Corynexochid Trilobites

Trilobite Orders Ptychopariida and Corynexochida both have related taxa that could be placed in Redlichiida, making both orders paraphyletic (Lieberman, 2010). In fact, Ptychopariida Superfamily Ellipsocephaloidea was once placed within Order Redlichiida, but is now accepted as an evolutionary step from the Redlichiids to the Ptychopariids. A trait of greatest importance for placement of a trilobite in Corynexochida is that the protaspis' hypostome is conterminant and remains so during development and in the holaspid period. This is in contrast with
Order Ptychopariida, in which the hypostome is also conterminant in the protaspis, but becomes natant early in development and remains natant in the holaspid.

Corynexochida was greatly expanded by Fortey (1990) compared with the 1959 Treatise (Rasetti and Moore, 1959), where it contained only Cambrian families, to include Leiostegioidea and Scutelluina. To unite the expanded order, Fortey used the characteristically clavate glabellar shape, often concavesided, and frequently with splayed glabellar furrows, and conterminant hypostomal attachment. Because Ontogenies are well known for a number of genera (e.g., Chatterton, 1980), including late protaspides very like those of Illaenidae, the Illaenids were incorporated in the Corynexochida; hypostomes and rostral plate structures were also noted to be comparable between the two groups. Fortey also notes the primary differentiating characteristic between Cambrian and later corynexochids is that Cambrian Corynexochids are typified by the rostral plate that is fused to the hypostome, in contrast to Ordovician and younger representatives.

With the 1990 revisions, the morphological features uniting the Corynexochids are: opisthoparian sutures; elongate pestle-shaped glabella, splayed furrows (unless effacement is apparent) with typically backward-pointing hind pair and anterior pairs pointing increasingly forward toward the cephalon; sometimes with pit-like furrows; ledge like cranidial borders, either conterminant or (in more evolved derived forms) impendent hypostome, and large eyes (sometimes arcuate). The Corynexochid thorax typically has 7 to 8 segments, often with spinose pleural tips, but species with from 2 to 12 segments are known. The pygidium is usually large, isopygous or subisopygous, of variable form, and sometimes spinose.

Corynexochid Trilobite Fossils