Lichida

From Palaeos.org

o Arthropoda
`--o TRILOBITA
   |?-Agnostina
   `--+--o Redlichiida
      |  |--Olenellina
      |  `--Redlichiina
      |--Corynexochida
      |--Phacopida
      |--Lichida
      `--o Librostoma
         |--Proetida
         |--Harpetida [Harpoidea]
         |--Ptychopariida
         |?-Eodiscina
         `--Asaphida

The Middle Silurian trilobite Arctinurus most probably Arctinurus boltoni (Bigsby), known from the Clintonian stage of New York; illustration from Systeme Silurian du Centre De La Boheme par Joachim Barrande (circa 1852). Photo by Roger Perkins. c.f. Moore, Lalicker and Fischer, Invertebrate Fossils p.506 fig.10; length of original animal about 15 cm

Introduction

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.

Among the most extraordinary of these forms is the gigantic Early Devonian species Terataspis

It has been suggested that the spiny character of these trilobites came about as a defense against the rapidly evolving fishes of the Devonian period, though, this explanation can not account for the fact that the earliest lichids that arose during the Ordovician were already incredible spinose (some genera even lasted well into the Silurian and even Devonian, in fact), while fishes with jaws first arose some time during the early Silurian.

Links

• Lichida Guide - from the Guide to the 8 Orders of Trilobites by Sam Gon III.

Credits

Introduction by DanielCD Wikipedia