Geology Department

Quantitative estimates of time-averaging (age mixing) in gastropod shell accumulations from Quaternary (the late Pleistocene and Holocene) eolian deposits of Canary Islands were obtained by direct dating of individual gastropods obtained from exceptionally well-preserved dune and paleosol shell assemblages. A total of 203 shells of the gastropods Theba geminata and T. arinagae, representing 44 samples (= strati graphic horizons) from 14 sections, were dated using amino acid (isoleucine) epimerization ratios calibrated with 12 radiocarbon dates. Most samples reveal a substantial variation in shell age that exceeds the error that could be generated by dating imprecision, with the mean within-sample shell age range of 6670 years and the mean standard deviation of 2920 years. Even the most conservative approach (Monte Carlo simulations with a non-sequential Bonferroni correction) indicates that at least 25% of samples must have undergone substantial time-averaging (e.g., age variations within those samples cannot be explained by dating imprecision alone). Samples vary in shell age structure, including both left-skewed (17 out of 44) and right-skewed distributions (26 out of 44) as well as age distributions with a highly variable kurtosis. Dispersion and shape of age distributions of samples do not show any notable correlation with the stratigraphic age of samples, suggesting that the structure and scale of temporal mixing is time invariant. The statistically significant multi-millennial time-averaging observed here is consistent with previous studies of shell accumulations from various depositional settings and reinforces the importance of dating numerous specimens per horizon in geochrono logical studies. Unlike in the case of marine samples, typified by right-skewed age distributions (attributed to an exponential-like shell loss from older age classes), many of the samples analyzed here displayed leftskewed distributions, suggestive of different dynamics of age mixing in marine versus terrestrial shell accumulations.

Stable isotopes of oxygen and carbon extracted from fossil land snail shells have been used increasingly to interpret past environments. To evaluate the utility of this approach for low-latitude oceanic islands, populations of the modern helicid land snail Theba geminata – a species also abundant in the Quaternary fossil record of the region – were sampled at ten low altitude (< 300 m) sites from coastal areas of the eastern Canary Islands. The results include stable isotopes of (1) 17 aragonite shells of live-collected adult snails; (2) 17 body tissue samples from the same snail individuals; (3) 10 samples of carbonate sediments; (4) 69 plant tissue samples representing all 24 identified species; and (5) 7 rain water samples. The mean isotopic composition of the rain water is − 5‰ (V-SMOW) for δD and − 2‰ (V-SMOW) for δ18O, ranging from − 11‰ to + 2‰ (V-SMOW) and from − 2.6‰ to − 0.7‰ (V-SMOW), respectively. The local vegetation is heterogeneous, including C3, C4, and CAM plants. δ13C values vary from − 13.0‰ to − 29.0‰ (V-PDB) across plant species. Of the 24 species, five are C4, 15 are C3, and four are CAM plants. The δ18O values for shells represent a narrow range of values (from − 0.3‰ to + 2.5‰ [V-PDB]), which is consistent with the low climate seasonality typifying low-latitude oceanic settings. Hypothetical model of the expected δ18O value for shell aragonite precipitated in equilibrium suggests that the most negative δ18Oshell represent the closest estimate for δ18Orain water. The δ13C values of shells range from − 9.4‰ to + 1.7‰ (V-PDB). The most positive δ13C values are attributed to a diet based on C4 plants. The comparison of δ13C values of soft tissues and shells suggests that snails ingested notable amounts (from ~ 20% up to ~ 40%) of foreign carbonates. Consequently, fossil shells with the most negative δ13C values should be selected for radiocarbon dating in future geochronological studies of the region. The δ13C values of body tissues vary from − 12.0‰ to − 27.2‰ (V-PDB), indicating that land snails consumed C3 and C4 plants indiscriminately. The mean carbon isotopic composition, averaged across multiple fossil specimens, may thus provide a useful tool for reconstructing paleoclimates and paleoenvironments throughout the Quaternary history of the Canary Islands and other comparable low-latitude oceanic settings.

Napaeus lajaensis sp. nov. is the oldest Napaeus species found in the Canary Islands, with more than 130 ka. It is described from a Pleistocene aeolian deposit intercalated between two basaltic lava flows located at Mancha de La Laja (Tenerife Island). The new species is characterized mainly by the presence of two very prominent, spiral, semicylindrical ribs on the body whorl shell. The stratigraphic setting and taphonomic features of the land snail association to which N. lajaensis belongs, were also shown.

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 communitywide 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 ϳ42,500 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 timeaveraged 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.

