Abstract

Cambrian ptychopariids have defied most attempts at higher classification. Even at the generic level, there is no consensus as to what constitutes identification criteria in these morphologically conservative trilobites. The problem is more acute when there is a lack of knowledge about the different sclerites of species, the presence of species with morphologies intermediate between two genera, the naming of genera where incompletely known or poor quality specimens were used for type species, and the use of character checklists to generate diagnoses for higher taxa. This study on the kochaspid trilobites (Lower to Middle Cambrian) uses several available representatives of each genus to alleviate or assess some of these problems. Successive reweighting of characters is used for the assessment of 70 characters among 66 taxa as to their importance in forming clades. Results of this study include five important points about ptychopariid classification: 1) knowledge about noncranidial sclerites is important for the proper placement of taxa that have generalized morphologies; 2) in contrast, taxa that have more derived cranidial morphology can be accurately placed within a clade; 3) intermediate morphologies illustrate a close link between genera, but these forms can present nomenclatural problems in their placement; 4) although genera defined by type species based on incompletely described or poorly preserved specimens can create nomenclatural nightmares, other species assigned to these genera are not dependent on their previous taxonomic assignment for their placement in the cladogram; and 5) checklists of morphological characters cannot be used to recognize ptychopariid groups, since the level of homoplasy is too high and there are too many reversals of character states within a clade. Instead, a list of character states that typify a taxonomic group is more useful and will lead to a more stable classification.

This cladistic analysis of kochaspid trilobites indicates that 64 of the 70 characters were useful in the formation of clades. Fifty-six cranidial, librigenal, thoracic, and pygidial characters were useful in typifying larger and smaller clades. The analysis also indicates that Eokochaspis, Nyella, and Onchocephalus are polyphyletic. Reassignments based on the cladistic analysis include Schistometopus collaris to Caborcella, and Syspacephalus obscurus to Mexicella.

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