Article: Classification of the trilobite suborder Asaphina

We present a new phylogenetic classification of trilobites which can be included in a ptychoparioid suborder Asaphina, considerably extending the range of families included in the group as compared with existing classifications. Much of the group is known to be united by the possession of a distinctive type of inflated and effaced larva termed the asaphoid protaspis. The morphology and occurrence of this kind of protaspis is reviewed. All of the group has a ventral median suture, except where it was secondarily lost through fusion of the free cheeks, and most morphological evidence is considered to favour a monophyletic origin for this structure. Relationships between families having such a suture are based on the analysis of morphology; however, stratigraphy is relevant to the determination of the sequencing of characters within a family and to the identification of minor character reversals and parallelisms which can be discounted in the higher level analysis. Two methods of analysis are used. One produces a cladogram based on our weighted assessment of the most important characters. The other, a computer-based analysis using the PAUP program, uses a much wider range of characters to produce two trees which are equally likely. There is generally good agreement between the different methods of analysis. As thus defined, Asaphina includes Cyclopygacea (comprising Cyclopygidae, Nileidae, and Taihungshaniidae), Asaphacea (Asaphidae and Ceratopygidae), Remopleuridacea, and Dikelo-cephalacea, together with some more primitive families which are more difficult to classify: Dikelokephalinidae, Pterocephaliidae, and Anomocaracea. We make a case that the Trinucleacea are linked to the Asaphina by more characters than to any other group. Trilobites included within the suborder are discussed family by family. The supposed olenid Hedinaspis should be included in Asaphacea; on the other hand, the Olenidae, which were included in Asaphina by Bergstrom (1973), are unrelated to the families considered here. The Asaphina was diverse from the mid-Cambrian until the end of the Ordovician, when the group was particularly vulnerable to extinction; this may have been connected with the planktic specialization of the asaphoid larva.