Liberal Debutante

Trilobites

Pronounced: try-lo-bite meaning “three lobes”
Animal type: trilobite arthropod
Time: 520 - 248 mya
Size: 5 mm - 80 cm
Found: All over, yo.
Diet: Scavenger
Fossil:

These little guys are famous. They’re the biggest success story coming out of the palaeozoic era. They look a little like a termite you might find running around your downtown basement apartment but really they are an extinct order of marine arthropods who survived for an extremely long period of time. This long time frame resulted in over 15 000 different species of trilobites the largest being Isotelus found in 1998 by a Canadian scientist on the shore of Hudson Bay. The longest living was the Proetida, the only trilobite to not go extinct in the Devonian extinction, making it all the way to the Permian extinction.

Based on the similarities between the Spriggina and parvancoria from the precambrian it is possible that the trilobite may have evolved from these trilobitomorphs of the Ediacaran period. Which all probably shared a common ancestor that is currently unknown.

The trilobite bodies are divided into three different parts, the cephalon, a thorax and a pygidium. The cephalon is its head comprised of two preoral and four postoral segments all stuck together. The thorax is the trilobite’s flexible torso and the pygidium is the tail comprised of a few segments fused together with the telson. Each trilobite had two antennae, six legs and a gill branch used to breath while swimming.

Despite their heavy exoskeleton made of calcite and calcium phosphate minerals the trilobite was at the very bottom of the food chain being eaten by a whole range of predators. Many of the fossils have damaged shells where the trilobite was attacked by predators. These shells would fully shed off of the trilobite - which is also an explanation for the amount of fossils that are found of the trilobite, a fully shed shell should easily be mistaken for a not so well preserved fossil. To protect themselves some would use their flexible torso to curl up into a ball, others would live in burrows, or had spines or knobbly shells. They were also one of the first animals to develop eyesight. Their eyes were complex and delicate, as they were made up of dozens upon dozens of small crystal lenses. The eyes of the trilobite were not as advanced as most animals, but were sensitive to motion warning the trilobite of predators.

In some areas of the world, like in North America, the trilobite fossils have actually be preserved in three dimensions, complete with the fragile spines and delicate crystal eyes. For example in the Burgess Shale in Field, BC the rock outcrops have preserved not only the hard shell but also the soft parts of the trilobite body like their legs and internal organs. The abundance of trilobites helps with dating rock periods, scientists can date trilobite fossils to within a few million years.

Their exact reason for extinction isn’t know, but the number of trilobites started to decrease when the number of sharks and hinged jawed animals started to appear in the waters. The trilobites would have been an abundant and easy source of protein and food for these new marine animals. The closest current animals that the trilobites would be the ancient ancestor of is the horseshoe crab and the cephalocarids.

Extensive information here.