Editing Trilobite (section)

====Evolutionary trends====
Principal evolutionary trends from primitive morphologies, such as exemplified by ''[[Eoredlichia]]'',<ref name=Fortey&Owens97a /> include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy), and development of extreme spinosity in certain groups.<ref name="Clarkson98"/> Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.<ref name=Fortey&Owens97a>{{Citation |last1=Fortey |first1=R. A. |last2=Owens |first2=R. M. |contribution=Evolutionary History |editor-last=Kaesler |editor-first=R. L. |title=Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida |pages=[https://archive.org/details/treatiseoninvert0002unse/page/249 249–287] |publisher=The Geological Society of America, Inc. & The University of Kansas |place=Boulder, CO & Lawrence, KS |year=1997 |isbn=978-0-8137-3115-5 |url=https://archive.org/details/treatiseoninvert0002unse/page/249 }}</ref> Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures, and hypostome specialization.<ref name=Fortey&Owens97a /> Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).<ref name=Fortey&Owens97a />

Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend. Notable examples of this were the [[order (biology)|order]]s [[Agnostida]] and [[Asaphida]], and the [[suborder]] [[Illaenina]] of the [[Corynexochida]]. Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for [[taxonomist]]s since the loss of details (particularly of the [[Glabella (trilobite)|glabella]]) can make the determination of [[phylogenetics|phylogenetic]] relationships difficult.<ref name="efface">{{cite web|url=http://www.trilobites.info/trends.htm|title=Evolutionary Trends in Trilobites|author=Samuel M. Gon III|date=July 20, 2008|publisher=A Guide to the Orders of Trilobites|access-date=April 14, 2011|archive-date=May 17, 2013|archive-url=https://web.archive.org/web/20130517123118/http://www.trilobites.info/trends.htm|url-status=dead}}</ref>