Yurena Yanes

Testing limiting similarity in Quaternary terrestrial gastropods

Abstract The hypothesis of limiting similarity, which postulates that morphologically and/or ecologically similar species will differ enough in shape, size, or other variables to minimize competition, has been controversial among ecologists and paleoecologists. Many studies have reported the occurrence of limiting similarity in modern environments or in time-averaged fossil deposits; however, empirical high-resolution time series demonstrating limiting similarity over longer time scales are lacking. We have integrated radiocarbon-calibrated amino acid dating techniques, stable isotope estimates, and morphometric data to test the hypothesis of limiting similarity in late Quaternary land snails from the Canary Islands over a period of 42,500 years. We tested for both ecological character displacement (two closely related species will differ in size in order to minimize competition in sympatry and these differences will be minimized in allopatry) and community-wide character displacement (overdispersion of body size among competitors in a guild). Multiple proxies of body size consistently show that two endemic congeneric pulmonate gastropod species (Theba geminata and T. arinagae) maintained a difference in size from ∼42,500 b.p. through the last occurrence of T. arinagae 14,900 b.p., with a concomitant trend of a decreasing body size. Theba geminata body size did not converge on that of T. arinagae and variation in T. geminata body size did not increase significantly following the extinction of T. arinagae; therefore, ecological character displacement and release did not occur. Community-wide character displacement was found in only one time bin over the last 42,500 years. These results suggest that limiting similarity is a transient ecological phenomenon rather than a long-term evolutionary process. This study not only demonstrates the problems inherent in biological “snapshot” studies and geological studies of time-averaged deposits to test limiting similarity adequately, but it also presents a more adequate research protocol to test the importance of interspecific competition in the history of life.

The Microstructural Record of Predation: A New Approach for Identifying Predatory Drill Holes

Abstract Drill holes in prey skeletons are the most common source of data for quantifying predator-prey interactions in the fossil record. To be useful, however, such drill holes need to be identified correctly. Field emission scanning electron microscopy (FE-SEM) and environmental scanning electron microscopy (ESEM) were applied to describe and quantify microstructural characteristics of drill holes. Various specimens, including modern limpets and mussels drilled by muricid snails in laboratory experiments, subfossil limpets collected from a tidal flat (San Juan Island, Washington state, USA), and various Miocene bivalves collected from multiple European sites, were examined for microstructural features. The microstructures observed are interpreted here as Radulichnus-like micro-rasping marks, or predatory microtraces, made by the radula of drilling gastropod predators. The mean adjacent spacing of these microtraces is notably denser than the spacing of muricid radular teeth determined by measurements taken from the literature. Because the radular marks typically overlie or crosscut each other, the denser spacing of predatory microtraces likely reflects superimposition of scratches from repeated passes of the radula. One incomplete drill hole showed a clear, chemically aided drilling dissolution signature around its outer margin, while a number of other specimens showed similar, but ambiguous, traces of dissolution. The range of organisms examined illustrates the utility of scanning electron microscopy (SEM) imaging for identifying micro-rasping marks associated with predatory drill holes in both modern and fossil specimens. These distinct microtraces offer promise for augmenting our ability to identify drill holes in the fossil record and to distinguish them from holes produced by non-predatory means.

Shell Taphonomy and Fidelity of Living, Dead, Holocene, and Pleistocene Land Snail Assemblages

Abstract Variations in the taxonomic composition of ancient land snail assemblages can potentially reflect changes in past ecosystems. The use of fossil associations as a paleoenvironmental-paleoecological proxy assumes that the original biological signature is retained, but postmortem processes can distort it. In this study, the fidelity of land snail assemblages was tested by comparing taphonomic and ecological variables recorded by live and dead, middle Holocene and Upper Pleistocene land snail shelly assemblages from San Salvador Island (Bahamas). Shells of living organisms were practically unaltered whereas dead and fossil shells were primarily affected by fragmentation, ornament loss, color loss, and carbonate coating. Taphonomic features fluctuated across space and time likely due to variable environmental conditions and/or time of exposure prior to shell burial. Live assemblages showed good taxonomic agreement with dead assemblages, although the later exhibited a higher number of taxa and individuals than the former. Assemblages that were moderately (dead and Holocene) and strongly (Pleistocene) taphonomically altered did not differ in species abundances, suggesting that the original biological signal was preserved. In contrast, unaltered (live and some dead) assemblages differed taxonomically from moderately and strongly damaged assemblages, likely as a consequence of different scales of time-averaging rather than variable shell-specific destruction rates. Taxonomic richness and simple dominance of time-averaged land snail assemblages were similar at various interglacial time periods (∼125 ka, ∼5–6 ka, and today). Such apparently equivalent snail richness may suggest that the climatic-environmental and/or ecological conditions at those times were comparable to the present.

