Michael J. Simms Andrew J. Jeram, Waterloo Bay, Larne, Northern Ireland: Department of Geology, Mullaghdubh House, Ulster Museum, 27 Gobbins Path, Botanic Gardens, Islandmagee, The ammonites of the earliest Jurassic Belfast BT9 5AB, Co. Antrim BT40 3SP, Northern Ireland. Northern Ireland. [email protected] [email protected]
Ammonites are a conspicuous element of the earliest 24 34 Jurassic macrofauna at this site. Most taxa are represented by at least some 3-dimensional material, preserved either in pyrite or early diagenetic carbonate 33g concretions. 23 33f Caloceras sp. Selected specimens are illustrated here at actual size, unless indicated otherwise.
33e top Bed 27 22 The strata in the picture to the left encompass the top of
Psiloceras cf. plicatulum Caloceras johnstoni top Bed 26 the ‘Pre-planorbis Beds’, the erugatum Horizon (Bed 24) and the stratigraphic range of Neophyllites (beds 25 33d to 27). 21 top Bed 25 The site has significant potential for designation as a Bed 24 nodules Global Stratotype Section and Point for the base of the
33c Jurassic System. In addition, this part of the foreshore 20 top Bed 23 would make an ideal stratotype location for the erugatum, imitans and antecedens biohorizons, for which surface stratotypes have yet to be designated. Psiloceras plicatulum
19
33b Psiloceras plicatulum A diverse macro- and microfauna Specimen from lower 33a part of Bed 25 (ammonites, bivalves, gastropods, 18 Psiloceras cf. sampsoni (enlarged x3) This highly evolute example is the echinoids, crinoids, trace-fossils and 32 stratigraphically highest example of occasional vertebrates) is present in the sampsoni recovered from the site. upper Penarth Group and Lias Group at Diademopsis 31 Psiloceras cf. plicatulum 17 Larne and is currently being documented. tomesi 30 Some published stratigraphic data already 29 exist for fossil macrofauna and
16 palynomorphs through correlative strata in 28c the nearby Larne borehole (Ivimey-Cook Psiloceras planorbis 1975, Bull Geol Surv. G.B., 50, 51-69; 28b Warrington and Harland 1975, Bull Geol Surv. G.B., 50, 37-50). Psiloceras ? tilmanni metres above base of measured section 15 Modiolus cf. minimus 28a Neophyllites antecedens
Neophyllites sp. 14 27 Neophyllites antecedens Highly involute smooth Psiloceras. To download a copy of 26 Could this be a representative of the Psiloceras tilmanni group? these posters, and for Neophyllites imitans 13 25 further information about
24 the Triassic - Jurassic
23b boundary succession at
12 23a Larne, visit the website at
22d www.habitas.org.uk/larne Psiloceras erugatum Evolute ammonite with costae apparently developed on the venter. Does this indicate the presence of 22c Choristoceras, or is it merely a preservational artifact? 11 The authors of this poster 22b Crinoids as a proxy for fully marine salinity Ossicles of a small crinoid (Isocrinus sp. nov.) appear abruptly and in abundance enlarged x3 enlarged x3 contribution would be 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 happy to assist any visitors of columnals per noditaxis is consistently different from both Isocrinus bavaricus to the site, and freely 10 (Rhaetian Kössen Beds) and Isocrinus psilonoti (Hettangian, NW Europe), 22a indicating a distinct species. encourage others to Crinoids are exclusively stenohaline organisms. Their abrupt appearance at this level may indicate the establishment of fully marine salinity following the Psiloceras erugatum contribute to its scientific This is stratigraphically the lowest ammonite enlarged x3 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. recovered from the site. Ribbing persists investigation. 9 Isocrinus sp.nov. unusually late in this large example.