Lichida
Encyclopedia
Lichida is an order of typically spiny trilobite
that lived from the Ordovician
to the Devonian
period.
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.
es 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.
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
that lived from the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
to the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period.
Morphology
These trilobites usually have 8-13 thoracicThorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
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
Pygidium
The pygidium is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor...
(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 fishFish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
es of the Silurian and Devonian periods. However, this fails to explain why the lichid trilobites, especially those of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
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.