Holocene (~4.5–1.7 cal. kyr BP) paleoenvironmental conditions in central Argentina inferred from entire-shell and intra-shell stable isotope composition of terrestrial gastropods

The isotopic fingerprint of terrestrial gastropods has been increasingly used as a credible natural paleoenvironmental archive. Most published work has used this proxy at tropical and temperate latitudes of the Northern Hemisphere, and focused on entire-shell analysis. The present study provides entire-shell and intra-shell isotopic profiles to infer average and seasonal late Holocene environmental conditions in central Argentina (30°S). Shells of Plagiodontes daedaleus (Gastropoda: Odontostomidae) were retrieved from the Alero Deodoro Roca–Sector B site, one of the few archaeological sites in central Argentina rich in shells collected by pre-Hispanic hunter-gatherer groups. Ancient entire shells exhibited values that were ~2.5‰ higher in δ13C and ~1.8‰ higher in δ18O than modern individuals, pointing to higher abundance of C4 plants and overall drier conditions (lower relative humidity and/or higher rain δ18O) during 4.5–1.7 cal. kyr BP than today, in agreement with published regional proxies. Intra-shell isotopic profiles suggest that modern and fossil specimens deposited their shells throughout two-to-three summer/winter cycles. Intra-shell δ18O values varied ~5‰, matching with the seasonal variation of rain δ18O values. The extent of seasonality was similar during 4.5–1.7 cal. kyr BP and today. Intra-shell δ13C values varied ~2–3‰ and did not portray distinct seasonal cycles, depicting minimal seasonal variations in the snail diet. This work illustrates that South American terrestrial gastropods have great potential for paleoenvironmental studies.

ECOLOGICAL FIDELITY OF PLEISTOCENE–HOLOCENE LAND SNAIL SHELL ASSEMBLAGES PRESERVED IN CARBONATE-RICH PALEOSOLS

Abstract Studies that assess the ecological fidelity—preservation of the original community—of terrestrial shell accumulations are uncommon but essential to infer accurate changes in past ecosystems. When live-dead comparisons are unavailable, the taxonomic agreement between differing taphofacies may be used to evaluate the fidelity of ancient shelly assemblages. This approach was used to approximate the fidelity of Quaternary land snails preserved in carbonate-rich paleosols from the northeastern islets of the Canary Archipelago. Such macroscopic alteration as fragmentation, corrosion, carbonate coating, and color loss affected shells, however, microscopic analyses concluded substantial diagenetic alterations unlikely. The shell abundance negatively correlated with fragmentation, suggesting that a higher proportion of shells may be a consequence of higher shell input rate and lower shell destruction rate rather than lower sedimentation rate, as predicted by taphonomic models. Strongly and weakly altered taphofacies significantly differed in species abundances. Substantial taphonomic bias was improbable, however, because both taphofacies contained taxa with comparable durability. Temporal fluctuations in taphonomy and ecology suggest variable environmental conditions operated through time. The overall decline in shell abundance from the last glacial to interglacial paleosols may be explained by a decline in humidity and reduced island surface area resulting in lowered snail proliferation, and in turn, a decreased net shell input rate. This study emphasizes that the original community is preserved within the studied terrestrial shell accumulations regardless of the degree of taphonomic alteration. Measures of past taxonomic richness and diversity, therefore, may be used as a reliable measure of the original snail community.

