STRATIGRAPHIC TRENDS IN MORPHOLOGY: THE EVOLUTION OF BRISTOLIA (TRILOBITA, CAMBRIAN)

Bristolia is a genus of olenelloid trilobite confined to a single sequence stratigraphic cycle within Lower Cambrian shelfal deposits of the southwestern US. Specimens obtained by continuous sampling through the Latham Shale in the Marble Mountains of California were supplemented with collections from correlative sections in California and Nevada and from museums. This comprehensive sampling of Bristolia throughout its stratigraphic and geographic range permits detailed investigation of morphological evolution of the genus within a tightly constrained stratigraphic and environmental framework.

Cephalic morphology was summarized using traditional univariate measures and landmark-based geometric morphometric methods. Similarity of form among specimens was assessed by conducting principal components analysis of partial warp scores, following thin-plate spline analysis of their superimposed landmark configurations.

Univariate and multivariate analyses demonstrate that specimens fall within a gradational morphological spectrum including the B. mohavensis, B. harringtoni, B. bristolensis, and B. fragilis morphotypes. Bristolia insolens, with its extreme genal spine advancement and adgenal angle development, represents a unique morphotype occupying a separate area of morphospace.

The data show a dynamic morphological trend when examined within stratigraphic context. From the stratigraphically oldest B. mohavensis morphotype, collections show a trend of increased adgenal angle and genal spine advancement with loss of the preglabellar field, passing through the B. harringtoni morphotype and culminating in the B. bristolensis morphotype. This trend corresponds to relative water deepening during a transgressive systems tract. During the subsequent highstand relative shallowing, collections show a trend back towards less pronounced adgenal angle and genal spine advancement, passing from the B. bristolensis to the B. fragilis morphotype. Bristolia insolens represents an extreme extrapolation of the earlier trend and is restricted to a narrow stratigraphic interval at maximal flooding.

Such stratigraphically and environmentally consistent morphological trends raise the possibility of anagenetic evolution or ecophenotypy within a single phylogenetic lineage.