Ptychagnostus
atavus
Order
Agnostida, Suborder Agnostina, Superfamily Agnostoidea, Family Ptychagnostidae
Geological
Time: Middle
Cambrian
Size: 3
mm - 4.5 mm
Fossil Site:
Lower Wheeler Formation, Millard County, Utah
 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. Believed plantonic, they often lacked eyes, and their
cephalon and pygidium are of essentially the same size.
This
plate coming from the Middle Cambrian of Millard County, Utah contains
16 specimens of the species Ptychagnostus atavus.
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