Species of the Napaeus subgenera show incongruences between genital anatomy and molecular phylogenetic data. In this study, four new Napaeus species from the Canary Islands are described. Shell drawings of the 56 known Napaeus species were obtained from shell photographs for comparison. Differences between some genital system character-states of the species described here and those of the original subgenera descriptions are evident. Consequently, in the absence of phylogenetic analyses, the new species are not assigned to the Napaeus subgenera. In addition, the possible relationships among Macaronesian enids are preliminarily evaluated.Within the Macaronesian region, enids are exclusively present in the Canaries and the Azores. The absence of enids from Madeira, combined with the differences in genital anatomy between the Canarian and Azorean enids and the relatively large distance between these archipelagos, may indicate different immigration pathways (Africa/Europe, respectively) and possibly different phylogenetic origins for the Canarian and Azorean enid populations.

Quaternary aeolian deposits of the Canary Islands contain well-preserved terrestrial gastropods, providing a suitable setting for assessing the taphonomy and compositional fidelity of their fossil record over ~13 kyr. Nine beds (12, 513 shells) have been analysed in terms of multivariate taphonomic and palaeoecological variables, taxonomic composition, and the stratigraphic and palaeontological context. Shells are affected by carbonate coatings, colour loss and fragmentation. Shell preservation is size-specific: juveniles are less fragmented and show colour preservation more commonly than adults. In palaeosols, the adult shell density correlates negatively with the proportion of fragmented adults, negatively with the proportion of juveniles, and positively with the proportion of adults with coatings. High bioturbation intensity in palaeosols is associated with low shell fragmentation and high proportion of shells with coatings. These relationships imply that high adult density in palaeosols was driven by an increase in shell production rate (related to a decrease in predation rates on adults and a decrease in juvenile mortality) and a decrease in shell destruction rate (related to an increase in durability enhanced by carbonate precipitation). In dunes, the relationships between taphonomic alteration, shell density and bioturbation are insignificant. However, dune assemblages are characterized by a lower frequency of shells with coatings and higher rates of colour loss, indicating lower shell durability in dunes than in palaeosols. Additionally, non-random differences in the coating proportion among palaeosols imply substantial temporal variation in the rate of carbonate crust formation, reflecting long-term changes in bioturbation intensity that covaries positively with shell preservation. Dunes and palaeosols do not differ in species abundances despite differences in the degree of shell alteration, suggesting that both weakly and strongly altered assemblages offer data with a high compositional fidelity. Carbonate-rich terrestrial deposits originating in arid conditions can enhance the preservation of gastropods and result in fossil assemblages that are suitable for palaeoecological and palaeoenvironmental studies of terrestrial ecosystems.

Some snails disguise the shell with a covering of soil or other material. Among species in the Canarian enid genus Napaeus, some rock-dwelling species cover the shell with lichens; in one of these, N. barquini, the process is known to be active. In some ground-dwelling species the muddy covering may be acquired passively, as for example in N. variatus. Napaeus badiosus is a snail with an almost smooth shell which is not disguised in the field. It normally rests out of sight of predators. Four specimens of N. badiosus were transferred to a glass terrarium with a layer of loose, humid soil without stones and vegetation. The snails acquired a soil layer with prominent protuberances that covered the whole shell, reducing the risk of attack by visual predators. The appearance of the disguised N. badiosus shell is compared with those of N. barquini and of N. variatus. The disguise of N. badiosus is similar to that of N. barquini. This active disguise thus appears to be facultative, adopted when hiding places are not available. Mechanisms of disguise are briefly discussed; the behaviour of other disguised species should be investigated.

The isotopic composition of land snail shells was analyzed to investigate environmental changes in the eastern Canary Islands (28–29°N) over the last ~ 50 ka. Shell δ13C values range from −8.9‰ to 3.8‰. At various times during the glacial interval (~ 15 to ~ 50 ka), moving average shell δ13C values were 3‰ higher than today, suggesting a larger proportion of C4 plants at those periods. Shell δ18O values range from −1.9‰ to 4.5‰, with moving average δ18O values exhibiting a noisy but long-term increase from 0.1‰ at ~ 50 ka to 1.6–1.8‰ during the LGM (~ 15–22 ka). Subsequently, the moving average δ18O values range from 0.0‰ at ~ 12 ka to 0.9‰ at present. Calculations using a published snail flux balance model for δ18O, constrained by regional temperatures and ocean δ18O values, suggest that relative humidity at the times of snail activity fluctuated but exhibited a long-term decline over the last ~ 50 ka, eventually resulting in the current semiarid conditions of the eastern Canary Islands (consistent with the aridification process in the nearby Sahara). Thus, low-latitude oceanic island land snail shells may be isotopic archives of glacial to interglacial and tropical/subtropical environmental change.

