1 Introduction

Acavus is an endemic genus of arboreal land snails found primarily in the southwest Wet Zone of Sri Lanka (see map 1). The species is found in areas with annual precipitation exceeding 2500 mm, mainly in the foothills of the central massif at elevations below 1,600 m (Hausdorf and Perera 2000; Ratnapala 1984). These high-humidity conditions facilitate their distribution. In addition to their ecological importance, Acavus is significant in Sri Lankan archaeology due to their presence in prehistoric sites. Although such remains are common in the southwest wet zone, they have also been found in the Semi-Arid to Transitional Zones (including intermediate and dry zones) (SATZ), extending beyond their usual habitat (Fig. 1; Table 1). In this context, we will examine these anomalous occurrences, suggesting either paleoclimatic changes (Deraniyagala 1956a, b, 1958) or a prehistoric pattern of interregional trade (Deraniyagala 1992). The assessments of the present study are based on new findings from Kalametiya in the southern coastal semi-arid region and previous archaeological literature.

Fig. 1
figure 1

Distribution of Acavus species in Sri Lanka and distribution of archaeological sites reported with Acavus remains in relation to ecological zones. Refer Table 1 for relevant details of the numbered archaeological sites. The wet zone is delimited by the 2500 mm isohyet and the 1800 mm isohyet separates the intermediate and dry zones. (Acavus distribution source: pattern shaded data from Natural History Museum - http://www.nhm.ac.uk/nature-online/species-of-the-day/biodiversity/loss-of-habitat/acavus-superbus/distribution-habitat/index.html; Other Acavus distribution data: (Anon, 2009; Manamendra-Arachchi 2012; personal communications with Deepika Rathnayake, 2023)

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Table 1 Archaeological sites with acavus shells and freshwater-land malacofauna in each eco zone
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Most of the malacofaunal remains in Sri Lankan archaeology come from cave or rock shelters with prehistoric human habitations, indicating human involvement. Thus, it becomes increasingly difficult to distinguish between cultural and natural factors to explain the anomalous occurrence of the Acavus snails beyond the wet zone. Archaeological data from the Semi-Arid Zone coastal open-air sites in Southern Sri Lanka revealed that Acavus spp. lived there naturally before and during human occupation (Kulatilake et al. 2018; Siriwardana 2015a, b, 2020). Considering these findings, the present study aims to reevaluate the abovementioned two models concerning Acavus remains found in archaeological sites within Sri Lanka’s intermediate to semi-arid zones. Due to their sensitivity to climate shifts, land snails are valuable for analysing regional climatic changes. Their distributions are influenced by vegetation, rainfall, temperature, soil properties, and habitat diversity, operating at both broad and local scales in response to climate and habitat variations. This understanding is vital for accurate climatic analysis (Ratnapala 1984; Goodfriend 1992).

Quaternary land snails provide valuable paleoenvironmental data, including vegetation, plant distribution, soil moisture, temperature, and rainfall isotopes. This is a developing field worldwide (Goodfriend 1992; Martin, Magnin, and Kiss 2003; Yanes 2012; Sumegi et al. 2022), yet no attempts have been made in the context of Sri Lankan ecology or archaeology. This paper aims to explain the significant presence of the Wet Zone inhabiting land snail shells within the SATZ. We seek to re-evaluate whether this phenomenon of high snail shell abundance in SATZ was a product of human activity or whether it results from the natural behaviour of land snails by examining material collected from Semi-Arid Zone sediments and insights from prior research.

1.1 Ecogeography of Acavus spp. in Sri Lanka

Paleoenvironmental reconstruction from fossil species depends on understanding the ecological factors shaping modern land snail distribution (Goodfriend 1992; Yanes 2012). Sri Lanka is home to a diverse array of land snail species, with 135 endemic/indigenous types. These snails exhibit varying distribution patterns across distinct ecological zones. The southwestern wet zone, characterized by its lush rainforests, harbors an impressive 97 species, while the intermediate zone accommodates 33, and the dry zone supports 20 species (Naggs et al. 2003). It is crucial to note that these counts exclude exotic species, emphasizing the uniqueness of these distribution patterns. The high abundance of snail species in the wet zone can be attributed to their preference for humid climates. They are typically found at altitudes below 1,600 m, where ideal humidity conditions and greater environmental stability prevail. Interestingly, many snail species originally specific to either dry or wet zones exhibit remarkable adaptability, enabling them to occupy niches at the edges of the intermediate zone (Ratnapala 1984).

Acavus, a sizable arboreal snail, often exhibits green algae deposits on its shell. Within the Acavus genus, three noteworthy species stand out: Acavus haemastoma (including A. haemastoma ‘var. melanotragus), Acavus superbus (encompassing A. superbus ‘var. roseolabiata and A. superbus ‘var. grevillei), and Acavus phoenix, consisting of two subspecies: Acavus phoenix phoenix and Acavus phoenix castaneus (Hausdorf and Perera 2000; Raheem and Naggs 2006).

These Acavus snails, renowned for their conical shells in coffee-brown, pink, red, and white shades adorned with irregular stripes, captivate malacologists due to their distinctive shell characteristics. These include slightly convex whorls, a prominent keel, flat undersides, elongated apertures, and angular lips featuring a broad spire plate. Initially classified as a distinct subgenus known as Acavus by de Montfort (1810), these snails have gained significance in malacofaunal studies in Sri Lanka (e.g. Hartmann 1844; Perera 1992; Hausdorf and Perera 2000; Raheem et al. 2008; Bandara et al. 2013).

