INTRASPECIFIC VARIATION IN TRILOBITES FROM THE TETHYAN HIMALAYA AND THE CAMBRIAN BIOGEOGRAPHY OF EQUATORIAL PERI-GONDWANALAND

Much previous work on the Cambrian trilobite fauna in the Himalaya has utilized tectonically deformed specimens preserved in shale. This deformation has hindered taxonomic appraisal of these trilobites, and thus reduced their biogeographic and biostratigraphic utility. Collections recently made from Spiti and Zanskar have yielded specimens preserved in limestone that show little evidence of tectonic deformation. The preservational quality of these fossils allows morphometric techniques to be employed during systematic evaluation of the faunas. A locality at the base of the Spiti section, PO3, is one of these well-preserved limestone localities and is of latest Lower Cambrian or earliest Middle Cambrian age. PO3 has an abundant, low diversity trilobite fauna. There are two species of ptychoparoid, distinguishable by cranidium ornamentation, and a large number of dolichometropid trilobites that show a consistent set of meristic characters that are markedly variable in their relative proportions. An analysis of the dolichometropid trilobites, using both traditional morphometrics and geometric methods based on 14 cranidial landmarks, has shown that the morphological variation among these specimens was continuous. The distributions of the traditional morphometric ratios were approximately normal. The superimposition data were used to create a Procrustes score for each specimen and a mean score for the sample. The distribution of distances of individuals from the mean was approximately normal. The results of the morphometric analyses suggest that the dolichometropid trilobites at PO3 are all members of a single, but variable, morphological species. These results confirm the impression that previous work in the region has taxonomically over-split species from the shale fauna. The presence of continuous intraspecific variation indicates that a broader species concepts than has been used previously should be applied. The Himalayan faunas contain an equatorial peri-Gondwanan fauna, with elements in common with several outboard terranes, including the Yangtze block.