Celebrating the Richness of Paleontology through Fossil Hunting

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The Trilobite Gallery

Utah Trilobite Hunt
Fossil Hunting Trip Report

Agnostid Trilobite Gallery

agnostid trilobite fossils from Utah

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.

Phylum: Arthropoda
Class: Trilobita
Order: Agnostida
Suborder Agnostina
Superfamily: Agnostoidea
Family: Peronopsidae and Ptychagnostidae are common
Genus: Common ones are Peronopsis, Ptychagnostus, & Onymagnostus
Species: Numerous species
Locality Information:
There are numerous genera and dozens of species of agnostid trilobites. They appear to have a global distribution in Cambrian exposures, North America to Africa and Australia.

The agnostid trilobite fossils featured here, are either Peronopsis, Ptychagnostidae, and peraps Onymagnostus, come from the Cambrian Wheeler shale and Marjum formations from the House Range in Utah.
Diagram of an agnostid trilobite fossil - This shows the difference between the head and tail (cephalon and pygidium).

Agnostid Fossil Examples

A closeup of a plate of house range agnostid trilobite fossils from the Cambrian of Utah - House Range

A plate of agnostid trilobite fossils from the Cambrian of Utah - House Range

An Overview of the plate of agnostid trilobite fossils from the Cambrian of Utah - House Range

This is a little plate of agnostid trilobites. It's both the posative and negative sides of the plate.

On this plate, the largest agnostid is 7 mm (just over 1/4"), the smallest is 2 mm (.08").

On the plate is also a cephalon of a Bathyuriscus fimbriatus trilobite

Location:House Range, Utah
Age: Cambrian
Formation: Marjum

A Beautiful Red agnostid trilobite fossil from the Cambrian of Utah

Closer look at the Red agnostid trilobite fossil from Wheeler Formation in Utah

This is a rather large trilobite (for an agnostid). It's a whopping 9mm (almost 3/8") beautiful red peronopsis.

It could be a Peronopsis interstricta

A small protion of the pygidium is flaked off. (The left side is the pygidium, the right side is the cephalon).

Location:House Range, Utah
Age: Cambrian
Formation: Wheeler Shale

agnostid interstrictus trilobite fossil from utah

This is another red agnostid trilobite fossil. It is very 3-D looking, but has some damage to it.

Location:House Range, Utah
Age: Cambrian
Formation: Wheeler Shale