Initially, it was presumed that Acavus snails remained stationary, clinging to tree trunks, possibly due to their watery secretions impeding mobility (Ratnapala 1984). However, subsequent research uncovered their nocturnal mobility as they descend to the ground for fruit feeding (Naggs et al., 2005). Acavus snails exhibit distinct habitat preferences in the Wet Zone:

  • A. haemastoma is typically found in low-lying coastal areas, up to 100 m in altitude. They often inhabit tree trunks, particularly near the crowns of areca and coconut palms.

  • A. superbus prefers sizable trees with moss-covered trunks in moist areas with dense vegetation.

  • A. phoenix thrives in the south-central wet zone and along the coastal region from Kalutara to Colombo, ranging up to approximately 500 m in altitude.

  • A. phoenix castaneus is commonly found in the northern interior of the wet zone, spanning altitudes from around 30 m to over 600 m (Hausdorf and Perera 2000; Ratnapala 1984).

These distinctions underscore their ability to adapt to specific ecological niches in regions characterized by wet conditions.

Acavus species thrive under favorable conditions (Hausdorf and Perera 2000; Naggs et al. 2003), adapt to vegetation changes under similar climatic conditions (Raheem et al. 2009; Bandara et al. 2013; Bandara and Ranawana 2016), and live in marginal conditions (Ratnapala 1984; Hausdorf and Perera 2000; Manamendra-Arachchi 2012), but cannot cope with harsh dry conditions without humid-wet conditions (Priyadarshana 2016). Unlike A. haemastoma and A. superbus, A. phoenix shows higher adaptability to various zones except the northern semi-arid zone. They are well-established in the wet and intermediate zones and can partly adapt to cooler, more humid regions within the dry zone (Ratnapala 1984). As an example, isolated populations of A. phoenix castaneus are found on Ritigala mountain (750 m) in the northern central dry zone (Hausdorf and Perera 2000). A recent observation noted their ecogeographical expansion in the slopes of the north of central highlands toward Sera Alla in the last few years (personal communication, Deepika Rathnayake, 2023). Another observation reports other isolated Acavus populations at Kinchigune (500 m) on the southern slopes of the central highlands (Manamendra-Arachchi 2012). These show a broader historical ecogeographical distribution of Acavus populations and current trends.

1.2 Early archaeological models and explanations

Since the first discovery of Acavus in an archaeological site in Sri Lanka in 1885, scholars have explored various models and explanations for its ecological and cultural values, which can be summarized as follows:

  1. 1.

    Utilitarian Plane Snails: The Sarasins’ excavation in Nilgala cave in 1885 uncovered Helix (Acavus) phoenix shells, initially emphasizing their utility as plane snails (Hobelschnecken) (Sarasin and Sarasin 1908), although this interpretation has received limited consideration (Perera 2010).

  2. 2.

    Paleoclimatic Indicator of SATZ and Food Source: PEP Deraniyagala interpreted Acavus remains as evidence of shifts in SATZ’s paleoclimate, and their presence alongside Cyclophorus sp. at Bellan-Bandi Palassa hinted at a wetter past. This interpretation also has received limited consideration. He also recognized snail shells as a primary food source during times of limited game availability (Deraniyagala 1957, 1958), which has been reviewed on different grounds lately (Weliange et al. 2009; Perera 2010; Weliange 2010; Ruhiru 2017).

  3. 3.

    Paleoclimatic Indicator of Wet Zone: SU Deraniyagala’s culture-ecological model suggested that Acavus remains in Pleistocene sites as at Batadombalena cave (28,500 to 12,000 BP) and Kithulgala Belilena cave (13,000 to 10,500 BP) of the Wet Zone indicated a climate resembling the present Wet Zone conditions (Deraniyagala 1992). Perera found increased Acavus consumption in Batadombalena around 18th-14th kya (Perera 2010; Perera et al. 2011). However, recent investigations at Kitulgala did not find malacofaunal remains from pre-10,577–8029 cal. BP levels (Kourampas et al. 2009; Wedage et al. 2020).

  4. 4.

    Trade-ornament items transported to SATZ: SU Deraniyagala proposed that human activities, such as trade or ornamental use, introduced Acavus to SATZ due to its restricted natural distribution to Wet conditions. Therefore, he suggests that the Dry Zone didn’t experience a climate resembling the wet zone during the late Quaternary period (Deraniyagala 1992). Subsequent studies have followed this model to describe the behavioral complexity of Mesolithic hunter-gatherers (Chandimal et al. 2019; Kapukotuwa et al. 2020).

Among these, the Sarasins’ utilitarian account is detailed. However, a comprehensive examination of the shells’ technological and trace-ware aspects is needed to examine the island’s sites. The species Acavus serves as an indicator of paleoclimatic conditions in Pleistocene wet zone cave habitats. It is also clear that it had value as a food source in the wet zone.

Moreover, limited ethnological data exists about the diverse uses of Acavus. Its watery mucus is employed as a medicinal remedy for falls from trees (Naggs, Raheem, and Platts 2005). Also, snail shells, including Acavus, have cultural significance as a source of lime in various practices. These include their use in chewing betel leaves among the Vanniyalaetto, traditional forest dwellers of Sri Lanka, although alternatives like the calcium-rich bark of Terminalia chebula are also used. Another intriguing practice involves burning snail shells, particularly from Cyclophorus sp., and blending the resulting lime with slime from Habenaria macrostachya orchid roots for romantic purposes during joyous occasions (Nevill 1887; Sarasin and Sarasin 1892).

