Actatheria (lit. "beach beasts") are a clade of Pelagotheria.
Actatheria first emerged around 4225ma, when it diverged from the Thalattotheria branch of Pelagotheres.
Actatheres live most of their life in the water, but return to land in order to reproduce, as their ootheca must be kept outside of water for the embryos to survive. In all other aspects, they are adapted to a marine lifestyle.
Actatheres are roughly the same size as Eudolichognathans, measuring about a meter in height.