Subdivision of the Kimmeridgian Autissiodorensis Zone (Upper Jurassic, UK) John C.W. Cope a, *, Stephen M. Etches b a Department of Natural Sciences, Amgueddfa Cymru —National Museum Wales, Cardiff, CF10 3NP, United Kingdom b Museum of Jurassic Marine Life, Kimmeridge, Wareham, Dorset, BH20 5PE, United Kingdom A R T I C L E I N F O Article history: Received 30 October 2019 Received in revised form 18 March 2020 Accepted 20 March 2020 Available online xxx Keywords: Kimmeridgian Stage Autissiodorensis Zone Autissiodorenis Subzone Camericensis ubzone A B S T R A C T Ammonites hitherto included in the species Aulacostephanus autissiodorensis in fact belong to two quite separate forms that as well as being readily distinguishable morphologically have different stratigraphical ranges. A new species, Aulacostephanus camericensis, is proposed for forms which are equicostate throughout and the name A. autissiodorensis restricted to later forms that lose their ornamentation on the outer whorl. Two new subzones are proposed for the Autissiodorensis Zone: a lower Camericensis Subzone and an upper Autissiodorensis Subzone. © 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction The zonal division of the Kimmeridgian Stage, represented in Dorset by the Lower Kimmeridge Clay, was initially the work of Salfeld (1913). Some of his original zones have survived until the present day, but his upper two zones, of Aulacostephanus yo and Aulacostephanus pseudomutabilis have been abandoned, following work by Ziegler (1962) who reported that the former species, typical of the southern Paris Basin and north-west Germany and also known from the Boulonnais, appeared unknown in Britain (though it has recently been found at Kimmeridge; see Gallois et al., 2015), whereas Aulacostephanus eudoxus (d’Orbigny) was a common and widespread species at that level; additionally Neumayr (1873) had already cited a Eudoxus Zone, thus making it the senior-most available name for the lower zone. Ziegler (1961) had earlier realised that Salfeld had commonly misidenti- fied Aulacostephanus autissiodorensis (Cotteau) as A. pseudomuta- bilis (de Loriol); the latter species seems restricted to the underlying Eudoxus Zone and thus Ziegler (1962) made A. autissiodorensis the index for the uppermost zone of the Kimmeridgian Stage. 2. The Kimmeridge section Although much of the Lower Kimmeridge Clay is not exposed at Kimmeridge, the section from the upper half of the Eudoxus Zone through the remainder of the Kimmeridge Clay Formation and into the Portland Group is virtually uninterrupted in a magnificent coastal section along part of the World Heritage Site known as ‘The Jurassic Coast’. Since the adoption of the Tithonian Stage as the terminal Jurassic Stage, the underlying Kimmeridgian Stage can now only be interpreted in its shorter (sensu gallico) sense. For the beds that were formerly classified as Upper Kimmeridgian (sensu anglico) Cope (1993) suggested that Blake’s (1881) proposal of a Bolonian Stage be accepted, as it is not possible to correlate accurately the upper part of the Kimmeridge Clay and the Portland Group with the Tithonian Primary Standard. This is because of extreme provincialism of the ammonite faunas immediately above the Autissiodorensis Zone. Gallois and Cox (1976) recognised that lithological character- istics of the Kimmeridge Clay enabled them to identify units of widespread lithological similarities that appeared to have chro- nostratigraphical significance; these were later applied to the Dorset succession (Cox and Gallois, 1981). For the upper part of the Kimmeridgian Stage these divisions, which they named ‘Beds’, do indeed appear to have some such value, but for some lower horizons in the Kimmeridgian Stage (and more particularly in the Bolonian Stage) the scheme is not so satisfactory, basically because the numbering scheme was introduced in East Anglia where the succession is considerably attenuated. Additionally it cannot be used in correlation from basinal to nearer-shore sequences (as, for * Corresponding author. E-mail address: john.cope@museumwales.ac.uk (J.C.W. Cope). https://doi.org/10.1016/j.pgeola.2020.03.006 0016-7878/© 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved. Proceedings of the Geologists’ Association xxx (2019) xxx–xxx G Model PGEOLA 823 No. of Pages 7 Please cite this article in press as: J.C.W. Cope, S.M. Etches, Subdivision of the Kimmeridgian Autissiodorensis Zone (Upper Jurassic, UK), Proc. Geol. Assoc. (2020), https://doi.org/10.1016/j.pgeola.2020.03.006 Contents lists available at ScienceDirect Proceedings of the Geologists’ Association journa l homepage : www.e lsevier.com/loca te/pgeola