INTRASPECIFIC MORPHOLOGICAL VARIATION IN LOWER CAMBRIAN DORYPYGID TRILOBITES

“Pete’s Unusual Fauna” is a highly diverse mid-Dyeran (Cambrian Stage 4) trilobite fauna from Nevada, described by Palmer (1964) as containing at least a dozen species, with most specimens representing the Dorypygidae (Corynexochida). However, there is considerable within-sample variation in purportedly taxonomically informative morphological features, such that it is often ambiguous whether among-specimen differences should be interpreted as intraspecific variation or interspecific disparity. This study quantifies the nature and degree of morphological variation among these trilobites and evaluates the extent to which the currently proposed species diagnoses are supported.

Geometric morphometric landmark coordinates were collected from a new, large collection of trilobites from the Unusual Fauna. The data were examined using principal component analysis and several methods of hierarchical clustering to determine whether the taxa described as occurring in the Unusual Fauna can be recovered as diagnosable groupings.

Most of the historically recognized taxa represented by multiple specimens in this collection were returned as unique groupings by at least some clustering algorithms on the basis of pygidial morphology. Intraspecific morphological variation is present but not sufficient to render taxonomic assignment ambiguous. However, none of the dorypygid species are found to be distinguishable on the basis of cranidial morphology. Thus, although cranidial morphology is crucial to distinguishing family-level taxa within the Corynexochida, cranidial characters previously described as taxonomically informative within the Dorypygidae are subject to a large amount of intraspecific variation and do not correspond to clear divisions between species. Additionally, Bonnia caperata (Palmer, 1964) is likely a junior synonym of Bonnia columbensis (Resser, 1936).

In summary, morphometric analyses largely support the validity of the historically recognized taxa. However, dorypygid systematics should emphasize pygidial features, and species diagnoses presently reliant on cranidial morphology should be critically re-evaluated.