STRATIGRAPHIC CORRELATION AND MORPHOLOGIC VARIATION OF MIDDLE CAMBRIAN TRILOBITES IN THE HOUSE AND DRUM RANGES, WEST-CENTRAL UTAH

If morphologic variation in trilobite populations is considered as a proxy for genetic divergence, a key component of speciation, then comparison of morphology between populations may allow an inference of the degree of gene flow occurring between these populations, using the classic population genetic models of Ronald Fisher and Sewell Wright. Populations showing little morphologic diversity between populations may be inferred to possess high rates of gene flow between populations or may exist in environments where strong selective pressures constrain morphologic variance to low levels. Populations exhibiting high morphologic diversity between population may have limited gene flow between population or exist in environments with little selective pressure acting to constrain morphologic variation. Understanding the dynamics of population level morphologic variation is therefore useful, in combination with paleoecological assessments, in determining how environment and gene flow influence variation in trilobite populations of the region.

In working towards this goal the field collection of Ptychopariid trilobite specimens is combined with stratigraphic measurements at a scale of >1m in multiple localities of the Wheeler Shale. This mode of specimen collection allows an inference of relative time between deposition of the collected specimen populations of single locations along with determination of spatial distance along the Laurentian margin between temporally contemporary populations of collected specimens. Fossil specimens are digitally imaged at a known scale; the images are then used for landmark analysis of shape differences between populations of specimens from different localities and stratigraphic positions. Procrustes methods are used to isolate variance related to shape from variance associated with proportional size of specimens. Principal components derived from warp scores of measured specimen populations are compared and presented to demonstrate morphologic divergence among populations and the relationship of divergence to relative time and spatial distance between populations of specimens.