With all of the news about irreversible global warming and environmental collapse, it's time to take a look at one of the Earth's mega-survivors: the versatile Trilobite.
Throughout the Paleozoic era, which began 543 million years ago, Trilobites were extraordinarily successful. As we face all sorts of planetary crises, and worry about our long-term future on Earth, perhaps we should examine these hardy organisms over their three hundred million year existence.
Trilobites are arthropods - their closest living relative is the horseshoe crab - and were so common and diverse throughout much of their existence that they are often used by geologists and paleontologists to date the rock formations in which they are found. (1) Throughout the Cambrian period, they populated the seas with over six hundred separate species, before their dominance over the seas waned during the Ordovician period.
The term Trilobite is very descriptive - they are divided longitudinally into three sections - a central lobe, and two pleural lobes. Its thorax is divided into smaller segments that allow for a range of movement - some species could even roll into a tiny ball for protection. Their heads are called cephalons, and might contain eyes and defined cheeks; their tails are called pygidiums. Under the thorax, under each segment, was a pair of legs, and some species sported a number of spikes on top. (2)
What is remarkable about trilobites is their rapid evolution and growth to adapt to the many environments presented to them in the oceans of the Paleozoic. If you compare two species, one from the early and one from the later parts of the Cambrian, the differences are numerous. Earlier trilobites are far more primitive, lacking the more advanced features such as spines and eyes. (2)
As one moves up the column and time, the earlier, more primitive trilobites are slowly replaced by more sophisticated and diverting species. They shed segments, gained eyes and spikes, all essentially tailored to an environment where the trilobites found they could survive. Richard Fortey, in his book Trilobite: Eyewitness to Evolution, explains "punctuated equilibrium," a term which comes from the study of trilobites in the 1960s. Put simply, the theory holds that some species change very little over long periods of time, and then experience short spurts of intense change where one species branches out into several new ones.
Trilobites follow this pattern. The changes in trilobite species occurred gradually in regions, with no major events. But once a species diverged from another, they would endure. (4) Further studies of trilobites have come up with similar findings, and to the scientific community, these ancient creatures are essentially a study of evolution in action.
By the end of the Cambrian period, roughly 490 million years ago, trilobites had lost their dominance throughout the seas. During the later stages of the Ordovician period, the Earth was gripped by a vast ice age, and geologists have found fossils of the trilobites that survived this period. With this period of climate change came real changes for the species. Trilobites that existed in open water and were adapted to warmer climates died off, as their food supplies died with the cold. Other species, not adapted to open and deep water, flourished on the continental shelves, and thrived into the Silurian. (5)
During this time, vertebrate fish species evolved. While not likely a cause of the downfall of the trilobites, these new species would have likely presented problems for the trilobites. (6) By the Permian period, 290 million years ago, trilobites were in further decline, helped along by another ice age. But most likely their final extinction was caused by an event that is thought to have eliminated almost 95% of all life from the planet.
Theories vary, but it is thought that there were two separate extinction events. The first was the formation of Pangea, a single land mass that later broke up into the continents we know today. The consolidation of the super continent cut off global currents, essentially causing a global cooling event. Second, there was a period of massive volcanic activity on an almost unbelievable scale, which resulted in extremely thick outcrops of volcanic lava flows and ash over Siberia and China. This would have likely caused another period of warming and cooling in the early Jurassic period. Other theories include meteor hits, although that is certainly up for debate. Massive amounts of carbon dioxide in the atmosphere are thought to have killed off additional marine species. (7)
The trilobites didn't survive the Permian, but we we can learn several things from their long lifespan on the planet. The first is that these arthropods were extremely adaptable, able to change to meet a number of environments present in the ocean - from the deep ocean to the continental shelves. They ruled the seas, filling every environmental niche until they were stopped by several global cooling periods.
The second thing that we can learn is that even a hardy species has an incredibly difficult time with rapid climate change. Global warming periods pumped carbon dioxide into the oceans, which is thought to have helped to reduce their numbers, but also the extreme periods of global cooling further killed species off as they could not adapt in their specific environments fast enough to survive. Examining the effects of climate change through the eyes of a geologist or a paleontologist shows that these changes have come before, and that they have had devastating effects on organisms throughout the world's environments. Is humanity next, doomed because we cannot adapt quickly enough? Possibly.
And possibly because they force us to ask that question, trilobites have captured our imaginations. James Gurney, in his fantastic world of Dinotopia, shows a Devonian world, complete with trilobites, flourishing in a part of the world that escaped the mass extinctions. While this story is impossible in the context of global climates, it is an entertaining one. But, in Ken Macleod's 2000 novel Cosmonaut Keep, we briefly see that the trilobite has been resurrected by genetic manipulation in the near future.
Will we see the trilobite once again in our oceans? I certainly hope so.
1 – Donald R. Prothero and Robert H. Dott Jr, Evolution of the Earth, (McGraw Higher Education, Boston, MA: 2004), 197
2 - Cyril Walker and David Ward, Fossils, (Dorling Kindersley Books, London, United Kingdom: 1992), 56
3 - Fortey, Trilobite! : Eyewitness to Evolution, (Knopf Books, New York NY: 2000), 159
4 - Ibid, 163-164
5 - Ibid, 184-185
6 - Prothero, 262
7 - Ibid, 346-347
Supporting source: James R. Beerbower, Search for the Past: An Introduction to Paleontology, 2nd Edition, (Prentice-Hall, Englewood Cliffs, NJ: 1968)