The biostratigraphy and taxonomy of graptoloids from the Ordovician and Silurian of Britain

This thesis presents new graptoloid biostratigraphic range charts for the Ordovician and Silurian of Britain. The Ordovician graptoloid succession is represented by two separate biostratigraphic frameworks; one for England and Wales, another for Scotland, due to significant stratigraphic disparity between the graptoloid faunal assemblages of the two regions. The British Silurian sequence is represented by a single graptoloid biostratigraphic scheme reflecting more uniform graptoloid assemblages.;The reliability of the taxonomic foundation upon which the range charts are based is tested using the dominant Ludlow Series (Silurian) graptoloid subfamily Saetograptinae Urbanek. The results suggest that intraspecific variation has led to over-elaborate taxonomic subdivision of this group at the species and subspecies level. The poor preservation of some of the type material has resulted in problems of misidentification and reinforces the need for a full taxonomic review of the saetograptids. Despite this, three basic theca/spine relationships can be recognised which may serve as a basis for future species identification: a chimaera-type morphology whereby thecal spines project form the middle of the apertural margin; a leintwardinensis-type structure, in which the spine extends from the free ventral wall of the proceeding theca; and a colonus-type relationship whereby the lower part of the apertural margin is prolonged into a hook-like lip.;Graptoloid biodiversity data extracted from the new range charts highlights several periods of biotic crisis during the Ordovician and Silurian of the UK. These 'events' correlate with globally recognised intervals of major graptoloid extinctions, which in turn correspond with large-scale palaeoenvironmental fluctuations, such as eustatic sea-level and climate change. Patterns of morphological change within UK graptoloids are also linked to these biodiversity trends and consequently, to environmental controls.