Anthropogenic effect recorded in the live-dead compositional fidelity of land snail assemblages from San Salvador Island, Bahamas

Terrestrial malacofaunas that inhabit islands are vulnerable to human activities. Habitat destruction, introduction of exotic species, predators, etc. are distorting the composition and distribution of indigenous snail communities. Specifically, the taxonomic discordance between live and dead assemblages may be the consequence of anthropogenic disturbances rather than natural post-mortem processes. Live-dead fidelity may hence reflect the degree of human alteration in a given locality. This approach was used to study the relative abundance of live and dead land snails from pristine and urbanized localities around San Salvador Island (Bahamas). Thirteen species were encountered from 64 samples containing 5,343 specimens. Taxonomic composition was significantly different between areas of negligible anthropogenic disturbance and those impacted by humans. Although which species of land snails are endemic to San Salvador is unknown, all identified species are native to the Caribbean region and invasive species were not encountered. However, some synanthropic species, found exclusively in urban-developed areas, have not been recovered from the local fossil record, pointing to their relatively recent introduction possibly from nearby islands. Non-metric MDS, Jaccard-Chao index, and Spearman correlation analyses indicated that dead assemblages displayed a good compositional correspondence to live communities at pristine sites, whereas considerable disparity was sometimes observed at human-modified areas. Urbanized areas probably favored the proliferation of synanthropic species whereas dead assemblages may contain anthropophobic taxa that lived there prior to recent human modification. These findings are consistent with previous live-dead fidelity studies of marine molluscan and terrestrial small-mammal assemblages, and suggest that the taxonomic discordance between live and dead assemblages of land snails may be indicative of recent anthropogenic alteration.

Drilling predation intensity and feeding preferences by Nucella (Muricidae) on limpets inferred from a dead-shell assemblage

Although limpets are common in rocky intertidal shores, little is known about drilling predation on them. Drilling intensity and preferences by Nucella (Muricidae) on three Lottiidae species (Lottia pelta, L. digitalis, and Tectura scutum) were explored in a modern limpet death assemblage from False Bay (San Juan Island, Washington, USA). Of the 1,531 shells, only 61 (4%) were drilled, with drilling frequencies of 5.9% (L. digitalis), 2.4% (L. pelta) and 0.5% (T. scutum). The higher drilling frequency observed for L. digitalis may reflect spatial differences in prey distribution within the intertidal zone. Hole diameter correlated positively with limpet size, suggesting that larger predators drill larger prey. No differences in drilling frequency were observed due to prey ornamentation or size; however, drill holes were never observed on the largest and thickest L. pelta shells, suggesting a possible size refugium. The majority of holes occurred near the apex, indicating stereotypic attack behavior. Uniform frequency distributions across taphonomic grades and similar central tendencies between drilled and undrilled shells suggest that holes were not affected by taphonomic bias. The preservation of drilled and undrilled shells differed significantly, however; thus, drill holes may have negatively affected the preservation potential of shells, possibly by weakening the shell. Poor shell preservation indicates that biostratinomic effects may play a larger role in preservational biases and underestimation of predation frequencies than previously thought. Studies using drilling frequencies demand careful identification of predatory traces when shells are poorly preserved. In addition, careful evaluation of predation frequency is needed when predatory strategies that may not leave visible traces are possible.

SEASONAL VARIATION IN ECOLOGICAL AND TAPHONOMIC PROCESSES RECORDED IN SHELLY DEATH ASSEMBLAGES

Abstract Modern valves of Lucina pensylvanica (Bivalvia: Lucinidae) were analyzed for 18O/16O ratios, drilling predation traces, biometric measurements, and taphonomic descriptors to explore seasonal variations in ecological and taphonomic processes within a death assemblage from Grand Bahama Island. The &dgr;18O values at the shell margin ( = last growth episode) were used as a proxy for temperature at the time closest to the organisms death. Temperature estimates suggest that most individuals died in warmer months, whereas mortality appeared to have been lower during cooler seasons (<24 °C). Only drilled valves yielded the coldest temperatures at the shell margin (∼18.1–21.3 °C), whereas exclusively undrilled valves exhibited the hottest temperatures at the last growth episode (∼25.5–26.5 °C). Drilled valves were significantly smaller and exhibited higher taphonomic alteration than undrilled valves, pointing to postmortem transport and sorting. While seasonal variations in predation and growth rates may have affected the observed patterns to some extent, the results, at least partly, reflect hydrodynamic variations throughout the year, which may have induced seasonal variation in postmortem sorting of drilled and undrilled valves. This study illustrates that, by combining geochemical proxies and quantitative paleoecological data, more informed ecologic and taphonomic interpretations can be achieved for subfossil and fossil assemblages.

