Waterloo Bay, Larne, Northern Ireland: The ammonites of ...

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Waterloo Bay, Larne, Northern Ireland: The ammonites of the earliest Jurassic

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Caloceras sp.

Caloceras johnstoniPsiloceras 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 x3Crinoids 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 erugatumThis is stratigraphically the lowest ammonite recovered from the site. Ribbing persists unusually late in this large example.

Psiloceras cf. sampsoniThis 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. minimusPsiloceras ? 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. SimmsDepartment of Geology, Ulster Museum, Botanic Gardens, Belfast BT9 5AB, Northern Ireland. michael.simms@magni.org.uk

Andrew J. Jeram, Mullaghdubh House, 27 Gobbins Path, Islandmagee, Co. Antrim BT40 3SP, Northern Ireland. AnitaJeram@aol.com

Highly involute smooth Psiloceras.Could this be a representative of the Psiloceras tilmanni group?