|Asaphus, a trilobite genus common in the Ordovician|
Trilobites belong to the animal phylum Arthropoda. Arthropod means jointed legs, one of the characteristics that define the group of segmented, jointed-leggedness, hard-bodied animals that include the insects, spiders, scorpions, crabs, and lobsters.
|Horseshoe crab Limulus polyphemus from the Gulf of Mexico. |
Note the segmented body, carapace, and jointed legs.
These are characteristics of the Arthropods.
Trilobites appeared on the Earth’s fossil record since the early Cambrian, around 540 million years ago, and went extinct during the massive extinction event that took place in the Permian-Triassic, around 250 million years ago. Trilobites had suffered previous minor extinctions before. The first occurring soon after their origination, during the late Cambrian, around 485 million years ago.
Yet, their acme or time of greatest diversity and distribution, occurred during the Ordovician, and lasted for millions of years until the Devonian period, when fish-like predators began to keep them in check. Their large eyes, capable of all-around vision, and hard segmented bodies (carapace) likely arose out of the necessity to look out and scape from such predators. Their interesting and unique eye characteristics were reminiscent of frogs. For that reason, scientists gave them names such as Phacobs rana, or “frog-like eyes”, best known from fossils commonly found in northeastern North America.
|Phacops rana: trilobite species common during the north American Silurian-Devonian.|
Note its segmented, frog-like eyes with minute lenses.
Trilobites inhabited shallow, shelf, marine environments. By having eyes and eyespots on the top of their heads, trilobites could watch the world above their backs. The hard, segmented body, allowed for multiple forms of locomotion, swimming, and flexibility. Some species were more swimmers, others more bottom crawlers. Their tracks are known from rocks made from ocean bottom muds. They fed mostly on anything, from small organisms to organic matter and sea bottom detritus. This included dead organisms that fell down to the ocean floor. This made them, sort of, ocean bottom vultures or cockroaches. That goes besides their obvious physical similarity.
An analogous species today would be the horseshoe crab or lobsters.
|Horseshoe crab Limulus polyphemus from the Gulf of Mexico.|