Lichida

Lichida Temporal range: Ordovician–Devonian PreЄ Є O S D C P T J K Pg N
Terataspis grandis
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Trilobita
Order:	Lichida Moore, 1959
Superfamilies
Dameselloidea Lichoidea Odontopleuroidea

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

Morphology

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.

Evolution

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 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.

References

• Lichida Guide - from the Guide to the 8 Orders of Trilobites by Sam Gon III.
• Margaret J. Campbell & Brian D. E. Chatterton, 2009. Silurian Lichid Trilobites from Northwestern Canada: Ontogeny and Phylogeny of Lichids. Journal of Paleontology 83 (2), p. 263-79 DOI: 10.1666/07-140R1.1
• H. B. Whittington, 2002. Lichidae (Trilobita): Morphology and Classification. Journal of Paleontology 76 (2), p 306-20; DOI: 10.1666/0022-3360(2002)076<0306:LTMAC>2.0.CO;2