Devonian Arms Race
On a recent trip, we stopped at the Herrett Center for Arts and Science on the campus of the College of Southern Idaho. There I found, on display, a replica of Therataspis Grandis, one of the larger trilobites ever discovered. I have always been fond of trilobite fossils and have a good half-dozen purchased larger specimens and a number of smaller ones that I have picked up in some of my early rockhounding days in the mountains of Utah. What impressed me most was the completely armored aspect of the huge creature, and I began to wonder why. All trilobites have similar features, the tri-lobed body, armored head, and possibly the first compound eyes in nature. They appeared at the beginning of the Paleozoic era, the first great explosion of animal species in the ocean. They lived on this planet for a total of 270 million years, much longer then the dinosaurs.
The earliest trilobites got no bigger then an inch long. In the Cambrian period, 540 million years ago they were the early dominate species. Later during the Ordovician Period 485 million years ago, there was increased competition from other species and predation. They grew larger and developed tougher exoskeletons. The first world mass extinction event at the end of the Ordovician cut into their numbers and thousands of species became hundreds. Later in the Devonian Period when fishes really began to develop, things got increasingly difficult for trilobites. Plants had colonized the world and theropods developed the ability to walk on land and breathe air. In the ocean, an arms race was developing. Fish began to rapidly diversify, developing armor, cartilaginous skeletons, jaws and teeth. The first sharks appeared at this time and rapidly evolved.
Trilobites reacted to the increased competition by growing larger, developing even thicker shells and spines. Many of the species in the early Devonian are some of the most beautiful with remarkable and exotic spines and other defensive arrays. This cumulated in the Therataspis, probably the most armored of the day. I’m sure that with teeth or not, many predators would avoid this trilobite. At the end of the Devonian a couple of events created another mass extinction and trilobites suffered greatly. Only one order of trilobites survived and lingered on until the end of the Permian Period, 250 million years ago, when the largest mass extinction event on earth killed off 95% of all ocean life and 70% of all land species. Oddly enough, this paved the way for the development of the dinosaurs, who went on the rule the earth for the next 180 million years.
(A complete fossil of the Therataspis has never been found. But enough bits and pieces have been discovered for scientists to reconstruct the creature fairly accurately.)
