INTRASPECIFIC VARIATION AND MORPHOLOGICAL EVOLUTION IN THE EARLY CAMBRIAN TRILOBITE BRISTOLIA (OLENELLOIDEA)

Bristolia is a morphologically diverse trilobite genus common in Lower Cambrian shelf deposits across the southwestern US, offering an ideal opportunity to study patterns of morphological variation and evolution at a variety of geographic and temporal scales.

Continuous sampling through the Latham Shale in the southern Marble Mountains yielded many Bristolia specimens, permitting detailed investigation of patterns of morphological variation within a tightly constrained stratigraphic framework. These new collections were supplemented by material collected in stratigraphic context from correlative sections in Nevada, and by existing museum collections with little or no stratigraphic information.

Cephalic morphology was summarized using "traditional" univariate measures (intergenal angle, genal spine advancement) and landmark-based geometric morphometric methods. Similarity of form among specimens was assessed in geometric morphometric terms using relative warp scores, following thin-plate spline analysis of their superimposed landmark configurations.

Taken out of stratigraphic context, univariate and multivariate measures of morphological variance separate Bristolia specimens into two distinct groups: one representing B. insolens, the other comprising a gradational spectrum of morphologies including the B. mohavensis, B. harringtoni, and B. bristolensis morphotypes.

Examination of the second group within tight stratigraphic context reveals a dynamic morphological trend. From an initial B. mohavensis morphotype, the lineage undergoes gradational increase in intergenal angle and advancement of the genal spines, progressing through the B. harringtoni morphotype and culminating in the B. bristolensis morphotype. Stratigraphically higher specimens show a trend back to more acute intergenal angles and less advanced genal spines typical of B. fragilis. Collections from other localities demonstrate that this morphological trend was a geographically widespread phenomenon. Cladistic analyses support an evolutionary basis for the morphological trend.

Documentation of morphological variance and trends within and between collections in a tight stratigraphic framework has important ramifications in high-resolution phylogenetic and biostratigraphic studies.