Article: Ontogeny, hypostome attachment and trilobite classification
The high level classification of trilobites has proved particularly difficult. This paper discusses the classification of those trilobites which have been placed in the Order Ptychopariida, together with other relevant groups, including Agnostida. Although often considered 'generalized', the ptychoparioids have a distinctive derived character: the hypostome is not exoskeletally connected to the cephalic doublure (natant hypostomal condition). The polarity of the natant hypostome as a derived character is supported both by known ontogenies, and by comparison with other trilobites, and is unusual in primitive arthropods as a whole. The wide distribution within the Trilobita of the natant hypostomal condition is established. The primitive state, found in redlichiids, is conterminant, in which the hypostome is attached to the cephalic doublure and closely corresponds ventrally with the frontal glabellar lobe. Several trilobite groups which primitively had natant hypostomal condition became secondarily conterminant, e.g. among Asaphida and Proetida. All trilobites having natant hypostomal condition, together with those which were primitively natant and only secondarily conterminant or impendent, are believed to constitute a new monophyletic group termed the Subclass Libristoma. This group includes the majority of the trilobites. A number of major monophyletic groups within the Libristoma are recognized, and one or more autapomorphy for each group is described. The Ptychopariida can be reduced in scope to a paraphyletic group of primitive libristomates which cannot yet be assigned to a clade. The other major groups of Libristoma include: Asaphida, Proetida, Olenina, Harpina. There is slim evidence that the Phacopida also had libristomate ancestry. Some groups included in Ptychopariida in the 1959 Treatise can now be assigned to different higher taxa. Illaenacea (= Scutelluina) and Leiostegiacea probably belong within a large corynexochoid clade; Damesellacea are regarded as the primitive sister group of Odontopleurida. Agnostida are not libristomates, but they are trilobites; they are more advanced than olenellids, and comprise the sister taxon of Redlichiida + all other trilobites. An important autapomorphy defining Agnostida is the loss of calcification of an olenelloid-like rostral plate.Natant hypostomes are conservative in morphology, and it is often difficult to assign the hypostome among natant families. Specialized hypostomes are associated with the conterminant or impendent hypostomal condition. The natant condition is lost polyphyletically, and there is some evidence that secondarily conterminant trilobites arose when primarily conterminant groups became extinct. The relationships of subgroups within the Libristoma, and the relationships of other major conterminant taxa (corynexochoids, Odontopleurida, Lichida, Phacopida) to one another and to the Libristoma remain to be explored before the taxonomic status of major groups can be decided. The Redlichiida is probably another paraphyletic group having the same relationship to all higher trilobites as does the Ptychopariida within the Libristoma.