Agnostid Trilobite Facts and Information:
Agnostid trilobite fossils are very peculiar in that they have no eyes, the head (cephalon) and tail (pygidium) look very similar, and the thorax only has two segments. They look like a miniature peanut!
They are quite tiny! A small one can be under a millimeter, while a large one is about 10 mm. When found, they often look like flecks of pepper on the rock. A death plate that contains a dozen can be as small as a few inches in size!
The agnostida order, appears in the Cambrian and dies out in the Ordovician.
Because of their peculiar morphology, the ecology of this trilobite has been debated. Some think it was pelagic, meaning it floated in the water column with other plankton like organisms. Others think is was benthic, living on the bottom of the sea. They are often found in groups, and often found underneath and inside other fossils, suggesting they indeed lived on the sea floor.
The key characteristic in identifying a cephalon from a pygidium is the pygidial axis (middle of the pygidium) runs closer to the end of the pygidium than the glabella on the cephalon. If this sounds confusing, which it does, the diagram below will straighten it out.
As far as identifying species and genera, it appears to be a delicate science taht need the use of many journals. It will not be attempted here. While doing research, it appears many people blindly assign a species name or genera without knowing for sure.