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AGNOSTID TRILOBITES Facts and Information


Agnostid Triolobites
The Smallest Trilobites

agnostid trilobite fossils from Utah.

Fast Facts about Agnostid Trilobites

Name: Agnostid: This is an order of Trilobites. The name comes from the word Agnostic meaning "Unknowable." This referrs to the fact that the head and tail look nearly identical.

The Agnostid order is broken into the Agnostina and Eodiscina suborders, each of these suborders have numerous families, genus, and species which are all difficult to tell apart from one another.

Taxonomy: Phylum: Arthropoda - Class: Trilobita - Order: Agnostida - Suborder: Agnostina and Eodiscina - Common Families: Peronopsidae and Ptychagnostidae

Common Genus: Peronopsis, Ptychagnostus, & Onymagnostus

Age: Cambrian to Late Ordovician

Distribution: Global
Agnostid trilobites were very diverse and widespread.

Body Size:
Agnostid trilobites are TINY! They can range in size from under 1 mm to around 10 mm.

Benthic Particle Feeders
They most likely scavanged the ocean floor.

Physical Appearance:
Agnostid trilobites have a head

Fun Fact 1:
These are the worlds smallest trilobites. Mass mortality plates of dozens can fit in a few inch area.

Agnostid Trilobite Facts and Information

Diagram of an agnostid trilobite fossil - This shows the difference between the head and tail (cephalon and pygidium).

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 Late Ordovician.

Because of their peculiar morphology, the ecology of this trilobite has been debated. Based on the morphology and associated fossils, most people think they were 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. Also the lack of eyes is consistant with living on the ocean floor with very little light.

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 requires the use of many journal articles. Idntification to a genus and species level will not be attempted here. While doing research, it appears many people blindly assign a species name or genera without knowing for sure.

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.

Where to find Agnostid Trilobite Fossils

U Dig Quarry - An easy place to find Elrathia Trilobites.
U-Dig images by Albert B. Dickas, who is the author of some great Geology and Fossil Books!

Egnostid trilobites are found globally throught Paleozoic formations that contain trilobites. They are most abundant in Cambrian formations. Easy places to find them are the Wheeler Shale in Utah and Nevada. They look like little pepper flakes on the rocks.

To try and find your own Agnostid trilobites, I would suggest going to the U-Dig quarry near Delta, Utah.

Click here to read about my trilobite fossil collecing trip in the Wheeler Shale near Delta, Utah

Recommended Books

Trilobites: Common Trilobites of North America (A NatureGuide Book)
by Jasper Burns, 2000

Jasper Burns has some great illustrated fossil guide books. This one is on the Trilobites of North America. It has detailed illustrations of 41 species of North American trilobites and includes information about trilobite paleobiology, vision, growth, extinction, and much more! It's a nice little resource book for those interested in North American trilobites. It's also a very inexpensive book. Check it out!

The Trilobite Book: A Visual Journey
by Dr. Riccardo Levi-Setti, 2014

This is an updated (2014) hardcover (kindle available) of his famous 1994 book. It now has color images instead of black and white ones. The images are of prefectly prepared trilobites from all over the world. this book is geared toward the beginner and does not get overly technical. However, it's wonderful just to see the pictures and is a must for any trilobite enthusiast.

Trilobites for Sale:

Trilobites from Fossil Era
Trilobite fossils are some of the most beautiful and collectible fossils in the world! They make beautiful display and conversation pieces. They can make very affordable for gifts to fossil and paleontology enthusiasts. Fossil Era has a huge selection of top quality trilobites. It's fun just to browse through the inventory and look at all the different types!

Agnostid Trilobite Fossil Examples

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

Formation:Wheeler Shale
Location:House Range, Utah

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

Formation:Wheeler Shale
Location:House Range, Utah

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

Formation:Wheeler Shale
Location:House Range, Utah

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

Formation:Wheeler Shale
Location:House Range, Utah

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

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

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