Completeness and diversity patterns of Palaeozoic chondrichthyans

Abstract

Chondrichthyans, the cartilaginous fishes, are a highly successful clade of vertebrates that have occupied the aquatic realm for over 450 million years. Of these, the Palaeozoic represents a key time interval in terms of their origin and early diversification, yet much of their early macroevolutionary patterns remains unresolved because of the limitations of their fossil record. Their early fossil record is poor because their mostly cartilaginous skeletons are rarely well preserved and this, in turn, has restricted the data available for estimates of their evolutionary history. In this thesis, multiple statistical methods for quantifying the skeletal completeness and estimating diversity trends are combined to critically assess the quality of the Palaeozoic chondrichthyan fossil record. A novel metric is presented to quantify levels of soft tissue preservation next to skeletal completeness in the stem-chondrichthyan acanthodians which are known to preserve notoriously little skeletal features. The results identify a complex array of natural and human-driven biases that greatly influences our understanding of the Palaeozoic chondrichthyan fossil record. Variation in completeness impacts on the utility of characters for assessing phylogenetic relationships of the various chondrichthyan groups, specifically the stem-chondrichthyan lineages. An extensive assessment of early chondrichthyan diversity patterns give greater insight on their environmental and biological drivers but also highlight the impact sampling biases may have on them. Together, the quantified macroevolutionary patterns of completeness and diversity draw a more comprehensive picture of the evolutionary history and nature of the early chondrichthyan fossil record