8 NON-MARINE MOLLUSCS FROM THE TO'AGA SITE SEDIMENTS AND THEIR IMPLICATIONS FOR ENVIRONMENTAL CHANGE PATRIcK V. KIRCH INTRODUCTION NON-MARINE MOLLUScs, or "land snails," can occurin high frequencies at archaeological sites where deposidonal environments are favorable to their preservation (e.g., calcareous sands such as at the To'aga site). In general, these snails are diminutve (often in fte 1-3 mm size range), and their presence in archaeological deposits results from natual ratherthan cultual deposition. Since land snails are highly sensidve to microenvironmental conditions such as vegetation cover, changing fruencies of snail shells in sratdfied archaeological deposits can provide a proxy measure of environ- mental change. Snail analysis for the purposes of envionmental reconstruction has been carided out in English and European archaeology for many years (Evans 1972), but has been relatively neglected in the Pacific. Beginng in the 1970s, Kirch and Christensen began to apply snail analysis in Oceanic arhaeology with fruitful results (Kirch 1975, 1989; Christensen and Kirch 1981, 1986; Christensen 1983). bse studies have demonstrated that non- marine molluscs in Pacific archaeological sites can provide significant infoimation with regard to (1) local environmental change (especially in vegeta- ton), and (2) the intrducdon of exotic biota by preistoric humans. Local enviromnental changes are demostrated trugh changing assemblages of endemic and indigenous snail species which are sensitive to microenvironmental conditions. The introduction of exodc biota, on the other hand, is signalled by the presence of one or more species of synanthropic (or anthropophilic) snails that are closely associated with human habitations, gardens, and disuibed environments. Pacific malacologists recognized some time ago that a number of widely disseminated snail species in te Pacific had almost cenainly been dispersed trugh prehistoric human agency. C. Montague Cooke, who spent a lifetime reearching Pacific snail faunas, wrote that '"ere is no doubt that about a dozen species were carned by Polynesians in their migrations. At least four species were carried by the latter to the Hawaiian Islands. These for the most part are minute species of snails that are always found in situations just above high-water mark and are fairly uniformly distributed wherever Polynesians live" (Cooke 1926:2279). Among these synanthropic snail species are Gastrocopta pediculus, La,nellidea pusilla, Liardetia samoensas, Assiminea niida, and Lmnellaxis gracilis. Because tese widely dispersed species are highly character- istic of atolls, they have sometimes been described as an "atoll fauna" (Reigle 1964; Harry 1966), although they are just as common in lowland, antrpogenic habitats on high islands. During fte 1987 excavations at To'aga, I observed te presence of non-marine mollusc shells in a number of sntaigraphic contexts, and therefore undertook systematic sampling of fte 1987 main trench and several oter test units for snail shells. 116 The To'aga Site This chapter presents the results of analysis of these samples and discusses the implications of our results for paleoenvironerntal change and for the inrduc- tion of exotic biota at To'aga. MATERIAL AND METHODS Sediment samnples for non-marine mollusc analysis were bained from two columns at site AS- 13-1, one from the main trench of the 1987 excava- tion and the other from Unit 3. The main trch column was taken from the cleaned west face of Unit 9, and fte nine column samples spanned the strati- graphic sequence from Layer IIC thrugh Layer IIA- 1. No land snail shells were observed below Layer IIC or in the acidic colluvial sediments of Layer I. The position of the land snail column in the main tench is depicted on the main stratigraphic section shown in figure 5.5. For the most panl, the methods used in the analysis of non-manne molluscs at To'aga follow those developed and reported by Christensen and Kirch (1986:55-56) for use in Hawaiian archaeologi- cal sites. Sediment samples for snail extraction were taken as continuous columns. Prior to sieving, the sediment samples were air-dried, te weighed and measured volumetrically in order to assure quandta- tive comparability between samples. In the labora- tory, the sediment samples were wet-sieved through 4, 2, and 1-mm mesh nested geological screens, and the washed residues air-dried. Snail shells were then extacted by hand-sorting under a lOX binocular microscope. Snail shells were identified by referrng to published taxonomic monographs and to reference material in the collections of the Benice P. Bishop Museum (Honolulu). In the next section the various species of non- marine molluscs present in the To'aga samples are listed in taxonomic order, with remarks on their distribution and ecology. This is followed by the presentation of quantitative data on snail species frequency from the two stratigraphic columns. SYSTEMATIC REVIEW Family Helicinidae Pkuropoma sp. A few specimens of Pleuropoma are present in e To'aga samples. It has not been possible to identify these to species level. Family Assimineidae Assuimnea cf. nitida (Pease) The genus Assuimnea