However, the magnitude of Acavus snails’ importance in fulfilling these purposes among hunter-gatherer communities and the motives for transporting them to SATZ remains uncertain. Logistical challenges, including distance and geographical barriers like mountain ranges, could have affected the transportation of Acavus shells beyond their current distributional ranges. Given these considerations and the present study’s findings, we feel it is prudent to re-evaluate PEP Deraniyagala’s ecological-climatic shift model for SATZ.

1.3 Study area

Sri Lanka’s ecological regions are divided into four zones based on annual rainfall patterns: semi-arid, dry, intermediate, and wet (Fig. 1). Monsoon activity and impact vary by zone and are influenced by wind direction and elevation. The semi-arid zone in the island’s northwest and southeast lowlands below 900 m receives 760 to 1270 mm of annual rainfall. With less than 1,750 mm of annual rainfall, the dry zone experiences a distinct dry season from May to September. As the transitional zone, the intermediate zone receives 1,750 to 2,500 mm of moderate rainfall with a shorter, less pronounced dry season. The wet zone enjoys over 2,500 mm of annual rainfall, characterized by a lack of significant dry periods (Deraniyagala 1992; Punyawardena 2020). Unlike the lush Wet Zone, Sri Lanka’s Semi-Arid and Transitional Zones (SATZ) exhibit varying degrees of aridity, transitioning from intermediate to drier environments.

The current field study took place in the semi-arid zone within Kalametiya- Lunama Sanctuary in Hambantota district (Fig. 1: site 16). The site recorded as KM/Ex 1, situated along the coastline, features a rock outcrop. The coastal area has undergone significant changes, including the formation of shell beds during Holocene high stand events, with dates recorded at Kalametiya (Katupotha and Fujiwara 1988). Evidence of human occupation dating back to at least 2960 ± 160 BP is associated with aquatic conditions during the high stand phase (Deraniyagala 1992). Miniathiliya midden, located in the northern part of the study site, also had occurrences of Acavus (Kulatilake et al. 2018).

In addition to the coastal sites (Fig. 1) within the lower peneplain, significant inland sites such as Bellan-Bandi Palassa, Kuragala, and Udupiyangalge are all nestled within the Walawe Valley. These inland sites are situated on the expansive undulating plateau known as the ‘Southern platform’ of the central highlands (Cooray 1984). Separating it from the lower peneplain is the prominent Kaltota escarpment, reaching 325 m above the Walawe Valley. These localities fall within the catchment area of the Walawe River, a prominent Sri Lankan River spanning 138 km. This geographic positioning grants access to vital resources, including freshwater and a diverse range of aquatic and wild fauna. The region also boasts a deciduous rainforest, home to the Udawalawa sanctuary. Another significant site, Nilgala, is in the undulating terrain of eastern Sri Lanka and serves as a crucial watershed for Gal Oya. Nilgala features lowland tropical dry mixed evergreen forests and includes prominent peaks like “Yakun Hela” (700 m). Rajagala (346 m) produces the remotest record of Acavus, a major Buddhist monastic complex located east of Nilgala. Furthermore, in the dry zone of North Central Sri Lanka, we encounter three remarkable sites: Sigiriya, Ritigala, and Potana. These locations are characterized by inselbergs, rock outcrops, and hydrological connections with rivers originating from the northern slopes of the central highlands (Fig. 2). Each of these sites is of unique geological and historical significance.

Fig. 2
figure 2

The landscape of the regions where Acavus reporting sites are located. (a) Kalametiya lagoon and sea associated with coastal shell midden (KM/ Ex 1) and shell beds (semi-arid zone); (b) The mountain range of Udupiyan-Galge Cave (intermediate zone); (c) The undulating terrain with humid tropical rainforests (wet zone); (d) Batadomba-Lena Cave and a live Acavus snail (wet zone); (e) The southern slopes of the central highlands, Kaltota escarpment and the Walawe basin (intermediate and dry zones) (photos by Thilanka M. Siriwardana)

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2 Materials and methods

Our study comprises two main components: field data collection and a review of previous literature. We started our investigation by examining the coastal resource utilization patterns of Mesolithic hunter-gatherers in Southern Sri Lanka during the Holocene sea-level rise. This initial study provided valuable insights into the extent of human activity in these transformed coastal environments. Our fieldwork spanned from 2012 to 2014, and we have previously published preliminary findings separately (Siriwardana 2015b, 2018, 2020).

During our excavation efforts, we focused on a 1 × 1-meter sampling unit at our study site. Here, we systematically collected malacofaunal remains of Acavus spp. These collected shells underwent a detailed recording process, which included measuring shell dimensions such as height, width, and length. It is important to note that these measurements were exclusively taken from intact shells.

