Pelagotheria (lit. "sea beasts") are a clade of Deinognatha.
Pelagotheres emerged around 4221ma, evolving from Dolichognathans native to ocean coasts. A large contributor to their initial success was the change in climate, which disrupted the oceanic ecosystem and removed much of the competition.
The limbs of Pelagotheres are powerful flippers, which provide fast and stable swimming. Their pedipalps have reduced to an almost vestigal state, being held flat against the mandibles.
Pelagotheres are adapted to an aquatic lifestyle, and the species that can survive on land only return to the land for reproduction.