Diversity and ontogeny of Pitus tree-trunks in the early Mississippian rocks of the Isle of Bute, Scotland: The importance of sample size and quantitative analysis for fossil wood systematics

Abstract

In a recent paper, we described tree trunks of Pitus primaeva from early Mississippian (?late Tournaisian) strata of the Isle of Bute, Scotland based on a small sample size (n = 3) and semi-quantitative analysis. In this follow-up study, we analyse a much larger population of specimens (n = 34) and conduct Principal Component Analysis (PCA) of quantitative anatomical data to discriminate ontogenetic and phylogenetic variants. This larger study reveals the existence of six morphotypes, which represent ontogenetic variants of three distinct biological taxa, Pitus antiqua, P. withamii, and P. primaeva/ P. rotunda. Of these, P. primaeva/ P. rotunda is overwhelmingly dominant (~ 90%) and we hypothesise that the ontogenetic relationship of P. primaeva to P. rotunda, is that of a tree trunk to its stump or root. Our work, which uncovers an unusually high diversity of Pitus taxa for a single locality, emphasises the need for large, quantitative studies in order to capture total diversity and elucidate ontogenetic variability in fossil wood assemblages. It also significantly extends the lower stratigraphic range for P. withamii, which was previously known only from late Visean-early Serpukhovian strata. Facies analysis of the Montford site, coupled with growth ring observations, shows that the Pitus trees grew on seasonally dry, well-drained floodplains, and were joined by dense stands of Archaeocalamites during intermittent volcanic eruptions.