mya).

Sirenians grow to between 2.5 and 4 m in length and 1500 kg in weight. The recently extinct Steller's sea cow was the largest known sirenian to have lived, reaching lengths of 10 m and weights of 5 to 10 t.

Sirenians have a large, fusiform body which reduces drag through the water and heavy bones that act as ballast to counteract the buoyancy of their blubber. They have a thin

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