up to 11 tonnes, bigger than many modern whales. To put this into perspective, an adult male killer whale can come in at eight metres long and weigh up to six tonnes.