Strophomenida

Strophomenida is a large, extinct order of articulate brachiopods that existed from the lower Ordovician to the lower Jurassic period. It was the largest known order of brachiopods, encompassing over 400 genera, including the largest and heaviest of known brachiopod shells. The strophomenids lost the ability to attach by the stalk in adult specimens, so they either lay free, attached the ventral valve at the umbo to a firm substrate, or balanced with their spines sunken into a soft substrate. Typically the dorsal valve was either concave or flat, though occasionally it was convex; the ventral valve was convex. Typically, a member of this order was wider than it was long. In juveniles, there was a tiny hole at the animal's umbo for a stalk to emerge from. Tiny bumps cover the interior of the valves.