A qualitative approach is employed to analyze land snails in the assemblage and compare them to the current ecogeography of Acavus. To support this analysis, we compared the proportions of our snail assemblage with data from known live and fossil populations found in the relevant literature (Bandara and Ranawana 2016; Hausdorf and Perera 2000; Sumanarathna et al. 2016). This allowed us to explore the extent of size variation among Acavus spp. While we could not access the raw data from previous literature on live specimens to calculate quartiles and identify outliers precisely, we have created a visual representation that provides insight into the central tendency and spread of the data. To interpret land snail assemblages ecologically, it is assumed that they represent a distinctive habitat condition. By considering the current environmental associations of these extralimital species, conclusions can be drawn about deviations from the present environment, such as colder or drier conditions (Goodfriend 1992; Martin, Magnin, and Kiss 2003).

Further, we conducted radiocarbon dating on an Acavus shell from a midden level to determine the possible last occurrence date for this species in our study area. The radiocarbon dating analysis was performed by Beta Analytic Inc. in the USA. Radiocarbon dating of land snail shells presents challenges, as they often exhibit a 14 C age anomaly, where the apparent age exceeds the actual age (Goodfriend 1992). However, Abeyratne demonstrated the reliability of Acavus shells for radiocarbon dating by cross-checking with other materials and methods to establish a robust chronological framework (Abeyratne 1996; Abeyratne et al. 1997).

Here, we examine previously documented archaeological records (as in Table 2). This involves reviewing chronological information and noteworthy findings regarding the occurrence of Acavus and other terrestrial and freshwater malacofauna at these archaeological sites. Our methodology encompasses analyzing this data along with sea level changes (Katupotha and Fujiwara 1988; Lambeck et al. 2014; Shackleton 1987) and local climatic changes (Premathilake and Risberg 2003), which will be integrated into our subsequent discussions and analyses, contributing to a holistic comprehension of the subject matter.

Table 2 Acavus shell measurements (in mm) from the Kalametiya site 1 (KM/Ex 1)
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3 Results

3.1 Acavus spp. shells

We uncovered 41 Acavus shell remains within the test pit unit, including 12 complete shells. Complete and measurable shells are reported in Table 2. Context 11 yielded only a single fragment, while context 7, identified as the shell-midden level, provided 7 remains. Contexts 8 and 10 yielded 18 and 15 remains, respectively. It is worth noting that apart from two shells found in context 7, which display perforation in whorl, all other complete shells do not exhibit any such perforations or damage. Detailed measurements were taken for the complete shells, including width, length, and height, and the data are provided in Table 2 for reference.

The Acavus shells in question (Fig. 3) display a distinctive conical-globular shape characterized by three-and-a-half convex whorls. Notably, the body whorl exhibits a flattened or slightly concave profile. The surface of the shells bears inconspicuous growth ridges. All specimens display taphonomic alterations, including color loss with a subtle brownish hue and partial encrustation with darker brownish sediment. The aperture of the shells is notably flattened and lunate in shape, while the peristome is reflexed and displays a subangular form. No trace of the shell’s original natural coloration has endured over time.

Fig. 3
figure 3

Acavus haemastoma specimen identified from Kalametiya (KM/Ex 01)

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The known morphometric data of Acavus spp. are depicted in the box plots in Fig. 4. The length measurements of the shells from Kalametiya fall within the observed morphometrical ranges for both A. haemastoma and A. superbus. Notably, the individuals recovered from this assemblage exhibit body lengths that surpass the minimum length observed in A. haemastoma and A. superbus but fall within the lower range of the maximum length for these species. Additionally, the reported species show a relatively lower height than the Acavus species typically found in wet zone climatic conditions. Considering their morphometric characteristics and preference for coastal environments, we classify the identified specimens as A. haemastoma.

Fig. 4
figure 4

Comparative Analysis of extant Acavidae species from different geographical distributions in the wet zone with pertinent fossil specimens of Acavidae from Kalametiya (this study) (comparative data from Hausdorf and Perera 2000; Sumanarathna et al. 2016; Bandara and Ranawana 2016)

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This analysis of size distribution patterns enables us to make tentative reclassifications for other sites. For instance, specimens from Kabaragalge that do not exceed a body length of 4.5 cm, previously designated as A. splendens, may be reclassified as either A. haemastoma or A. superbus. Similarly, those with a body length of 6.5 cm, previously known as A. magnus, may belong to the phoenix species. This approach enhances our understanding of these ancient snail populations by considering size characteristics.

3.2 Chronology

One radiocarbon date was secured on an Acavus shell from the shell midden level (Level 7), which provides a firm upper boundary for the last human occupation at Site KM/Ex1. The sample KM/EX1/L7/S1(Beta Lab no. 671,483) from this level yielded a date 3240 +/- 30 BP calibrated to 3494 − 3382 cal BP (91.0%). The present date suggests that land snail species, as previously concluded from wet zone cave habitations, can be considered reliable materials for dating.

3.3 Other archaeological contexts

Excavation strategies are fundamental in shaping archaeological assemblages and are closely linked to dating and interpretation. Techniques for stratigraphic sequences organize data, influencing explanations. In Sri Lankan prehistory, regional sequences are emphasized, often sidelining minor findings like invertebrate fauna. While the Acavus genus is a critical malacofaunal species in Sri Lankan archaeology, there are certain lapses in the regional archaeological record.

The cave habitations receive more focus due to distinct stratigraphy; open-air sites receive less attention, like Bellan-Bandi Palassa (Table 1; Fig. 5). Only three Acavus records are open-air, while 14 are cave records, biasing the present database. Further, challenges arise in addressing microchanges within these sites. They’re significant locally but may seem irrelevant on broader scales. Therefore, observations and interpretations connecting sites for comprehensive models are complex. Methodological and interpretive biases are influenced by researchers’ backgrounds. Those with natural science and ethnological backgrounds often focus on lesser remains, such as malacofauna. The archaeological literature reflects diverse research approaches and varying data reporting quality.

