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Trillions of Trilobites

I pull off to the side of the road at a steep rocky slope in the House Range of western Utah. Getting ready to collect the first samples for my PhD dissertation on half a billion year old trilobites and their environment. The slope consists of thin limestone beds ranging from about a centimeter to several centimeters thick, interspersed with shale layers that are usually somewhat thicker. The slope goes up more than a hundred feet and there are hundreds of these limestone layers, stretching as far as I can see East and West along the highway. They once also stretched over my head, before the pass was eroded by wind and water. Each layer consists of many thousands of tightly packed trilobite shells, disarticulated skeletons. The head piece, or cephalon, is actually five parts, four of which are small and not too commonly seen. The thoracic segments, something like 14 or 15 in number, are all separate, but there are so many of them that even though they're thin and fragile we find plenty. The pygidium covered the tail, and is fairly sturdy.

The skeletons were torn apart and redeposited in windrows by storms sweeping across a shallow marine shelf like that off the eastern coast of North America today. Each layer consists of fossils of a single species of the trilobite Ehmaniella. (Different layers have one or another of several clearly distinguishable species.) Nothing else, except calcite crystals that cemented them together after deposition. What did they eat? Algae? Tiny worms? These things would of course have left no skeletons at all. The mature trilobites were small: a 5 or 6 centimeters long. The smallest babies had one-piece exoskeleton; these are about 1/3 of a millimeter long. The fossils have been naturally replaced by silica, so I got them out of the limestone by immersing them in hydrochloric acid. Fun times! NOTE: DO NOT TRY THIS AT HOME! I did this under a chemical hood and used dilute acid.

Alas, I no longer have many of the photos I took nearly 50 years ago. Here is one, of two adult cranidia (the large head shields). They are each about a centimeter long.

Here's an illustration from my first paper about Ehmaniella. Some of the scale bars didn't copy, but the fossils are arranged from smallest in the upper left to largest in the lower right. All the scale bars are 0.1 mm. The top row are the four protaspid stages. These are the babies, in which the exoskeleton is a single piece. At every molt, they got bigger and changed a bit in shape. On the bottom row are cranidia; the head shield minus the four small sclerites (parts). These are accurate drawings of seven of the specimens I collected.