The Malacologist-

RESEARCH GRANT REPORT

Molluscan biostratigraphy and radiocarbon dating of Irish Holocene tufas

George R. Speller

Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, gspe...@gmail.com

The impoverished nature of the Irish fauna and flora compared to Britain and the rest of Europe has stimulated much discussion since at least the third century AD (Cabot, 1999). Amongst the vertebrates, snakes are probably the most famous absentees, but many other groups also show deficiencies. Terrestrial molluscs, for example, are depauperate, with only 76% of the British land snail fauna, excluding casual introductions, occurring in Ireland (Table 1). Notable absentees from the Irish land snail fauna include many species frequent in southern Britain such as Abida secale and Azeca goodalli. Other species present in Ireland, such as Arianta arbustorum, Cepaea hortensis and Vallonia costata, are far less common than in Britain, displaying greatly restricted distributions (Kerney, 1999), whereas Pomatias elegans is known only from a single site in the Burren (Platts, 1977). This species and also Helicigona lapicida (known from a single Irish site; Philips, 1914) are probably recent introductions, but what about the others? Discussion relating to the origins of the Irish biota has in the past been complicated by consideration of the so-called ‘Lusitanian’ elements. These organisms, mostly plants but including the Kerry slug (Geomalacus maculosus), are characteristic of south-west Ireland but which elsewhere are known principally from Iberia. The problems surrounding the origin of the Lusitanian elements concern, to a lesser extent, the whole of the Irish biota. Four principal (non-exclusive) scenarios have been put forward to explain how Ireland acquired its present flora and fauna:

1	Pre-glacial or inter-glacial entry and subsequent survival in ice-free refugia in, or to the south of, Ireland during the time of the last glacial period.
2	Early Holocene colonisation by a terrestrial route from southern European refugia.
3	Accidental introduction during the Holocene by some form of trans-marine passive dispersal, possibly wind, birds or humans.
4	Deliberate introduction during the Holocene by humans.

Much of the early speculation concerning the nature of the Irish fauna and flora was based simply on perusal of species distribution maps. For example, Praeger (1950) postulated that the restricted northern range of Arianta arbustorum in Ireland was suggestive of a former landbridge between Ireland and Scotland but such arguments are speculative without empirical geological support. The value of the fossil record in providing a factual basis to any discussion on the origins of the Irish biota has repeatedly been stressed. Land snails have an excellent fossil record in calcareous deposits, such as tufa (a calcium carbonate precipitate formed in low-energy environments in the vicinity of springs) but their potential in furnishing detailed Holocene successions has hitherto received relatively little attention in Ireland, where most of the existing incomplete records have come from caves and sand dunes. The dating of many tufa deposits proved difficult because of the general absence of datable materials such as wood or charcoal. The advent of radiocarbon dating using accelerator mass spectrometry (AMS), enabling the dating of tiny amounts of charcoal or individual shells, has transformed this situation. The primary purpose of the present project has been to increase the number and geographical coverage of well-dated Holocene molluscan successions from Ireland, with the specific aim of shedding light on Irish biogeography. To this end over 86,000 shells have been analysed in nine profiles from three extensive Irish Holocene tufa deposits. Two of the sites, Graffy and Cartronmacmanus, are located in Co. Mayo in western Ireland; the third (Newlands Cross) is situated in the outskirts of Dublin in eastern Ireland. The work at Newlands Cross builds on an earlier study (Preece et al., 1986), with the analysis of five further profiles at a much-improved sampling resolution. The establishment of a secure dating framework for the tufa profiles was crucial if any meaningful conclusions could be sought. Since organic material was scarce a chronology was provided by 15 AMS dates based on the shells of the genus Cepaea in conjunction with 4 dates on charcoal. The grant from the Malacological Society helped fund some three of these essential dates (obtained from the University of Waikato Radiocarbon Dating Laboratory, New Zealand).

The sequences from Newlands Cross and Co. Mayo are the most detailed molluscan successions yet reported from Ireland, providing a record of faunal history that spans virtually the whole Holocene, from around 9700 radiocarbon years before present (years BP) to sometime later than 2100 years BP (Figure 1). Unfortunately, the records do not cover the very early Holocene, so it has not been possible to conclusively address the perplexing problem of Irish biogeography. Tufa formation has, however, persisted well into the late Holocene at these sites, providing a unique opportunity to obtain detailed molluscan data from this period. The sequences have shown that early Holocene molluscan assemblages from Ireland were broadly similar in terms of species composition to those from Britain and north-west Europe. Molluscan assemblages across north-west Europe appear to have obtained their regional character during the early to mid-Holocene. Thus, ‘oceanic’ species, such as Leiostyla anglica, that today are characteristic of the Irish fauna, seemed to arrive relatively early in the Holocene, particularly in comparison to south-east England. This suggests that colonization occurred from the south-west, probably from an Iberian refuge or in land now submerged off the western coast of Europe. Many other species common in southern Britain and the continent during the mid-Holocene, including Cochlodina laminata, Helicigona lapicida and Pomatias elegans, did not make it to Ireland, with their dispersal from more eastern refugia presumably being impeded by the Irish Sea. Other late Holocene immigrants in southern Britain, such as Monacha cantiana, are also absent from the Irish molluscan fauna adding to its slightly impoverished nature. Although the molluscan sequences from Newlands Cross and Co. Mayo have shed some light on the development of the Irish fauna, more well-dated land snail successions, especially from the Late-glacial and early Holocene, are required from Ireland before the thorny issue of Irish biogeography can be discussed in greater detail.

References

Anderson, R. (2005). An annotated list of the non-marine Mollusca of Britain and Ireland. Journal of
	Conchology 38, 607-638.
Cabot, D. (1999). Ireland: A natural history. HarperCollins, London.
Kerney, M.P. (1999). Atlas of the land and freshwater molluscs of the British Isles. Harley Books,
	Colchester.
Phillips, R.A. (1914). Helicigona lapicida in Ireland. Irish Naturalist 23, 37-38.
Platts, E. (1977). The land winkle Pomatias elegans (Müller) confirmed as an Irish species. Irish
	Naturalists' Journal 19, 10-12.
Praeger, R.L. (1950). Natural history of Ireland. Collins, London.
Preece, R.C., Coxon, P. and Robinson, J.E. (1986). New biostratigraphic evidence of the Postglacial
	colonization of Ireland and for Mesolithic forest disturbance. Journal of Biogeography 13, 487-509.

Acknowledgments

I would like to thank the Malacological Society of London for the centenary research grant which funded three of the radiocarbon dates used in this study. Richard Preece provided superb supervision during the project for which I am eternally grateful. I am also indebted to Liz Platts for useful discussion.

Table 1. Number of non-marine mollusc species in Ireland compared to Britain, excluding hothouse aliens but including two now considered extinct (Fruticicola fruticum and Cernuella neglecta). Possible recent introductions (i.e. those that lack a fossil record) are also included. Freshwater species include some brackish elements. * Semilimax pyrenaicus: status unknown (probably introduced). † Includes Geomalacus maculosus and two recent additions to the Irish list, Arion occultus and Arion fuscus, likely to be found in Britain. Data taken from Anderson (2005).

	Ireland	Britain	%	Native in Ireland but absent in Britain	Native in Britain but absent in Ireland

Land snails	71	93	76	1*	23
Slugs	32	35	91	3†	6
Freshwater snails	60	81	74	0	21
Total	163	209	78	4	50

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Figure 1. Schematic diagram showing the history of some key species in the molluscan sequences at Newlands Cross and Co. Mayo. The chronology is based on 15 AMS dates based on shells of Cepaea and 4 dates on charcoal..

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