Chapter 4 Molluscan Remains from the Valley Cores

Chapter 4 Molluscan remains from the valley cores Katrin Fenech, Chris O. Hunt, Nicholas C. Vella & Patrick J. Schembri 4.1. Introduction respectively. These studies showed the progress of the Holocene marine transgression and the infilling of the Molluscs often have quite specific environmental estuaries, and Fenech (2007) also showed the persis- requirements (Evans 1978, 82; Giusti et al. 1995; Schem- tence of open, exposed terrestrial environments in the bri et al. 2018). Many species require only a few square catchment of the Marsa estuary over c. 7000 years. At metres of habitat, so they are excellent micro-habitat the Neolithic Xagħra Brochtorff Circle (Schembriet al. indicators. Their shells can be dispersed, for instance 2009) and the Neolithic and later temple site at Tas-Silġ by running water, but generally, compared with other (Fenech & Schembri 2015), molluscan analysis demon- biotic materials used in palaeoecology (such as pollen strated long histories of anthropogenic disturbance grains or seeds), they do not disperse far from their life and sparse vegetation since the later Neolithic, but a habitat and therefore provide important indications considerable portion of these studies was done on shells of local environments. Alkaline sediments, which are recovered by troweling and dry sieving with a large very common in the Maltese Islands, will preserve fraction and therefore subject to a form of taphonomic molluscan shells and other calcareous biogenic mate- bias caused by the exclusion of most very small taxa. rial over thousands of years. This makes the analysis Analysis of a cave fill near Victoria on Gozo, based on of molluscan shells potentially a very important tool assemblages recovered by sieving, identified a phase for the reconstruction of past environments in Malta. of spectacular erosion caused by Classical period agri- Geologists and archaeologists recognized the cultural practices, followed by a more stable grazed value of molluscs as palaeoenvironmental indicators landscape in the Medieval and post-Medieval periods as early the first quarter of the nineteenth century ad (Hunt & Schembri 2018). Inevitably, the research done (Conybeare 1824; Preece 1998; Evans & O’Connor 2005, before the start of the FRAGSUS Project was very partial 41). Molluscan analysis is still, however, comparatively in coverage. The environmental history of the Maltese rare as a palaeoenvironmental tool, and for instance is Islands was still largely unknown. less commonly used than pollen analysis (e.g. Preece To investigate the environment and landscape of 1998, 158; Fenech 2007). the Maltese Islands and the changes they underwent In the Maltese Islands, the application of the over time, a full molluscan analysis was made of the technique has been limited and there has been no material extracted from sectioned sediment cores. The comprehensive palaeoenvironmental study using shell assemblages found in the FRAGSUS cores were molluscan analysis. Trechmann (1938), Giusti et al. controlled by their depositional environment, sur- (1995) and Hunt (1997) used the sporadic occurrence rounding habitats and taphonomic factors. They often of land snails in Maltese Quaternary deposits as an included molluscs from all four major habitat-groups: indication that these had accumulated in open, exposed marine, brackish water, freshwater and terrestrial. conditions. The highly cemented Quaternary deposits Although it is logical that species would be limited precluded anything other than the production of spe- to particular major habitats and that live individuals cies lists by these authors. Pedley (1980) suggested a from other major habitat types would not occur (for brackish depositional environment for the Pleistocene example, a freshwater habitat would not be expected Fiddien Valley Tufa on molluscan evidence. Fenech to also support live marine and terrestrial species), (2007) and Marriner et al. (2012) analysed cores taken in the result of the analyses of molluscan assemblages Holocene estuarine deposits at Marsa and Burmarrad, from cores is not always easy to relate to the type 115 Chapter 4 of environment when species from different major from habitats dominated by trees or shrubs, and from habitats are admixed. In the present study, it was garrigue. The Maltese low garrigue is also an exposed not always straightforward to determine which snail habitat but quantitatively and to some extent qualita- group is autochthonous (i.e. occurred naturally as live tively, its molluscan assemblages differed from those individuals), and which groups are allochthonous (e.g. of other exposed habitats presumably because of its washed into the coring site from elsewhere by run-off very open nature. However, this study also showed during storm