Trilobites, the Dinosaurs of the Arthropod Family

Everything You Ever Needed to Know About Trilobites

trilobite
A fossil of a typical trilobite, Arctinurus boltoni (courtesy of Trilobites.com).

Tens of millions of years before the first dinosaurs walked the earth, another family of strange, distinctive, weirdly prehistoric-looking creatures, the trilobites, populated the world's oceans--and left an equally abundant fossil record. Here's a look at the ancient history of these famous invertebrates, which once numbered in the (literal) quadrillions.

The Trilobite Family

Trilobites were early examples of arthropods, a vast invertebrate phylum that today includes such diverse creatures as lobsters, cockroaches and millipedes. These creatures were characterized by three main body parts: the cephalon (head), thorax (body), and pygidium (tail). Oddly, the name “trilobite,” which means “three-lobed,” doesn’t refer to this animal’s top-to-bottom body plan, but to the distinctive three-part structure of its axial (left-to-right) body plan. Only the hard shells of trilobites are preserved in fossils; for that reason, it took many years for paleontologists to establish what these invertebrate's soft tissues looked like (a key part of the puzzle being their multiple, segmented legs).

The trilobites comprised at least ten separate orders and thousands of genera and species, ranging in size from less than a millimeter to well over two feet. These beetle-like creatures appear to have fed mostly on plankton, and they inhabited a typical array of undersea niches: some scavenging, some sedentary, and some crawling along the ocean bottom. In fact, trilobite fossils have been discovered in pretty much every ecosystem on hand during the early Paleozoic Era; like bugs, these invertebrates were quick to spread and adapt to various habitats and climatic conditions!

Trilobites and Paleontology

While trilobites are fascinating for their diversity (not to mention their alien appearance), paleontologists are fond of them for another reason: their hard shells fossilized very easily, providing a convenient “road map” to the Paleozoic Era (which stretched from the Cambrian, about 500 million years ago, to the Permian, about 250 million years ago). In fact, if you find the right sediments in the right location, you can identify the various geologic eras by the types of trilobites that appear in succession: one species may be a marker for the late Cambrian, another for the early Carboniferous, and so on down the line.

One of the interesting things about trilobites is the Zelig-like cameo appearances they make in ostensibly unrelated fossil sediments. For example, the famous Burgess Shale (which captures the strange organisms that began to evolve on earth during the Cambrian period) includes its fair share of trilobites, which share the stage with bizarre, multi-segmented creatures like Wiwaxia and Anomalocaris. It's only the familiarity of trilobites from other fossil sediments that decreases their Burgess "wow" factor; they are not, on the face of it, any less interesting than their less-well-known arthropod cousins.

They had been dwindling in numbers for a few tens of millions of years before then, but the last of the trilobites were wiped out in the Permian-Triassic Extinction Event, a global catastrophe 250 million years ago that killed off more than 90 percent of the earth's marine species. Most likely, the remaining trilobites (along with thousands of other genera of terrestrial and water-dwelling organisms) succumbed to a global plunge in oxygen levels, perhaps related to massive volcanic eruptions.