includes a number of estuarine and standline-dwelling species (Abbou 1958). The most abundant land-snail species in the To'aga site sediments is an Assuimnea (fig. 8.1) which very closely resembles A. nida (Pease). This species is known to be widely distibuted from Soufteast Asia eastwards into Oceania. Family Achatinellidae Lamnellidea pusilla (Gould) Lamellidea pusilla is (fig. 8.1) one of several species in the Achadnellidae that have extensive distributions throughout the inner Pacific. Cooke and Kondo remarked that "'there is little doubt that the wide distribution of L. pusilla is due to human agency. It was probably tnsported from island to island during the Polynesian migrations" (1960:188). The species is distributed from the Maranas and Palau groups in the westem Pacific and as far eastward as Mangareva (Cooke and Kondo 1960: fig. 81). Family Pupillidae Gastrocopta pediculus (Shuttleworh) This small pupilHid (fig. 8. 1) is also abu tin the To'aga samples. Pilsbry made the following remarks on the distribution and geographic origin of Gastrocopta pediculus: Adaptability to life around habitations has, no doubt, led to the vast Polynesian distribution of G. pedculus. I infer that it has been carried from island to island, sticking to native impedimenta, cocoanuts, or other food mateials, in the thousand years or more of inter-island canoe voyages of the Polynesians. Its original habitat may have been somewhere between the Philippines and New Caledonia (1916-18:140-41). Generally distributed in coastal environments, G. pediculus shows a strong liking for coconut groves, often being found in large numbers "under leaves and sprouting cocoanuts" (Pilsbry 1916-18:148). Family Succineidae Succinea sp. Only a single example of this species was encountered in the To'aga site sediments, in Unit 3, even though living Succinea sp. were observed in leaf liUtter samples in the arboricultural zone. : e :9!Ek v 500rt7% ? ? 3 mm . igure 8.1 Photomicrographs of terrestrial gastropods recovered from the To'aga site sediments: a, Sinployea cf. allecta; b, Gastrocopta pediculus, c, Lamellidea pusilla; d, Assirrinea cf. nitida; e, Lamellaxis gracilis. Non-Marine Molluscs 117 118 The To'aga Site Family Caropidae Sinployea cf. allecta (Cox) A few specimens of a caropid snail of the genus Sinployea are present in several samples (fig. 8.1). This is most likely S. allecta, although the closely related species S. clausa (Solem) is also present in the Manu'a Group. Solem (1982:127) has described a subspecies, Sinployea allecta tauensis, from Ta'u Island which closely matches the remains found at the To'aga site. These species are apparently all lowland dwellers, primarily in leaf litter. Family Helicarionidae Liardetia samoensis (Mousson) This species occurs rarely in the To'aga samples. Liardetia samoensis is the most widely distributed member of the genus, ranging from te Bismarcks eastward to the Marquesas (Solem 1959:96). Baker, who monographed the zonitid snails of the Pacific, observed that the genus Liardetia "contains species that have been widely disseminated, probably trugh the agency of man" (1938:12), among them Liardetia samoensis. Solem (1959) regards the probable center of distribution of Liardetia as Indonesia. Family Subulinidae Lamellaxis gracilis (Hutton) This distinctive species (fig. 8.1) is reasonably well represented in te To'aga samples from both the 1987 main trench and from Unit 3. Pilsbry (1906) remaked ta L. gracilis is "probably the most widely distributed land snail in the world," and suggested that its dispersal throughout the inner Pacific may have been due to the Polynesians. Other malacologists (Cooke 1934; Solem 1978) regarded its wide distribution as a result of modem commerce. Christensen and Kirch (1981) demonstrated, how- ever, that Lamellaxis gracilis was present on the Polynesian outlier of Tikopia by 900 B.C., where Lapita colonists presumably introduced it inadvert- ently. Subsequently, specimens of L. gracilis have been identified from prehistorc contexts in Tonga (Niuatoputapu, in association with Lapita sites, Kirch 1988:233-34), the Marquesas (Rolett 1989, table 5.14), and Hawai'i (Christensen and Kirch 1986:60). The dissemination of this synanthrmpic species probably began with the Lapita expansion and continued with the Polynesian dispersal into the far eastem Pacific. RESULTS The frequencies of non-marine mollusc species in the various sample units from the 1987 main excavation, and from Unit 3, are presented in tables 8.1 and 8.2. In addition to the raw frequency counts per sample, the tables provide infornation on sample weight and volume, and on te density of snail shells per cubic liter of sediment. The nine sediment samples from the main trench (Unit 9) spanned te Layer II depositional sequence as follows: Samples 1-3, Layer IIC; Samples 4-6, Layer UB, the main occupation horizon; Samples 7- 8, Layer IIA; and Sample 9, Layer IIA-l. Several significant pattems are evident from te data pre- sented in table 8.1. First, the assemblage is domi- nated by a suite of synanthropic species, particularly Assiminea cf. nitida, Gastrocopta pediculus, Lamellidea pusilla, and Lamellaxis gracilis, but