Trilobite evolution - Morphology

Redlichia 

Grand daddy

Crotalocephalina  Elongated body	Harpillaenus  Transverse form	Balcoracania Increased segments	Thoracocare  Decreased segments

Sam Gon III Trilobige page

Sam's Trilobite pages contain wonderful materials that introduce the work of evolutionary biologists and teaches also us laypeople how fossil evidence is studied in detail to suggest patterns of evolution. There is beautiful, almost mathematical, logic in the variations of the forms shown in these drawings.

Sam says
Through the 300 million years that trilobites existed, prior to their extinction in the Permian, there were many opportunities for diversification of form, starting from the presumed primitive morphology exemplified by a species such as Redlichia.

This typical primitive morphotype had a small pygidium, well developed eye ridges, a simple, lobed glabella, several thoracic segments, and a rather flattened body form. The first trilobites were characterized by this primitive form.

Among the over 20,000 species of described trilobites there are species in which aspects of morphology have diverged greatly from the primitive state. Thoracic segments were reduced to as few as two or increased to over 100, overall body shape was greatly elongated in some, or rendered transverse (widened) in others.
Sam