Ptychagnostus praecurrens
Trilobites
Order Agnostida, Family Ptychagnostidae
Geological
Time: Early Cambrian (~520 million years ago)
Size: 7 mm
Fossil Site:
Burgess Shale, Stephen Formation, Burgess Pass, British Columbia, Canada
|  Description:
When first discovered in Walcott’s quarry, this trilobite
was named Triplagnostus buregessensis. It was later determined to
be the same as an earlier agnostid named Ptychagnostus praecurrens.
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. It is remarkably detailed for such a tiny specimen,
and fully articulated.
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