Fig. 5
figure 5

Known chronological records of Acavus spp occurrences (marked with dots) projected onto global sea-level change over the past 40,000 years and local humid phases (purple shades) from the Terminal Pleistocene. The line between dots indicates the range of the chronology from the sites. Dots are not placed accordingly to the sea-levels. Sea-level data sources are as given in the figure. Archaeological data sources as given in Table 1. Local climate event data: Premathilake and Risberg 2003; Holocene climate optimum and LGM data: (Reuter, Harzhauser and Piller, 2020) ©Thilanka M. Siriwardana

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The limited radiocarbon record poses challenges. Many sites lack radiometric dates, hindering analysis. Therefore, the concept of continuity in site use in the SATZ is poorly understood. Gap periods are rarely suggested (e.g., Nilgala and Udupiyangalge), even with vastly different radiocarbon dates in close sediment proximity. The limited radiocarbon dates in the Wet Zone extend from 33,000 to 8,000 BP. The SATZ dates are from 15,000 to 2,000 BP, mainly in the Holocene period.

We observe significant overlaps by summarizing the available data, including chronological information and terrestrial/freshwater gastropod malacofauna (Fig. 5). This hints at a potential connection between Acavus occurrences in the SATZ and the activation phases of the Southwest Monsoon and Holocene high stand phases.

4 Discussion

This study unveils the presence of Acavus haemastoma during the late Holocene in its natural coastal lowland habitat despite the now semi-arid climate in Southern Sri Lanka. Intriguingly, this species appears to have served a minor role in the diet of Mesolithic gatherers around 1500 BC in the study site. When we juxtapose these findings with early archaeological literature, it challenges our understanding of the palaeoecological conditions in the SATZ and calls for a re-evaluation of certain archaeological assumptions.

4.1 Deposition of shells

Snails in archaeological contexts serve as localized indicators of past environments, assuming limited post-mortem transport. Factors like shell middens and selective predation by animals could introduce bias in species abundance (Goodfriend 1992; Erlandson and Moss 2001). The kitchen midden at Kalametiya contained fewer land snails than the lower contexts with less cultural evidence. Moreover, shells in these lower levels lacked perforations. In contrast, such body whorl perforation is more common in cave findings, which is interpreted as an index of trade-related transportation of primitive currency cum ornament (Deraniyagala 1992; Perera et al. 2011). Given optimal foraging behaviors and patch choice models (Hill and Kaplan 1992; Hames 2001), it’s puzzling to comprehend the significant investment of time and effort in transporting these snails over distances exceeding 30 km from their closest known habitats. The geographical separation of other sites, ranging from 20 to 110 km from the current Acavus boundaries, suggests a similar local depositional pattern. Considering snail meat’s relatively low calorific value (Heerden 2019), compared to other readily available food sources, human transportation likely involved moving snails only from nearby environments to caves or open-habitation grounds rather than long-distance transport. Therefore, the findings at Kalametiya provide valuable insights that enable us to infer that the occurrence of Acavus in the SATZ is primarily a result of their natural behavior, as opposed to being influenced by anthropogenic factors.

Beyond potential human predation, we must consider common predators of Acavus, such as the greater coucal (Centropus sinensis), Crested Serpent Eagle (Spilornis cheela), toque monkey (Macaca sinica) consuming snail mucus, and mongoose (Herpestes sp.) (Perera 1992; Vandercone and Santiapillai 2003; Fred Naggs, Raheem, and Platts 2005; Bandara and Ranawana 2016). No reported evidence suggests that animals transported Acavus over long distances in Sri Lanka. Thus, the Kalametiya deposit contains only shells of snails in the immediate area.

4.2 Humid phases in SATZ

Chronological data surrounding Kalametiya and other pivotal dates reveal patterns in Acavus distribution and concurrent climatic shifts in SATZ. At least three major phases align with local high humid-wet climatic events based on palynological data from the island’s Central Highlands and regional paleoclimatic records (Premathilake and Risberg 2003; Tripathi et al. 2014; Veena et al. 2014). These phases correspond to shifts between semi-arid and humid conditions, driven by alterations in the monsoon system, particularly the Southwest Monsoon (SWM).

4.2.1 Phase I: terminal pleistocene to mid-holocene (15 − 6 ka bp)

This phase is reconstructed primarily based on previous studies on Kuragala (Eregama 2022; Stock et al. 2022), Bellan-Bandi Palassa (Perera 2010), Rajagala (Kapukotuwa et al. 2020; Wedage et al., 2016) and Udupiyangalge (Somadeva et al. 2018; Deraniyagala 1940) prehistoric habitations in the southern slopes of the central highlands and eastern parts of the island. These sites yielded Acavus shells and other sympatric malacofauna, including swamp snail Pila sp., stream snail Paludomus sp., and wet zone land-dwelling Cycloporous sp. Initially, PEP Deraniyagala proposed the extinction of the first two species from the Bellan-Bandi Palassa to indicate climatic change around the Mesolithic period or later (Deraniyagala 1957, 1965). However, these species still inhabit a broader region on the island, with very few reported from SATZ, showing higher abundance in suitable aquatic environments in the Wet Zone (Starmuhlner 1974).