Calibration of the stable isotope composition and body size of the arid-dwelling land snail Sphincterochila candidissima, a climatic archive abundant in Mediterranean archaeological deposits

Land snails recovered from archeological deposits may be used to deduce climatic conditions during prehistoric occupation because their aragonitic skeletons are usually well-preserved and document valuable climatic information in the form of isotope codes. Since the snail Sphincterochila candidissima is common in archeological sites along the western Mediterranean but has been minimally investigated, the present work examines the relationship between the oxygen (δ18O) and carbon (δ13C) stable isotopes of modern specimens and relevant meteorological data. Individuals of Sphincterochila were regularly live-collected throughout 1 year (from September 2013 to October 2014) in Tarragona, NE Spain, and the δ18O and δ13C values were measured at the shell lip or aperture, which depicts the last growth episode closest to specimens’ collection date. Shell margin δ18O values varied from +3.1‰ in September to −0.8‰ in April. Average shell margin δ18O values per collection date positively correlated with monthly averaged rainwater δ18O, and negatively correlated with monthly averaged maximum relative humidity, while other variables (i.e. temperature and precipitation amount) did not explain monthly–submonthly isotopic variations in the shell lip. Shell margin δ13C values ranged from −11‰ in September to −4.8‰ in March, pointing to detectable variations in snails’ diet between seasons. Interestingly, snail body size positively correlated with shell margin δ18O and δ13C values, suggesting that larger snail size is reached under drier conditions. This work shows that S. candidissima from the western Mediterranean is a valid paleoprecipitation δ18O and/or paleohumidity proxy in the region at the monthly–submonthly scale. Although shell margin δ18O recorded fall and spring climate conditions only, limiting therefore their use to estimate the season of harvest, snails that grow year-round (e.g. at subtropical–tropical regions) may be used usefully for such analyses.

On the transition from hunting-gathering to food production in NE Morocco as inferred from archeological Phorcus turbinatus shells

Processes behind the shift from hunting-gathering to food production lifestyle are multifaceted and not yet completely understood. The Mediterranean coast of NW Africa provides an eclectic transitional pattern, namely, a very hesitant transition to food production. The distribution and abundance of early Neolithic domesticated species is disparate and region specific. Climate and environmental change have been often considered as an important influencing factor for this transition. This hypothesis was tested using archeological shells of the rocky intertidal gastropod Phorcus turbinatus recovered from the Ifri Oudadane site in NE Morocco. The oxygen isotope composition (δ18O) of the shell was used to examine whether the hesitant transition to food production was linked to a local climate shift in the Mediterranean Maghreb. Intrashell δ18O values suggest a marked temperature increase from >7.6 to ~7.0 cal. ka BP, the time when Neolithic innovations first appear on site. An additional increase in temperature from ~7.0 to <6.8 cal. ka BP matches with the beginning of the main occupation phase and the doubtless breakthrough of cultivation at Ifri Oudadane. This apparent warming trend, although considered preliminary, seems to match well with warming tendency observed in several published regional climate proxies. Therefore, a temperature shift may have played a role in the timing and implementation of food production in the area. Last growth episode δ18O values suggest that shellfish were harvested throughout most of the year, with noticeable intensification during the cooler half of the year. This preliminary pattern was fairly consistent throughout the Epipaleolithic and early Neolithic phases, pointing to a probable near year-round site occupation rather than a single season settlement. Future research on Ifri Oudadane and other NW African archeological records are much needed to assess whether these patterns persist in Morocco and other Epipaleolithic and early Neolithic settlements in the western Mediterranean Maghreb.