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Waterloo Bay, Larne, Northern Ireland: The ammonites of the earliest Jurassic 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 22a 29 28c 28b 28a 27 26 25 24 23b 23a 30 31 32 33a 33b 34 33c 33d 33e 33f 33g 22b 22c 22d 9 metres above base of measured section Caloceras sp. Caloceras johnstoni Psiloceras cf. plicatulum Psiloceras plicatulum Psiloceras plicatulum Psiloceras cf. plicatulum Psiloceras planorbis Neophyllites antecedens Neophyllites antecedens Neophyllites sp. Neophyllites imitans Psiloceras erugatum enlarged x3 Isocrinus sp.nov. enlarged x3 enlarged x3 Crinoids as a proxy for fully marine salinity Ossicles of a small crinoid (Isocrinus sp. nov.) appear abruptly and in abundance 2.95 metres below the first ammonites (Psiloceras erugatum). They are common throughout the succession up to at least the lower Johnstoni Subzone. The number of columnals per noditaxis is consistently different from both Isocrinus bavaricus (Rhaetian Kössen Beds) and Isocrinus psilonoti (Hettangian, NW Europe), indicating a distinct species. Crinoids are exclusively stenohaline organisms. Their abrupt appearance at this level may indicate the establishment of fully marine salinity following the quasi-marine conditions prevalent in the Penarth Group. It suggests that whatever factor controlled the appearance of ammonites in the region, it was not salinity. Evolute ammonite with costae apparently developed on the venter. Does this indicate the presence of Choristoceras, or is it merely a preservational artifact? Psiloceras erugatum This is stratigraphically the lowest ammonite recovered from the site. Ribbing persists unusually late in this large example. Psiloceras cf. sampsoni This highly evolute example is the stratigraphically highest example of sampsoni recovered from the site. top Bed 26 top Bed 25 top Bed 23 Bed 24 nodules top Bed 27 Ammonites are a conspicuous element of the earliest Jurassic macrofauna at this site. Most taxa are represented by at least some 3-dimensional material, preserved either in pyrite or early diagenetic carbonate concretions. Selected specimens are illustrated here at actual size, unless indicated otherwise. The strata in the picture to the left encompass the top of the ‘Pre-planorbis Beds’, the erugatum Horizon (Bed 24) and the stratigraphic range of Neophyllites (beds 25 to 27). The site has significant potential for designation as a Global Stratotype Section and Point for the base of the Jurassic System. In addition, this part of the foreshore would make an ideal stratotype location for the erugatum, imitans and antecedens biohorizons, for which surface stratotypes have yet to be designated. A diverse macro- and microfauna (ammonites, bivalves, gastropods, echinoids, crinoids, trace-fossils and occasional vertebrates) is present in the upper Penarth Group and Lias Group at Larne and is currently being documented. Some published stratigraphic data already exist for fossil macrofauna and palynomorphs through correlative strata in the nearby Larne borehole (Ivimey-Cook 1975, Bull Geol Surv. G.B., 50, 51-69; Warrington and Harland 1975, Bull Geol Surv. G.B., 50, 37-50). Diademopsis tomesi Modiolus cf. minimus Psiloceras ? tilmanni Specimen from lower part of Bed 25 (enlarged x3) To download a copy of these posters, and for further information about the Triassic - Jurassic boundary succession at Larne, visit the website at www.habitas.org.uk/larne The authors of this poster contribution would be happy to assist any visitors to the site, and freely encourage others to contribute to its scientific investigation. Michael J. Simms Department of Geology, Ulster Museum, Botanic Gardens, Belfast BT9 5AB, Northern Ireland. [email protected] Andrew J. Jeram, Mullaghdubh House, 27 Gobbins Path, Islandmagee, Co. Antrim BT40 3SP, Northern Ireland. [email protected] Highly involute smooth Psiloceras. Could this be a representative of the Psiloceras tilmanni group?