Despite their scarcity in the current environment, the Rajagala cave findings reveal a high abundance of Pila sp. (n = 114) and Paludomus sp. (n = 44) in depositional associations with Acavus remains, including Cyclophorus sp. (n = 9) (Kapukotuwa et al. 2020, 2021). Though we do not have proper chronology from Nilgala, located in the proximity of Rajagala, the Sarasins also reported Cyclophorus (Litostylus) ceylanicus, Paludomus neritodes, and P. (Tanalia) loricata, with the first two in high abundance. Cyclophorus, like Acavus, prefers lowland rainforest conditions and is reported only in habitations in the southwest lowland wet zone of the island (Raheem and Naggs 2006; Raheem et al. 2008; 2009). These malacofaunal pieces of evidence show significant humid phases in the SATZ occurred during the Terminal Pleistocene to early Holocene, in several events. These tendencies may indicate a metaclimax forest landscape, where various subsystems within the forest landscape functioned simultaneously (Martin, Magnin, and Kiss 2003), housing aquatic, ground-dwelling, and tree-top malacofaunal species during the same period.

Three key local climate events marked with varying higher humidity levels existed during this phase: 17,600 − 16,000 cal yr BP, 13,600 − 12,000 cal yr BP, and 10,200–9900 cal yr BP. Toward the end of this phase, the most intense Southwest Monsoon (SWM) rains occurred around 8,700 cal yr BP (Premathilake and Risberg 2003). Based on the decline in chemical weathering intensity observed since 6,200 BP from Kerala, South India, it has been proposed that wetter conditions preexisted with intensified monsoons (Veena et al. 2014). Further stratigraphic data on snail remains with a proper chronological sequence will be needed to establish an appropriate sequence of the events in these sites.

4.2.2 Phase II: late holocene (3.6-2 ka bp)

In the late Holocene, another humid phase was evident conformably based on field data. Radiocarbon dating of Mesolithic habitation levels at Kalametiya revealed an A. haemastoma shell dated 1545 − 1433 cal BCE (3494 − 3382 cal BP). This finding contributes to our understanding of Acavus’ last known occurrence in the semi-arid zone of Southern Sri Lanka. It aligns closely with previous dates for Miniathiliya, which yielded a date of 3600 BP (Kulatilake et al., 2018). In addition to the coastal areas, early radiocarbon dating provides further evidence. Bellan-Bandi Palassa (open air) yielded a date of 2050 cal BP (Deraniyagala 1992), while Potana provided a date of 4000 BP (Chandimal et al. 2019). These dates coincide with a humid phase during the Late Holocene, initially from 3600 to 2000 cal yr BP, marked by increased humidity in the central highlands (Premathilake and Risberg 2003). Regional records also support this shift, with evidence of strengthened Southwest Monsoon (SWM) in Kerala, South India, around 3900 − 1900 cal BP (Veena et al. 2014) and similar climatic fluctuations in eastern India, ranging from 4,380 to 3,230 cal BP (Tripathi et al. 2014). This land snail data originating from the southern and northern slopes of the central highlands, which are currently characterized by drier conditions, helps us to propose an expansion of the wet zone isohyet toward SATZ during the late Holocene.

The coastal evidence from this second phase also corresponds with the period between two Holocene high stands (HS). Holocene sea-level change records in Southern Sri Lanka show two mid to late Holocene high stands not exceeding + 1 m of present sea level, where 1st HS is between 6485 and 5370 cal yrs BP, while 2nd HS is between 2902 and 1558 cal yrs BP, based on 14 C dating of coral and marine shells (Katupotha and Fujiwara 1988; Katupotha 1994; Woodroffe and Horton 2005). All the kitchen middens reported from the region occur in the gap period of HS’s spanning from 3190 BP to 5260 BP. In most cases, these are overlying the naturally deposited shell beds (Katupotha and Fujiwara 1988; Deraniyagala 1992; Kulatilake et al. 2014; 2018; Siriwardana 2014; 2020). The coastal conditions of the semi-arid region of southeastern Sri Lanka between these two high stands might have created a favorable phase for wet zone biota and human subsistence, evidenced by increased shell-midden formations and the distribution of Acavus snails. These periodic monsoon patterns and coastal changes significantly affect vegetation and humidity. Utilizing sediment samples from coastal lakes in Sri Lanka, Gayantha et al. (2020) suggested a pattern of shifting vegetation, with wet and humid phases dominated by trees typical of moist evergreen forests, while dry phases saw an increase in mangrove vegetation.

4.2.3 Phase III: historical period of holocene (since 2 ka bp)

The presence of Acavus snails at Bellan-Bandi Palassa until 2,000 bp (Deraniyagala 1963) and evidence from the 1400 bp cave 18 at Sigiriya (Deraniyagala and Kennedy, 1972 suggest that humid conditions persisted in the northern and southern slopes of the central highlands. The disappearance of land snails from the southern slopes may correspond to the noticeable decrease in precipitation observed since 2,000 BP, when the Southwest Monsoon rains weakened, as indicated by the gradual decline of the Upper Montane Rainforest (UMRF) and a shift towards shrub vegetation (Premathilake and Risberg 2003). However, alternative conditions might have prevailed in northern regions of the island, with the proposed wetter climate starting around 1500 years BP in South India (Veena et al. 2014) and from 1860 to 1300 BP in eastern India (Tripathi et al. 2014) based on palynological data. These microclimatic shifts occurred on a smaller scale and in specific regions. While the expansion of species dwelling in the wet zone was hindered in one region, it might have continued in another.