Hemicycla eurythyra O. Boettger 1908, endemic to Tenerife, was rediscovered near its type locality. It is redescribed and compared with two conchologically similar taxa: H. pouchet (A. Férussac 1821) and H. pouchadan Ibáñez & Alonso 2007. H. eurythyra is grouped in the subgenus Hemicycla s. str. by the presence of a diverticulum in the bursa copulatrix complex.

Four new species of Napaeus are described from La gomera (Canary islands), which is one of the smaller islands of the archipelago but has the highest number of Napaeus species. the four new species can all disguise the shell with a cover of lichens, soil, or both, to reduce predation. When the covering is made of hygroscopic lichen, it might function as a water or humidity reservoir as well as for camouflage.

Neolithic, Copper Age and modern land snail shells from the Los Castillejos archeological site (37°20′N), SE Iberian Peninsula, were analyzed for 13C/12C and 18O/16O ratios to estimate the paleoenvironmental conditions during shell deposition. Modern and ancient δ13C values ranged from −9.2‰ to −7.7‰ and from −13.6‰ to −5.7‰, respectively, and they differed among taxa. δ13C values were similar for Neolithic (δ13C = −8.1 ± 2.3‰), Copper Age (δ13C = −8.4 ± 0.6‰) and modern (δ13C = −8.4 ± 0.5‰) herbivorous Cernuella specimens. Alternatively, fossil shells of the herbivorous Ferussacia folliculum (δ13C = −9.9 ± 1.2‰) and the omnivorous Rumina decollata (δ13C = −11 ± 1.2‰) showed substantially lower δ13C values than modern specimens, which may suggest lower water stress during the early to mid Holocene than today. The δ18O values from modern specimens ranged from −1.1‰ to +0.6‰ while the δ18O values from fossil specimens ranged from −6.7‰ to +0‰ and they differed among species. Cernuella exhibited shell δ18O values that increased from the Neolithic (δ18O = −2.3 ± 1.8‰) and Copper Age (δ18O = −1.7 ± 0.6‰) to the present (δ18O = −0.3 ± 0.5‰). The δ18O values of fossil shells of F. folliculum (δ18O = −3.7 ± 1.0‰) and R. decollata (δ18O = −4.0 ± 1.6‰) were also lower than modern shells. Calculations of a published snail-flux balance mixing model for δ18O values indicate that early to mid Holocene shells precipitated during times when relative humidity was greater than today. The SE Iberian Peninsula was noticeably wetter ∼7200 cal BP, and experienced drier conditions thereafter. The results are consistent with other regional paleoclimatic proxies and reinforce the potential use of land snail shells as paleoenvironmental archives.

Four new species of Napaeus are described from La gomera (Canary islands), which is one of the smaller islands of the archipelago but has the highest number of Napaeus species. the four new species can all disguise the shell with a cover of lichens, soil, or both, to reduce predation. When the covering is made of hygroscopic lichen, it might function as a water or humidity reservoir as well as for camouflage.

From July to November 2009, concentrations of CO2 in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ13C values varied from −8.9‰ to −19.4‰. The CO2 from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ13C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO2 exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ13C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO2 gas. Collectively, the δ13C values of the indoor CO2 samples were linearly correlated with the reciprocal of CO2 concentration, exhibiting an intercept of −21.8‰, with r2 = 0.99 and p < 0.001 (n = 78). This high degree of linearity for CO2 data representing 18 interior spaces (with varying numbers of occupants), and the coincidence of the intercept (−21.8‰) with the average δ13C value for human-exhaled CO2 demonstrates simple mixing between two inputs: (1) outdoor CO2 introduced to the interior spaces by ventilation systems, and (2) CO2 exhaled by human occupants of those spaces. If such simple binary mixing is a common feature of interior spaces, it suggests that the intercept of a mixing line defined by two data points (CO2 input from the local ventilation system and CO2 in the ambient air of the room) could be a reasonable estimate of the average δ13C value of the CO2 exhaled by the human occupants. Thus, such indoor spaces appear to constitute effective “sample vessels” for collection of CO2 that can be used to determine the average proportions of C3 and C4-derived C in the diets of the occupants. For the various groups occupying the rooms sampled in this study, C4-derived C appears to have constituted ∼40% of the average diet.The average concentration of outdoor Dallas atmospheric CO2 was ∼17 ppm higher than the average of CO2 concentrations measured on the same campus 10 a ago. In addition, Dallas outdoor CO2 concentrations at both times were higher than the contemporaneous global atmospheric CO2 concentrations. This observation, plus the fact that the increase of ∼17 ppm in the average concentration of Dallas outdoor CO2 was comparable to the global increase of ∼18 ppm over the same 10-a interval, is consistent with a significant role for urban CO2 “factories” in the global atmospheric CO2 budget.► 13C of indoor CO2 indicates proportion of C4-derived carbon in occupants’ diet. ► Flux balance model for ventilated rooms shows rapid approach to CO2 steady-state. ► From extant indoor CO2 data more dietary C4 carbon in American than European diets. ► Local outdoor urban CO2 increase of 17 ppm in ten years, no change in average 13C.

