Hypagnostus and Ptychagnostus, Blind Trilobites from Cambrian Utah

Description: Coming from the Cambrian Marjum Formation deposits of Millard County Utah this is an example of Hypagnostus parvifrons (largest specimen) with several examples of the trilobite Ptychagnostus cujanus. 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! 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. They are remarkably detailed for such tiny specimens, and fully articulated.