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

The outcrop of San José de Gracia, in Sonora, Mexico, is a well researched Cambrian stratigraphic sequence whose diversity has been studied for the past few decades. Although the fauna of agnostid trilobites in San José de Gracia has been restricted to the genus Peronopsis and Pentagnostus, the diverse morphology that has been reported for Peronopsis and the fragmentary nature of the material may hide the actual diversity of the agnostid fauna in San José de Gracia. In this paper, we assess the use of phylogenetic analyses for fragmentary material when performing stratigraphic calibration. The stratigraphic framework of San José Gracia has been thoroughly understood in the past decade, and we can use it as a reference point to explore the potential value of phylogenetics in stratigraphic calibration. Specimen-level phylogenetics and stratigraphic congruence utilising the data sets available in 2013, at the point of the collection, show that the conclusions on stratigraphic calibration coincide with the stratigraphic framework outlined a decade later as more material was collected and studied. Thus, fragmentary material should not be discarded a priori without assessing the potential phylogenetic signals that could provide scenarios and generate new hypotheses to test.