Lichida

Lichida is an order of typically spiny trilobite that lived from the Ordovician to the Devonian period.

These trilobites usually have 8-13 thoracic segments. Their exoskeletons often have a grainy texture or have tubercles. Some species are extraordinarily spiny, having spiny thoracic segments that are as long or longer than the entire body, from cephalon (head) to pygidium (tail). The sections of the pygidia are leaf-like in shape and also typically end in spines.

It is often said and quoted that the spiny character of these trilobites could possibly have come about as a defense against the rapidly evolving jawed fishes of the Silurian and Devonian periods. However, this fails to explain why the the lichid trilobites, especially those of the family Odontopleuridae, first began evolving large spines (and in some cases, spiny spines) during the start of the Ordovician period, long before the advent of jawed vertebrates.