Aminochronological and aminostratigraphical methods have been used to study the Quaternary aeolian deposits from the islands located east of the Canary Archipelago (Fuerteventura and Lanzarote islands and La Graciosa, Montañ a Clara and Alegranza islets). The extent of racemisation/epimerisation of four amino acids (isoleucine, aspartic acid, phenylalanine and glutamic acid) was measured in land snail shells of the genus Theba. The age calculation algorithms of these amino acids have been determined to permit the numerical dating of these deposits. Eight Aminozones, each defining dune/palaeosol-formation episodes, have been distinguished and dated at 48.6 AE 6.4, 42.5 AE 6.0, 37.8 AE 4.6, 29.4 AE 4.8, 22.4 AE 4.5, 14.9 AE 3.6, 11.0 AE 4.0 and 5.4 AE 1.1 ka BP, the first five of them defining cycles of 5-7 ka.

Napaeus lichenicola sp. nov. from Jandía, Fuerteventura Island, is described, the main differences with the nearest species are discussed and data on distribution and conservation status are presented. The functional meaning of the epiphallar caecum and a chambered epiphallar region are inferred for the accommodation, respectively, of the spermatophoral spurlike hook and sharp-pointed denticles.

Stable isotopes of oxygen and carbon extracted from fossil land snail shells have been used increasingly to interpret past environments. To evaluate the utility of this approach for low-latitude oceanic islands, populations of the modern helicid land snail Theba geminata – a species also abundant in the Quaternary fossil record of the region – were sampled at ten low altitude (< 300 m) sites from coastal areas of the eastern Canary Islands. The results include stable isotopes of (1) 17 aragonite shells of live-collected adult snails; (2) 17 body tissue samples from the same snail individuals; (3) 10 samples of carbonate sediments; (4) 69 plant tissue samples representing all 24 identified species; and (5) 7 rain water samples. The mean isotopic composition of the rain water is − 5‰ (V-SMOW) for δD and − 2‰ (V-SMOW) for δ18O, ranging from − 11‰ to + 2‰ (V-SMOW) and from − 2.6‰ to − 0.7‰ (V-SMOW), respectively. The local vegetation is heterogeneous, including C3, C4, and CAM plants. δ13C values vary from − 13.0‰ to − 29.0‰ (V-PDB) across plant species. Of the 24 species, five are C4, 15 are C3, and four are CAM plants. The δ18O values for shells represent a narrow range of values (from − 0.3‰ to + 2.5‰ [V-PDB]), which is consistent with the low climate seasonality typifying low-latitude oceanic settings. Hypothetical model of the expected δ18O value for shell aragonite precipitated in equilibrium suggests that the most negative δ18Oshell represent the closest estimate for δ18Orain water. The δ13C values of shells range from − 9.4‰ to + 1.7‰ (V-PDB). The most positive δ13C values are attributed to a diet based on C4 plants. The comparison of δ13C values of soft tissues and shells suggests that snails ingested notable amounts (from ~ 20% up to ~ 40%) of foreign carbonates. Consequently, fossil shells with the most negative δ13C values should be selected for radiocarbon dating in future geochronological studies of the region. The δ13C values of body tissues vary from − 12.0‰ to − 27.2‰ (V-PDB), indicating that land snails consumed C3 and C4 plants indiscriminately. The mean carbon isotopic composition, averaged across multiple fossil specimens, may thus provide a useful tool for reconstructing paleoclimates and paleoenvironments throughout the Quaternary history of the Canary Islands and other comparable low-latitude oceanic settings.