Orders Ptychopariida and Corynexochida both have
taxa that could be placed in Redlichiida, making
2010). In fact, Ptychopariida Superfamily Ellipsocephaloidea
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
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.
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.
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.
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
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.