The Cuesta sea cow (Hydrodamalis cuestae) is an extinct herbivorous marine mammal, and the direct ancestor of the Steller's sea cow (Hydrodamalis gigas). They reached up to 9 metres (30 ft) in length, making them among the biggest sirenians to have ever lived. They were first described in 1978 by Daryl Domning when fossils in California were unearthed. Its appearance and behavior are largely based on that of the well-documented Steller's sea cow, who, unlike the Cuesta sea cow, lived into modern times and was well described.