The "bugs" that ruled the Paleozoic

 kw: book reviews, nonfiction, science, geology, paleontology, trilobites, paleozoic era

If there were no dinosaurs, everybody's favorite fossil animals would be trilobites. Soooo, they are second-favorite. They are not big like dinos. Most are between an inch and 4 inches long (25-100 mm).

What they lack in size, they make up for in panache! They had eyes (most of them), lots of legs, and a distinctive, unique look.

Though they vaguely resemble familiar animals such as horseshoe crabs and pillbugs (roly polys or woodlice), the last trilobites became extinct 251 million years ago. They have no modern descendants.

The three "lobes" of their designation are not arranged front-to-back, as many assume, but side to side. The central lobe is flanked by a right and left lobe consisting, in front, of cheek plates, and behind, of shell plates that covered the legs. A tail-end called a pygidium may be no more than a tiny blob, or may be as large as the head (called a cephalon).

Literally the most common trilobite is named Elrathia kingii. These attractive "bugs" are found in rocks  of middle Cambrian age, 515-499 million years old. Most specimens are found in Utah, with shells consisting of very dark gray calcite on a lighter gray matrix. They are rather small, usually an inch (25 mm) or less, but range up to 1.6 inches (39mm).

This is enough to whet the appetite a little. The range of species is immense: 25,000 named trilobite species so far, ranging from the early Cambrian, 521 million years ago, to the end of the Permian 251 million years ago, when they and many other marine animals were wiped out in the Great Dying that killed 99% of everything.

Several hundred species are pictured in Travels with Trilobites: Adventures in the Paleozoic by Andy Secher. Mr. Secher is one of the premier collectors of trilobites, having more than 4,000 specimens. Many of the photos in the book are of specimens in his collection. He is affiliated with the American Museum of Natural History as a Field Associate.

The book is arranged first by time. Trilobites are such a characteristic marker of Paleozoic rocks that it made sense to set chapter by chapter in the periods of the Paleozoic era. Four Chapters cover the Cambrian, Ordovician, Silurian and Devonian periods. A single chapter covers the rest, the Carboniferous and Permian periods, as they are known worldwide except North America, where the Carboniferous is divided into Pennsylvanian and Mississippian periods. The reason for this grouping is that trilobites were becoming less common, and many orders and families had gone extinct by the beginning of the Carboniferous period.

Within each chapter we find extended treatments of classic and prolific collecting localities. Naturally, the unequalled Burgess Shale, of middle Cambrian age, around 508 million years ago, is discussed in loving detail in the Cambrian chapter. Interspersed among the locality descriptions we find topical subjects such as "Trilobite Soft Tissue Preservation" (The Burgess Shale was the first major locality of this kind, preserving much more than trilobites) and "Trilobite Enrollment" (they could roll up like pillbugs or armadillos). Each chapter ends with several "Rapid Reports" of localities not discussed in detail, and a photo gallery of specimens from the chapter's geological period. The page below is from the Ordovician chapter.

These photos illustrate some of the variety of preservation styles. While these are among the best specimens in existence of their respective species, they show how some trilobites are more flattened, and some have been preserved in the round.

If you go to any locality where trilobites are common, will you find specimens this beautiful? It's rare. Mostly what we find are portions of the shell. For every trilobite that died intact and was preserved whole, there are thousands or millions of shell parts from molting. Their shells had sutures, as do the shells of crabs; periodically the animal would pop off the shell in parts, and, as rapidly as possible, grow a larger one. They were very vulnerable in the "soft shell" condition…as are soft shell crabs!

When a complete trilobite is found, it is seldom all visible. The first illustration above, for example, shows how a preparator had to remove surrounding material to expose the animals. The one in the upper middle, for example, was at a different depth (I don't know how the preparator knew that), so a lot of matrix had to be removed to reveal it.

Preparators are the behind-the-scenes heroes of the trilobite trade. They may start with a nodule that shows nothing more than a couple of tiny spots of shell, and spend hours removing material until the whole is exposed…or not. Many times the result is not what was expected. But they soldier on. A specimen such as this one requires a lot more work than the flatter ones most of us are accustomed to. This is a Walliserops, found in Morocco, where in recent years many incredible 3-D specimens of many species have been unearthed.

The last chapter of the book touches on many topics, including preparation. One side matter is "Fake Trilobites". Some preparators are more sculptors than extractors. If you want to buy some of the more common trilobites, you have little to worry about. But with others that are more rare, well, watch out. Get help from an expert!

Rather than go on and on, let it suffice that this almost-coffee-table-size book is well worth reading, well worth looking over again and again, and a terrific introduction to these fascinating Paleozoic "bugs".