2024-B3 Tetrapod diversity in the Purbeck Limestone Group of England: implications for Jurassic–Cretaceous faunal turnover

Overview

The Jurassic–Cretaceous (J/K) transition is an interval of important faunal turnover in Mesozoic terrestrial ecosystems, occasionally hypothesized to represent a poorly understood mass extinction event. Substantial faunal changes occur most notably within dinosaurs, where the sauropod and stegosaur-dominated herbivorous communities of the Late Jurassic are replaced in the Early Cretaceous by ornithopod-dominated communities. Other tetrapod groups such as lepidosaurs (lizards and snakes), amphibians and mammals show evidence for significant diversification around the J/K boundary. However, understanding of faunal change has been limited by the generally poor global record of latest Jurassic and earliest Cretaceous fossils.  

Rocks of the Purbeck Limestone Group crop out along the eastern end of the Jurassic Coast World Heritage Site in Dorset, UK, and date to the latest Jurassic to earliest Cretaceous. They formed in marginal and shallow marine environments, and have yielded >60 species of turtles, lepidosaurs, crocodilians, pterosaurs, dinosaurs and mammals. This diversity makes the Purbeck perhaps the best site globally to examine tetrapod communities and faunal change during the J/K transition. Most fossils are small and have most typically been collected using screenwashing to extract small teeth, jaws and other bone fragments from mudstone and marl horizons. Although this has been highly successful in increasing understanding of Purbeck tetrapod diversity, it has not been applied systematically throughout the formation to examine large-scale patterns of community structure and faunal change.   

In this field- and lab-based project, the student will systematically sample suitable horizons within the Purbeck Limestone Group both vertically through stratigraphy, and laterally at different outcrops across the coast and inland. Vertebrate microfossils will be extracted via screenwashing, identified, and combined with historical data to assess the diversity and ecological structure of Purbeck ecosystems, and their stability and variation through time. Re-examination of historically collected fossil material may be undertaken using the collections of the Natural History Museum in London and CT scanning approaches. All these data will be drawn together and combined with those from Late Jurassic (e.g. Morrison Formation, USA) and later Early Cretaceous (e.g. Wealden Group, England) ecosystems to better understand changes in community composition and ecological diversity through the J/K transition.  

Figure 1: Reconstruction by Dr Mark Witton of the Purbeck ecosystem, showing early mammals, a small theropod dinosaur and sauropod dinosaurs.