Our research primarily focuses on microclimatic conditions favoring humid phases in intermediate-dry zones, but it is essential to acknowledge the potential for the wet zone to experience arid-dry conditions. There’s evidence of such conditions benefiting specific fauna in cave habitats, including the sloth bear (Melursus ursinus inornatus) in the South-West Wet Zone in the past (Deraniyagala 1965). Furthermore, researchers suggested significant climate variations in the wet zone during the Last Glacial Maximum (LGM) and the transition from the Terminal Pleistocene to the Early Holocene, impacting species like Gray Langurs (Rodrigo and Manamendra-Arachchi 2020). Climate change has also affected the geographical range and sizes of land snails in the lowland wet zone at present (Priyadarshana 2016), aligning with paleoclimatic records and indicating the impact of monsoon changes on vegetation and humidity (Premathilake and Risberg 2003). This phenomenon is ongoing, with recent reports of gradual plant and animal species migration from lowland dry regions to higher elevations on the island (Kottawa-Arachchi and Wijeratne 2017).

The need for a deeper understanding of the timing of these sequences is evident. The hunter-gatherer cave habitations in SATZ could be linked to the monsoon cycle, with traditional hunter-gatherers migrating to riverbeds in the dry season and caves during high-humidity phases to avoid malaria (Nevil 1887). Recent archaeological research in Sri Lanka has shifted its focus to human activities in marginal zones during the mid-Holocene period, revealing that Mesolithic hunter-gatherer communities strategically adapted to climatic fluctuations by intensifying plant gathering and developing advanced tools (Somadeva 2014; Somadeva et al. 2018). Around 3100 BP, the emergence of a prehistoric Early Iron Age culture in Sri Lanka marked the inception of practices such as iron production and paddy cultivation (Deraniyagala 1992). Subsequently, early rain-fed agriculture expanded into large-scale irrigation cultivation, closely linked to the prevailing humid climate conditions of the Late Holocene (Premathilake and Risberg 2003), and such activities increased in the SATZ. These adaptations underscore the intricate interplay between environmental factors and social-ecological resilience.

4.3 Microenvironmental natural refuges?

In previous archaeological studies, examining the species-area relationship in ecology has often taken a broad perspective when interpreting the presence of smaller fauna. Specifically, the coexistence of Dry Zone faunal remains such as Axis axis, Melursus ursinus, and Testudo elegans alongside wet zone snails within caves in dry conditions has been used to model the human agency in long-distance transportation of land snails (Deraniyagala 1992).

However, considering the isolated distribution of Acavus in SATZ, we need to think through a crucial concept known as the ‘small island effect.’ Initially highlighted by Lomolino (2000), this concept suggests that very small islands do not adhere to the conventional species/area relationship but instead exhibit unique characteristics determined by the array of habitats they offer. The size of an island plays a pivotal role in determining whether resource levels can sustain populations of highly energy-intensive species while smaller, less energy-demanding species may flourish (Lomolino 2000; Myšák et al. 2013).

Considering the presence of wet zone-favoring snails in contrast to dry condition-favoring large fauna, we can postulate that humid-wet conditions were restricted to smaller segments of the environment. These segments supported species with smaller home ranges. We propose that these segments resembled small, isolated refuges along the interconnected river basin catchments, enabling land snail expansion beyond their current distribution ranges. Consequently, unlike palynological data, which contributes to establishing a broader palaeoecological history, snail shells can provide valuable insights into more localized ecological conditions.

Figure 6 illustrates such hypothetical distribution corridors by considering factors like elevation, river networks, and coastal conditions, along with the varying ability of these snails to adapt to certain marginal conditions. The changes that facilitated the spread of A. phoenix castaneus to less cool and humid regions within the dry zone, such as the Ritigala mountains (Hausdorf and Perera 2000; Ratnapala 1984), may have also created conditions for potential corridors extending eastward into the Gal Oya River valley. Notably, archaeological records from the Nilgala cave within the valley report the presence of A. phoenix with sympatric malacofaunal species (Sarasin & Sarasin 1908). Unlike A. phoenix, the other two subspecies prefer wetter conditions, indicating wetter conditions in southeast Sri Lanka, including the Walawe River Valley. All three species and sympatric malacofaunal species were found from the southern slopes of the central highlands, bordering the intermediate zone toward the semi-arid coastal belt. The isolated populations in Kinchigune’s riverine forests of the Walawe Basin (Manamendra-Arachchi 2012) may be refuges of the said early dispersals. The occurrence of wet zone-dwelling Philautus variabilis frogs in Ritigala and some locations of eastern Sri Lanka also supports this view (Dutta and Manamendra-Archchi, 1996). The diversity of species in such island refuges may have been influenced by the interplay of stochastic events, including local weather conditions, which drive population dynamics (Lomolino 2000; Myšák et al. 2013). Specifically, the changing nature of monsoon patterns and coastal changes during the Holocene need to be considered in further studies.

