Name Meaning: Beautiful Cover
Geologic Era: Late Ordovician to Silurian
Size: 7 cm (about 2.5 inches)
Location Found: Morocco
Extinction Date: Later Permian (last trilobites)
Calymene trilobites are among the most common trilobites found, with a worldwide distribution. Like the Eurypterids, fossil trilobites are often shed exoskeletons, though full animals are also commonly found. Trilobites are often found in large groups, though they may or may not have died at the same time. Ocean currents could have deposited them in one place, or washed them up on beaches. Most Calymene trilobites are relatively small, though larger species could reach sizes of up to 70 centimeters (28 inches).
Our specimen still has a good deal of matrix clinging to it, and I am hesitant to try my luck removing it. The fossil itself is very light, and probably very brittle. Much as I would like to clean it up a bit, I probably won’t until I get some professional equipment.
Trilobite bodies can be divided up in two ways. The first is laterally, shown in the first diagram. The “head” region is the cephalon (blue), the thorax region (yellow) is the body, and the pygidium (tail) is shown in purple. Any eyes present would have been on the cephalon, while leg-like appendages would have been present on the thorax. The head and tail segements were fused, and allowed for limited movement, while the thorax was reasonably flexible. Some trilobites are found enrolled, perhaps trying to protect their undersides, which also attests to the flexibility of the middle segment.
The second way a trilobite can be divided is longitudinally. In this case, the middle segment is called the axis lobe or axial lobe, while the lateral segments are called pleural lobes. This is where the name “trilobite” comes from. Trilobites could have employed any movement style from crawling in sediment to open water swimming. Some trilobites had long spines on their bodies, while others had enormous eyes.
Trilobites were some of the most diverse and successful animals in Earth’s history, with around 15,000 species being identified to date. This amount of diversity rivals even modern animals. Some species, such as Calymene, are so common they are used as index fossils. Trilobites went into decline after the Silurian period, like the Eurypterids, being out-competed by bony and cartilaginous fishes. Some species held on until the end of the Permian, but also died in the Permian-Triassic extinction event.
Body Diagram:By Sam Gon III, derivative by Obsidian Soul [CC0], via Wikimedia Commons