Archaeoplacostraca (lit. "old plate shells") were a clade of Diplostoma and the first group of Placostracans. Archaeoplacostraca also forms a branch clade called Placostraca (lit. "plate shells") with itself and Euplacostraca.
Early Species Edit
Archaeoplacostraca first emerged around 4180ma, becoming the first clade of large terrestrial herbivores. While dwarfed by later species, they were easily the largest animals of their time, with some measuring around 2 meters in length. Their top two oral tentacles were elongated, allowing them to reach for food, while their bottom four were broader and shorter, lined with rows of teeth to aid in mastication.
In response to the arrival of the macropredator Coleostraca, they evolved a thicker shell, but otherwise remained identical in anatomy.
Archeoplacostraca were driven to extinction around 4205ma, having been out-competed by the Megalobrachids. They were one of the several Diplostoma branches that were wiped out by the emergence of Osteopods.