MORPHOLOGICAL CHANGE IN CLOSELY-RELATED CAMBRIAN TRILOBITES DURING AN INTERVAL OF ENVIRONMENTAL CHANGE

High-resolution stratigraphic sampling of silicified limestone beds from several sites across east-central Nevada yielded undeformed ontogenetic material for three species of the Early Cambrian trilobite Zacanthopsis. These species occur in stratigraphic succession during an interval of increasing water depth. The primary morphological change in the cranidium during this interval is the expansion of the palpebral lobes relative to the width of the anterior facial sutures, a change also associated with increasing water depth in other trilobites. Principal components analysis of geometric morphometric data reveals that smaller specimens of all three species overlap in morphospace but diverge as specimen size increases. The youngest species shows a distinct decrease in rate of shape change at 1.4 mm in cephalic length, coincident with the onset of divergence between all three species. Growth vectors extracted from Procrustes coordinates of specimens larger than 1.4 mm show a significantly different pattern of shape change between the oldest and youngest species but not between successive species where the degree of morphological divergence is smaller. These results suggest that morphological change occurred through allometric repatterning, possibly in concert with increasing water depth. Phylogenetic analysis based on morphology yields several most parsimonious trees, a subset of which is consistent with this interpretation.