Corynexochid
Trilobites
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within: Trilobita Also see: Trilobite Classification |
| The members of trilobite Order Corynexochida (pronounced Cory-nex-o-chee-da) comprise a large and morophologically diverse and complex trilobite clade with a biostratigraphy range from the Lower Cambrian to the Middle Devonian. The Corynexochida are divided into three suborders, Corynexochina, Illaenina and Leiostegiina that have distinct differentiating characteristics. Strong evidence supports the well-accepted phylogeny that the corynexochids descended from the Redlichiina in the Lower Cambrian.
Order Corynexochida was greatly expanded by Fortey (1990) from the 1959 Treatise (Rasetti in 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 differenciating characteristic between Cambrian and later corynexochids. 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 Corynexochida are: opisthoparian sutures; elongate pestle-shaped glabella, played furrows (unless effacement evident) 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 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. (see trilobites glossay of terms)
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