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

The Jurassic-Cretaceous (J/K) transition is an interval of important faunal turnover in Mesozoic terrestrial ecosystems, often hypothesised 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, our 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 outcrop along the eastern end of the Jurassic Coast World Heritage Site in Dorset, and date from the latest Jurassic to earliest Cretaceous. They formed in marginal and shallow marine environments, and have yielded more than 60 species of turtles, lepidosaurs, crocodilians, pterosaurs, dinosaurs and mammals. This diversity makes the Purbeck Limestone 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.

This project involves the systematic sampling of 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 diversity through the J/K transition.

Fieldwork will systematically target clay-rich horizons within the Purbeck, extending both vertically and geographically along the coast and inland. Sediment collected at each of these horizons will be processed using standard lab techniques to extract microvertebrates. These will be identified using literature and comparisons to the collections of the Natural History Museum (NHM).

Where new taxonomic groups or species are identified these will be published as standalone papers. CT scanning will be used to re-examine historically collected material of key groups to better constrain taxonomy. These data will be combined with historical collections from literature and museums to quantify ecological diversity and structure within individual horizons and through the Group and to allow wider comparisons with Late Jurassic and Early Cretaceous communities from other localities globally.

This project will provide a broad base of palaeobiological skills. Field- and lab-based work will include the development of skills in planning and executing fieldwork, sedimentology, palaeoenvironmental interpretation, lab-based microvertebrate processing, microfossil imaging, taxonomy and anatomy. CT-based examination of historical material will develop additional skills in CT scanning, segmentation and 3D visualisation.

The analytical phase of the project will develop skills in statistical programming and quantitative analysis of diversity change.