The agnostid trilobite assemblage from San José de Gracia, Sonora, México, and the implications of phylogenetics in stratigraphic calibration

Abstract

ABSTRACT The outcrop of San José de Gracia yields an agnostid assemblage that has been assigned to the middle Cambrian. Although agnostid trilobites are important biostratigraphic markers, the mosaic evolution of this group makes species-level determination difficult, and thus biostratigraphic correlations become cumbersome. The assemblage is described here using morphogroups of the ‘peronopsid’-like agnostids, and specimen-level phylogenetics is used to assess the statistical significance of assigning different ages to the final topology (stratigraphic fit), including the morphogroups as operational taxonomic units. A proper phylogenetic analysis requires considering modularity and ontogenetic development in agnostids, but the overall topology can respond to different stratigraphic calibrations even if the individual relationships are not resolved. Using the distribution of several stratigraphic fit metrics, namely SCI, GER and MSM*, we find that the biozone Plagiura-Poliella (509–598.4 Mya) is the one that offers the best stratigraphic fit for the outcrop of San José de Gracia, and it is consistent with the current biostratigraphic framework that has been built for the middle Cambrian of Sonora.