with Liardetia samoensis also represented. Indeed, the dominance of synanthropic species in xse samples is remarkable, greater than any oter archaeological example that I am aware of from the Pacific. The two indigenous/endemic species present, Pleuropoma sp. and Sinployea sp., are represented only in very limited numbers. The temporal distribution of species is also striking. The oldest sample contains only one species, Assiminea cf. nitida. This species is joined by Lamellidea pusilla and Gastrocopta pediculus in Sample 2, and by Lamellaxis gracilis and Liardetia samoensis in Sample 3. Thus, by the time of occupation represented in Layer IIB, ca. 2500 cal B.P., five species of synanthropic, human-transported land snails had been introduced to, and become established on, the coastal terrace of Ofu Island. Because Lamellads gracilis is particularly associ- ated with human gardening sites, its appearance by 2500 cal B.P. is important in tenns of the economic prehistory of the To'aga site. As can be seen from table 8. 1, the density of non-marine molluscs is extremely low (8 snaiLs/I3) in the lower part of Layer IIC, rising rapidly to a peak density of 400 snails/I3 in the middle of Layer IIB, the occupation horizon. Shell density then drops Non-Mari'ne Molluscs 119 Table 8.1 Non-marine Molluscs from the 1987 Main Trench, To'aga Site Sample No. Taxon 1 2 3 4 5 6 7 8 9 Pleuropoma sp. 1 1 1 1 2 Assiminea sp. 3 5 17 19 51 59 46 47 84 Lamnellidea pusilla 4 26 12 10 5 4 1 14 Gastrocopta pediculus 9 26 37 40 17 18 8 19 Sinployea sp. 3 Liardetia samoensis 1 Lamellaxis gracilis 5 6 9 8 4 3 11 Total snails counted 3 18 76 75 110 90 73 64 128 Snails/13* 8 65 276 269 400 321 203 197 366 * standardized density, adjusted for sample volumes. somewhat in Layer IHA, but rises again to 366 expected with the establishment of human habita- mails/13 in the Layer IIA-I deposit. This distribution tions in Layer IIB and of a stable vegetated soil is sigficant, since it correlates well with the sedi- surface represented by Layer IIA-1. mentological history of the column and is precisely The land snail data for Unit 3 are presented in the sort of overall density distribution that would be table 8.2. The overall pattem resembles that in the Table 8.2 Non-marine Molluscs from Unit 3, To'aga Site Sample No. Taxon 2 3 4 5 6 Pleuropoma sp. 1 1 Assiminea sp. 10 27 44 110 15 Lamellidea pusilla 2 6 3 18 Gastrocopta pediculus 6 9 7 27 Succinea sp. 1 Sinployea sp. 1 2 4 Liardetia samoensis 1 Lamellaxis gracilis 3 8 10 13 3 Total snails counted 21 52 66 175 18 Snails/13* 71 184 280 735 68 * standardized density, adjusted for suample volumes. 120 The To'aga Site main trench, except that the suite of synanthropic snails is already established at the base of the column (Sample 2). The increasing density of land-snail shells and the dominance of synanthropic species suggest the establishment of a stable, anthropogenic vegetation over the To'aga coastal terrace following human occupation of the island. CONCLUSION The analysis of non-marine molluscs from the To'aga site provides important conroborative evi- dence for several interpretations made elsewhere in this volume. That the snail assemblages at To'aga are dominated by a suite of synanthropic species (the so-called "atoll fauna') known to have been widely disseminated by human agency highlights our interpretation of the To'aga coastal terrace as a strongly anthropogenic habitat. The absence of endemicfindigenous species below Layer IIC in the main trench supports our interpretation of the coastal terrace as a narrow, exposed, calcareous depositional environment prior to human colonization. After human settlement and the establishment of crop plants and other introduced vegetation at the begin- ning of the first millennium B.C., the coastal terrace began to sustain an adventive snail fauna of synanthrpic species. The high concentration of these snails in Layer HA- I is also consistent with our interpretation of this deposit as a fonner stable, vegetated A1 horizon paleosol. The snail assemblages from To'aga further document of the early spread of synanthropic species into central Polynesia. Such species as Lamellaxis gracilis, associated with Polynesian gardening environments, were already known to have been present in Lapita contexts (Christensen and Kirch 1986; Kirch 1988). The presence of this and four other species at To'aga reflects the propensity of early Pacific colonizers to create "transported landscapes." REFERENCES CITED Abbott, R. T. 1958. The gastropod genus Assiminea in the Philippines. Proceedings of the Academy of Natural Sciences of Philadelphia 110:213- 78. Baker, H. B. 1938. Zonitid Snailsfrom Pacific Islands, Part 1. Bemice P. Bishop Museum Bulletin 158. Honolulu. Christensen, C. C. 1983. Analysis of land snails. IN Archaeological Investigations of the Waimea- Kawaihae-Mudlane Road Corridor, Island of Hawaii: An Interdisciplinary Study of an Environmental Transect, eds. J. T. ClaIk and P. V. Kirch, pp. 449-71. Departnental Report 83- 1, Anthropology Departnent, Honolulu: Bernice P. Bishop Museum. Christensen, C. C., and P. V. Kirch 1981. 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