August 29. The Last Trilobites

We haven’t talked about trilobites for a while. From their incredible abundance, and even dominance in Cambrian and Ordovician time, trilobites declined in both numbers and diversity. They were still really quite abundant and diverse in Silurian and Devonian time, but the extinction at the end of the Devonian had a pretty significant impact on them. The decline continued, until at the start of the Permian, there were only three families of trilobites in existence. Earlier in the Paleozoic, there were dozens of trilobite families. 

Permian trilobite from Kansas, about 1.5 cm long. See below for image credit.

The Permian trilobite families were divided into about 30 subdivisions at the genus level. They suffered two blows during the Permian – first, in the Middle Permian, about 266 million years ago, when more than half the living genera were wiped out. Combined with earlier losses, that meant that there were only five trilobite genera that survived until the end of the Permian, when they too were destroyed in the mass extinction that eliminated more than 90% of all the species on earth. We’ll talk about that extinction the day after tomorrow, when the Permian ends.

Trilobites survived in one form or another for almost 300 million years, making them one of the most successful animal groups in all of earth history. Their decline and ultimate extinction was probably the result of many factors, including competition from other kinds of organisms, more predatory organisms, climate change, and loss of habitat, the typical reasons any plant or animal goes extinct. There’s some speculation that poor and inconsistent molting, combined with increases in animals that could – and did – eat trilobites while they were in the vulnerable molting state contributed to their demise over a period of millions of years. But ultimately, I don’t think we have a really good handle on the reasons trilobites declined. Their final extinction, at the end of the Permian, is no surprise, since that event caused wholesale destruction. But their slow decline until that final blow probably represents a combination of various factors.

—Richard I. Gibson

The Last Trilobites

Nice drawing showing the expansion and decline of trilobites through the Paleozoic 

Poor molting style

Photo by Dwergenpaartje under Creative Commons license : proetid trilobite Ditomopyge decurtata from Permian of Kansas