Order Agnostida, Family Ptychagnostidae
Order Agnostida, Family Spinagnostidae
Time: Middle Cambrian
Marjum Formation, Millard County, Utah
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. Believed plantonic, they often lacked eyes, and the cephalon
and pygidium are of essentially the same size.
michaeli, with its five spines, is one of the rarest agnostids
in the Marjum Formation, and only recently have good specimens
been found. Hypagnostus hypagnostus is of moderate rarety.
all, a very nice Cambrian trilobite association plate with two rare
Ptychagnostus michaeli and a Hypagnostus parvifrons.
fossil pictures to enlarge