ABSTRACT Graptolites of the Pleurograptus linearis Biozone have long been known to straddle the boundary between the British Caradoc and Ashgill series (C/A) of the Ordovician. This is largely based on observations of these graptolites in the Whitehouse Subgroup of the Girvan area in Scotland. However, in the type Cautleyan-Rawtheyan area of northern England, graptolites of the same biozone range into the Rawtheyan Stage of the Ashgill Series, impacting the utility of the P. linearis Biozone for regional and global correlation around the C/A boundary. Here we use chitinozoan microfossils to correlate the strata in Girvan and Cautley in an independent manner. We present new chitinozoan data from an exhaustive sampling effort across the Whitehouse Subgroup from several locations in Girvan and correlate these with other key sections that contain graptolites from the P. linearis Biozone and index chitinozoans of Baltoscandian–Avalonian and Laurentian zonal schemes. During the Late Ordovician, Girvan occupied a pivotal geographical position allowing linkages between Laurentian faunas and those of both Baltica and Avalonia. Our samples yielded some typical Laurentian chitinozoan species (e.g., Hercochitina cristata , Kalochitina multispinata ), next to several well-known index species of mixed Baltoscandian–Avalonian affinity, relatable to the Fungochitina spinifera and Tanuchitina bergstroemi biozones and their subzones; the latter can be traced into key areas of Ordovician stratigraphy in England and Wales on Avalonia, and Denmark and Sweden on Baltica. These correlations suggest that the P. linearis Biozone at Girvan correlates to an interval starting in the Onnian and extending into some part of the (lower) Cautleyan Stage at Cautley, northern England. The Girvan district is confirmed as a key area to document mixed faunas from Laurentian–Avalonian–Baltic realms, and has potential as a stepping stone between rather distinct chitinozoan provinces.
LISSENS et al. (Wed,) studied this question.