Fig. 6
figure 6

Hypothetical expansion ranges of land snails under favourable conditions. Red = A. phoenix, Yellow = All three Acavus species

Full size image

4.4 Morphological changes

Land snail shell morphology changes, including variations in diameter, height, thickness, and color, are frequently linked to environmental differences. Consequently, it is possible to infer past climatic conditions by examining the morphology of fossil land snails (Goodfriend 1992). More data is needed to fully understand the morphological adaptations of Acavus during their expansion into the SATZ. Shells from our study show a smaller height range (26–34 mm, x̄= 29.5 ± 2.45) compared to live specimens (> 30 mm). This size difference is similar to what Sumanarathna and others (2016) observed in live and fossil specimens of A. superbus in the Wet Zone at Batadombalena. Furthermore, Siran Deraniyagala reported two smaller Acavus specimens, like A. superbus, in the driest parts of the Semi-Arid zones near Arnakallu in the northwest and Bundala in the South (Deraniyagala 1992). These specimens appeared more slender than live Acavus, suggesting shell structure changes to adapt to harsh conditions. When examining size differences in the same Acavus species across various ecological regions (Fig. 4), these variations may result from environmental influences (ecophenotypic). Interestingly, larger snail species exhibit slower temperature adjustments to their external environment than smaller species (Ratnapala 1984). This phenomenon might explain the snail shells’ tendency to be smaller than their wet zone dwelling relative. As proposed by Goodyear, environmentally induced traits can serve as valuable indicators for studying rapid climatic changes, as they exhibit no delay between the onset of environmental change and the corresponding morphological response. The morphology of the land snails in the southern semi-arid coastal conditions can be induced by such a change that occurs in a short-lived humid phase toward the late Holocene.

Considering height alone, its reduction resulting from long-term evolutionary trends may be an adaptation to certain levels of harsh conditions. In such cases, the favorable conditions do not necessarily have to be highly humid-wet conditions but can be marginal. Even if these evolutionary and climatic changes are minor, we can still assess the overall trend of environmental change from fossil fauna (Goodfriend 1992; Yanes 2012). Genetic diversity within the same species and dietary differences could also play a role in shell morphology, which will need further studies.

4.5 Limitations and further directions

Interpreting fossil land snail assemblages is challenging due to our reliance on a limited and non-quantitative modern land snail ecology database and archaeological data. While we used a uniformitarian approach with some improvements in geographic coverage, our knowledge of land snail environmental associations, feeding behaviors, growth response to monsoonal changes, and isotopic variations associated with monsoons is limited. Further research, including stable isotopes or Amino Acid Racemization, may face challenges due to low organic concentration and less favorable preservation conditions in tropical dry environments. Additional data from cave habitations and open sediments are needed, as cave snail remains may represent selective populations and can shift between thin depositional levels. It is important to conduct further studies on the correlation between human activities that affect the environment and the behavior of associated animals, such as snails. Even the slightest unintentional impact on water, vegetation, or soil can influence these small creatures. Therefore, it is necessary to understand the historical ecological aspect of this relationship and its impact on the overall ecosystem.

While we have used modern land snail ecology data to understand the observed morphological adaptations, it is essential to acknowledge biases. This data may not cover all ecological regions and can be biased toward well-studied areas. Changes in habitats and land use over time may also affect modern snail populations, making it necessary to re-evaluate the historical ecological conditions. Also, the present study did not consider the Acavus sp. shells found from Mantai (Joglekar 2013), as the findings are limited to a single photo without any descriptions.

Additional data are essential to assess whether these snails possess the necessary behavioral and physiological adaptations to inhabit regions with dry conditions. This will help determine whether they naturally adapted to such areas or migrated to regions with more favorable conditions. While this study is an initial effort to provide distributional data for recorded land snails, future research should prioritize extensive sampling and publishing relevant statistical data, particularly in the dry and intermediate zones. Such studies can lay the foundation for comprehensive investigations into important taxa’s biology, ecology, and physiology, shedding more light on the intricate relationship between ancient humans and their environment.

5 Conclusions

This study raises the question of whether the presence of Acavus species outside their preferred wet zone habitat serves as evidence of microclimatic humid conditions during the Holocene period, allowing them to expand their geographical range. As we have argued, it is improbable that Mesolithic hunter-gatherers transported these snails to distant regions in the intermediate and dry zones. Based on the findings, we make the following concluding remarks.

  1. 1.

    Acavus haemastoma lived in the natural conditions of the present-day Semi-Arid coastal zone of Kalametiya at least until the mid-4th millennium BP, indicating that wet-humid conditions prevailed in the region.

  2. 2.

    Findings from Kalametiya help us consider the natural expansion of the eco-geographical boundaries of Acavus and other sympatric climate-sensitive malacofauna under favorable conditions in SATZ.

  3. 3.

    Different humid events occurring in SATZ may correspond to the occurrence of wet zone-dwelling land snails in prehistoric habitations; hence, anthropogenic transportation of such fauna from the wet zone is unlikely to be plausible.

Even the smallest organisms hold significant scientific value. It is highly likely that the often-overlooked remains of small organisms, such as shells, play a more pivotal role than larger animals in deciphering historical patterns and microclimatic changes that have led to their displacement from previous habitats, restricting them to their current limited areas. As demonstrated by Acavus, this underscores the importance of further research, offering a dual benefit. Firstly, it allows us to trace human-animal responses to subtle changes in the past. Secondly, it provides a foundation for future-oriented, conservation-focused policies where archaeology can collaborate with natural sciences to promote resilience and sustainability.