Seeking Early Trilobites

The beginning is the most important part of the work. –Plato

Olenellus clarki, Latham Shale, Cambrian Period, Cadiz, California
Olenellus clarki, Latham Shale, Early Cambrian Epoch, Cadiz, California. Specimen is 1.7 cm across the genal spines.

I remember a field trip to the Cretaceous of Montana when I was an undergraduate geology student. The professor instructed the class to prospect the uppermost part of the Hell Creek Formation: He was interested in finding dinosaur fossils as close as possible to the Z Coal, the boundary with the overlying Paleocene Tullock Formation, to see if dinosaurs disappeared before the K/T extinction event. Wanting to find fossils, I kept drifting lower in the section. He noticed and yelled and waved me higher in the section. I yelled in reply, “But there’s nothing up there!” He glared back.

I had the same problem in reverse during childhood. When prospecting in the Cambrian of southeast Minnesota I usually found nothing. Occasionally a lingulid brachiopod or an isolated trilobite free cheek or pygidium would turn up. Prospecting the Ordovician or Devonian was an entirely different matter, however. Some localities were bristling with fossils.

Of course, there are highly fossiliferous Cambrian localities, the famous Burgess Shale around Mount Stephen in British Columbia, for example. Or Ruin Wash, Nevada in the Pioche Shale. This deposit straddles the Lower/Upper Cambrian boundary and is loaded with fossils, mostly olenellid trilobites–which were on the way out by this time.

Olenellus gilberti, Pioche Shale, Cambrian Period, Ruin Wash, Nevada
Olenellus gilberti, Pioche Shale, Cambrian Period, Ruin Wash, Nevada. Specimen is 4.0 cm across the genal spines.

But in general, the diversity (number of taxa) and abundance of shelly invertebrate fossils increase as you move up into the Ordovician–which is why I was so puzzled when I first read Wonderful Life (1989) by Stephen Jay Gould. One thesis of this book was that animal disparity (morphological variation) peaked during the Cambrian Period. Most of the evidence for this proposition came from Burgess Shale animals that Gould portrayed as surpassingly strange. The author, with few exceptions, concentrated on “phylum-level disparity.” Class-level disparity, such as the difference between a bat and whale or a Great Auk and a hummingbird mattered not.

In this context, Gould was often obsessed with the number of appendages coming from one or another body sclerite (or the presence of exotic appendages) in clearly arthropod-like animals. In some cases, the number and placement of appendages did not conform to the situation in later groups. In Gould’s mind, this meant that these Cambrian creatures didn’t belong to the Arthropoda sensu strictu.

But isn’t this is because Arthropoda was initially defined without knowledge of these Cambrian forms, without knowledge of the disparity they displayed? I felt that had some phyla, Arthropoda included, been defined with a full anatomical knowledge of Burgess and other Cambrian forms, taxonomists surely would have decided that these “weird” Cambrian animals belonged within more broadly defined higher-order taxonomic groupings, such as a different Arthropoda that could encompass an Anomalocaris or Opabinia. 

No matter your opinion of what constitutes “diversity” or “disparity,” the Cambrian is a fun place to visit, either in the mind’s eye or the field. But . . . something is to be said for places like Jbel Issoumour or just about any outcrop in the Pennsylvanian of Kansas where the fossils literally crunch beneath your feet . . . .

Democephalus granulatus, Weeks Formation, Late Cambrian Epoch, Millard County, Utah
Democephalus granulatus, Weeks Formation, Late Cambrian Epoch, Millard County, Utah. The Late Cambrian Epoch is the high water mark of family-level trilobite diversity. Many other major invertebrate groups such as gastropods, bivalves, and brachiopods continue their evolutionary radiations into the Ordovician Period, however. Specimen is 2.4 cm long.

©2017 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

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