Systematics, biometry of the species Opsidiscus from the Middle Cambrian succession of the Spiti basin, India

The Middle Cambrian succession of the Parahio Valley in general and Debsakhad section in particular is dominated by trilobites. Genus Opsidiscus, along with other trilobite forms dominates the lower Middle Cambrian succession. In the present study two new species of Opsidiscus, Opsidiscus wadiai and Opsidiscus srikantiai are reported from the lower Middle Cambrian succession of Debsakhad section. The species were differentiated on their morphological features and their multivariate analysis. The multivariate analysis applied here is used as a supplement method to qualitative analysis in order to differentiate between the cranidial characters of Opsidiscus. The qualitative study of each species studied individually shows more or less close affiliation as is observed by different quantitative methods. The presence of Opsidiscus has a great stratigraphic significance in this region, as they first appear from informal Stage 5 of Series 3 and goes up to the Drumian Stage of Series 3 of the Cambrian System in the Debsakhad section, which helps to correlate this section with other well known sections of the Middle Cambrian.