Spiny Ptychagnostus michaeli Trilobite from Utah

Description: The order Agnostida comprises trilobites that appeared in the Lower Cambrian, became widespread and then declined to become rare in the Ordovician prior to their complete extinction by the end of the Ordovician. They are so unusual that some experts have suggested that one of the two suborders, Agnostina, should not even be included in Class Trilobita. The members of the Agnostida are mostly thought to have been planktonic in nature. Most are blind, and have a pygidium equal in size to the cephalon (ispopygous), making it hard to tell which end was which! P. michaeli, with its five spines, is one of the rarest agnostids found in the Marjum Formation, and only recently have good specimens been found.