events or by waves during storms or even that assemblage composition is determined more by by tsunamis). Thus for instance, a storm might have the micro-habitat preferences of the species, especially overtopped a bay-bar leading to insertion of marine the spatial and temporal availability of shelter and taxa into a non-marine lagoon, but at the same time shade, than by macro-habitat type. Many species were run-off from heavy rain might have washed land found in more than one macro-habitat, albeit in dif- snails into the same deposition site. Additionally, ferent relative abundances, very likely because of the allochthonous recycled material might arrive on a site extreme mosaic nature of the present day landscape of as sediments and their contained molluscs may have the Maltese Islands where different habitats may occur been eroded and redeposited. In some cases, it was within very close proximity to each other, allowing a possible to discern which species were autochonous mixing of molluscan assemblages, especially of the and which had been transported to the deposition site less stenoecious species. The mixing that occurred in from elsewhere. the deposits studied here, compounds the problem A further complicating factor is that some dep- of the natural ‘mixing’ of assemblages and therefore osition sites were at times affected by erosion, for only allows broad generalizations to be made about instance scour during fluvial or mud-flow events or the environment of the catchments of the coring sites. by marine storm waves. These erosion events led to Stenotopic species (those with narrow environ- non-sequences or hiatuses, sometimes recognizable mental tolerances) are most informative about the as very marked and abrupt changes of the molluscan environments present and we have used these as fauna, and by abrupt sediment changes. indications that particular habitats were present at Previous studies of non-marine molluscs from in the catchment of the coring sites. Habitats whose various (undated) Quaternary deposits around the presence was inferred in this way included brackish Maltese Islands revealed the occurrence of several water (with such species as Hydrobia spp. and Ovatella freshwater or wetland species that are now extinct, myosotis), running freshwater (e.g. Pseudamnicola (s.str.) indicating the previous occurrence of rare periods moussonii), slower moving water and ponds and lakes characterized by much wetter conditions, at least (e.g. Lymnaea truncatula, Planorbis spp., Gyraulus crista, locally (Hunt & Schembri 1999).1 On the other hand, Bulinus truncatus, Ancylus fluviatilis), freshwater wet- several species that occur today, such as the woodland lands (e.g. Carychium schlickumi, Oxyloma elegans, indicator Lauria cylindracea, had no palaeontological Vertigo antivertigo), sheltered habitats (e.g. Oxychilus record (see Trechmann 1938; Thake 1985a & b; Giusti draparnaudi, Oxychilus hydatinus, Vitrea spp.), wooded et al. 1995; Allen & Eastabrook 2017; Hunt & Schembri habitats (Lauria cylindracea), and garrigue (e.g. Tro- 2018). These previous studies form the basis of the choidea spratti and Muticaria macrostoma). research undertaken, together with more recent studies The molluscan analyses undertaken here comple- of the ecology of non-marine molluscs in local environment the pollen analyses (see Chapter 3) by providing mental regimes (Schembri et al. 2018) that now allow data about the environment and landscape. The chang- various species to be more or less reliably assigned to ing compositions of the molluscan assemblages further a specific habitat. One of the aims of these ecological suggest how climatic and weather events and patterns studies was to analyse the habitat and micro-habitat of human activity affected the depositional environ- distribution of current species and assemblages in ment and the catchments of the respective coring sites. order to determine their specificity to different habi- While the resolution of environmental reconstruction tat types. On the assumption that the ecology of the provided by analysis of the molluscan assemblages species would not have changed significantly in the may not be as high as that provided by pollen, given time spans considered, species and assemblages from the greater number of plant species involved and the archaeological and older contexts might then be used habitat specificity of many plants, molluscan assem- to reconstruct past environments and to study envi- blages have the advantage of reflecting local conditions, ronmental change. since the transport of molluscs is usually over much Brackish and freshwater assemblages apart, four shorter distances than for pollen. present-day terrestrial assemblages were recognized: This chapter will describe

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