The trouble with Trilobites is pretty much the same trouble we have with understanding why any animal that was once so abundant can totally disappear because Trilobites were survivors. The history of these creatures shows that they practically invented survival in order for them to finally go extinct and leave no direct descendants. Nature had tried to kill them like four different times.
Let’s start about 540 million years ago, just before the dawn of the Cambrian period. Back then all life on Earth was in the sea and most of it was squishy like Sponge, Worms and Jellies. Yes, there were some Mollusks and simple shelled creatures like Brachiopods. But by and large, if you were an animal you didn’t have to worry about protecting yourself from predators. Then about 521 million years ago the first know Trilobites appeared in a place which is now called as Siberia. They may have evolved from small thick-skinned segmented worms like Spriggina. But, Trilobites took the benefits of physical complexity much further. They had legs and they developed complex eyes and a sophisticated digestive system. Plus they were covered from their round heads to their many segmented feet in an exoskeleton made of calcite and chitin. The Trilobite was a true arthropod, a founding member of the group that includes today’s arachnid and crustaceans and every insect. By equipped with all of these awesome new features it dominant the Cambrian seas.
Fisting on a diet of worms and other unprotected invertebrates, it flourished and diversified quickly. Within 40 million years of its first appearance in the fossil record, there were at least 60 different taxonomic families of Trilobites ranging from googly-eyed Asaphus to the familiar Elrathia. For millions of generations they thrived by lying low on the ocean floor, but the seas of the Cambrian were becoming increasingly hostile.
The Cambrian, after all, was when predation first appeared on Earth, with animals hunting other animals and Trilobites left behind lots of Fossils with bite marks on them to show it. But by the time the Cambrian came to a close Trilobites like many other animals had picked up some new tricks. The cutest of them Enrolling. Judging by the segmentation of their bodies, some kinds of Trilos like the genus Flexicalymene developed the ability to curl themselves up into little balls like Modern pill bugs or armadillos do, it made it harder for new-fangled predators to sink their teeth into them. But, not all the threats to our little-Armoured friend came from other animals.
Starting around, 445 million years ago, Earth’s climate dealt a double blow to life in the seas. First, a dramatic cooling changed the ocean currents and choked off the supply of warm water foods like algae. Then a period of glaciation locked up lots of Earth’s water, which dramatically lowered sea levels. This twofold impact as the Ordovician-Silurian Extinction, wiped out about 25% of all taxonomic families, including about half of Trilobites families. The families of Trilobites that remained were mostly ones that were well adapted to this new world like Dalmanites, which fared well in cooler seas. Then, of course, Trilobites had another problem to deal with.
As the environment continued to change, other organisms had to survive too, under constant evolutionary pressure to adapt or die. The new adoration that had come about during the Cambrian (like pincers, spikes and claws) was really starting to come in handy. But there is one adaptation that may have been Trilobites kryptonite “Jaws”. About 420 million years ago, the first jaws fish appeared and some experts think that these new predators put even a more evolutionary pressure on the Trilobites. Sure enough, about 20 million years after the first appearance of the jaw, a new variation of spiny, spiky Trilobites started showing up in the fossil record like Dicranurus. But Earth’s environment had still more surprises in store, starting around 375 million years ago when we don’t know what happened exactly. But we do know the results were, suddenly big swaths of carbon-rich sediments were lower down and oxygen levels in the water started to droop. Reef communities began to collapse and those new fish with jaws managed to get by as did new life forms on land like plants and insects. But, most jawless bottom feeders, including the Trilobites, vanished. In the end, these series of unfortunate events known as the Late Devonian Extinction wiped out about 20% of the animal families in the sea, including all but four families of Trilobites.
252 million years ago something changed radically in our planet’s atmosphere. Some scientists think it was an asteroid impact. Other points out that it was also a time of tremendous volcanic activity and on top of that Earth’s landmasses were changing, which altered climate patterns yet again. Whatever came knocking for Trilobites, it came quickly. Within probably less than a million years, 70% of the species on land were gone and 95% of those in the sea, including the last surviving species of Trilobites.
Anyway, the tale of the Trilobites is really the first success story for the entire animal kingdom. Sure they are gone now, but at the time they were the most advanced animal the world has ever seen and they went on to become one of the most diverse groups of extinct organisms on record with over 15,000 described species. They survived for way longer than many other types of animals. They were around longer than the non-avian dinosaurs and longer than mammals have been, including us. So the question to ask about Trilobites might not be why they extinct, but how they managed to endure for so long. After all, today we are one of the planet’s most successful animals, so the Trilobites trouble may someday be our own.