Fossil Molluscs Research Articles

More species in warm waters

Paleoecology Past patterns of diversity can be used as a baseline for understanding how current human-induced changes are affecting biodiversity. Womack et al. looked at mollusk fossils from 40 million years ago in New Zealand to determine how ocean temperatures influence species richness and functional redundancy, a measure of how many species fill similar ecological roles. Both richness and redundancy increased in periods with warmer water, meaning that there were more species and that those species often filled similar ecological roles. Such ecological redundancy can increase ecosystem resilience, and understanding its relationship with temperature can help us determine where human activities are driving change. Science , abf8732, this issue p. [1027][1] [1]: /lookup/doi/10.1126/science.abf8732

Paleorecords reveal the increased temporal instability of species diversity under biodiversity loss

Ongoing climate change threatens biodiversity and directly affects the stability of plant-animal communities. However, it is unclear how the species diversity of biological communities responds to natural climate warming, and whether the temporal stability of biodiversity in the face of climate warming varies with environmental gradients in different communities due to the lack of long-term data. We present three high-resolution records of terrestrial mollusk fossils dating back 25 kyr along a climatic and vegetation gradient from southeast to northwest across the Chinese Loess Plateau. The records show that deglacial warming increased the mollusk diversity by ∼30% in species-poor steppe communities; however, in high-diversity forest-steppe communities, the effect was less, and diversity's dynamic stability was even maintained. The high-diversity communities adjusted their composition by substituting thermo-humidiphilous taxa for cold-tolerant taxa, with the compositional turnover reaching ∼60%. The mollusk diversity of species-poor communities is shown to be more sensitive to the last deglacial warming and associated with the strengthening of the East Asian Summer monsoon than that of diverse communities. The increased temporal instability and sensitivity of biodiversity in species-poor communities in the face of climate disturbances, from a long-term perspective, highlights the potential risk of collapse of soil communities under future anthropogenic biodiversity loss.

Palaeo-hydrochemistry reconstructed from fossil mollusc shells from dammed palaeo-lake sediments in the Yarlung Tsangpo valley, Tibet

Radix sp., Gyraulus sp. and Pisidium sp. are aquatic mollusc taxa. Although their fossil shells occur widely in dammed palaeo-lake sediments in the Yarlung Tsangpo valley in Tibet, their hydrochemical and hydrological implications are not fully understood. Moreover, vital effects on the trace element and stable isotope composition of the aragonite shells of these molluscs are unclear. We measured the values of δ13Cshell, δ18Oshell, Sr/Cashell and Mg/Cashell of the fossil shells of Radix sp., Gyraulus sp., and Pisidium sp. collected from the dammed lake sedimentary sequences at Xigazê and Lhainxiang in Tibet, with the aim of reconstructing the palaeo-hydrochemistry and palaeo-hydrology of the dammed palaeo-lake. The results show that the values of δ18Owater, Sr/Cawater and electrical conductivity (EC) of the dammed palaeo-lake water are higher than those of the modern Yarlung Tsangpo River water. Nevertheless, the dammed palaeo-lake still exhibited a through-flow status, together with freshwater characteristics. The values of δ18Oshell and δ13Cshell of Gyraulus sp. and Radix sp. have a positive relationship, but their ratios of Sr/Cashell and Mg/Cashell are not correlated. Moreover, δ18Oshell in the Radix sp. shells is ~2.97 ‰ more enriched and has a stronger relationship with δ18Owater than is the case for shells of Gyraulus sp. The values of δ18Oshell and δ13Cshell of Pisidium sp. are similar to those of the shells of Radix sp., and we conclude that the shells of Radix sp. are the most suitable mollusc shell for recording the δ18Owater of the ambient water of water bodies in the Tibetan Plateau.

Pleistocene Shallow-Water Whale-Fall Community from the Omma Formation in Central Japan

We report a single whale bone associated with many molluscan fossils from the Omma Formation, Lower Pleistocene shallow marine deposits, along the Sai-gawa River, Kanazawa City, in central Japan. Most molluscan species which are commonly found in the Omma Formation show disarticulated and/or damaged shells, indicating semi-autochthonous or allochthonous modes of occurrence. However, the assemblage contained chemosynthetic bivalves, such as lucinid, solemyid and thyasirid bivalves, which are rare in the Omma Formation. The lucinids and solemyids show a high articulation ratio, along with some predatory and scavenging gastropods, such as naticids, nassariids and borsoniids whose well-preserved shells indicate an autochthonous mode of occurrence. In addition, most of the lucinid bivalves show an umbo-upward position similar to the life position of Recent species. Recent lucinid, solemyid and most thyasirid bivalves harbor chemosymbiotic bacteria in their gills and are well known members of the chemosynthetic community. These lines of evidence indicate that the community, mainly comprising lucinid bivalves and other autochthonous molluscan species associated with the whale bone, is an ancient whale-fall community. This shallowest fossil whale-fall community differs from deep-water cases in the dominance of infaunal bivalves, such as lucinids, and in the lack of epifaunal and semi-infaunal chemosynthetic bivalves, such as bathymodiolins and vesicomyids. This community supports a previous suggestion that the difference in characteristic species of the whale-fall communities depends on the water depth.

Drilling predation on Antarctic tusk shells: first records on Recent scaphopods from the Southern Hemisphere

AbstractDrillholes on shells are one of the few predation marks preserved in fossil molluscs, providing an opportunity to study and quantify predator-prey interactions in the palaeontological record. Among these, reports of drilling predation on scaphopods are rare, and such information from Antarctica is non-existent. We describe the finding of drillholes on scaphopods recovered from Recent mollusc assemblages between depths of 246.5 and 454.0 m in West Antarctica. The predation traces located in the middle sectors of two preyed shells belonging to Siphonodentalium dalli and Dentalium majorinum are identified as Oichnus and are interpreted as produced by naticids, probably Pseudamauropsis aureolutea. These new records constitute the first reports of drilling predation on scaphopods in Antarctica and also for Recent scaphopods of the Southern Hemisphere.

Early Permian macroinvertebrate assemblages from the Siang and Subansiri districts, Arunachal Pradesh: Implications on the regional stratigraphy, palaeoenvironment, palaeoecology, and palaeobiogeography

The Early Permian marine strata exposed at the Siang and Subansiri districts, Arunachal Pradesh are known for brachiopod and molluscan fossils. A stratigraphic review and palaeoenvironmental, palaeoecological, and palaeobiogeographic interpretation of these Early Permian fossils have been dealt here on the basis of additional fossil and rock materials. Our study confirms the following: (1) the brachiopod-dominated assemblage represents Early Permian, most likely the Sakmarian and younger, age; (2) all marine fossils were suspension feeding groups, representing normal marine condition. Based on the associated sediments and ecologies of taxa, the autecology of the assemblage represents an oxic, soft substrate condition, typical of shallow shelf environment; the plant remains of the upper Khelong/Bhareli Formation, however, indicate a coastal environment; (3) the depositional paleoenvironment indicates the waning phase of a marine glacial episode; and (4) this Early Permian bivalve-brachiopod assemblage show Gondwanan affinity, having strong faunal similarity with the East and West Australia, Nepal and North India (Kashmir) which were part of the Indoralian Province.

佐渡島（新潟県）の下部中新統最上部下戸層から 産出した砂底および岩礁性軟体動物化石

佐渡島（新潟県）の下部中新統最上部下戸層から 産出した砂底および岩礁性軟体動物化石

Late Pleistocene interstadial sea-levels (MIS 5a) in Gulf St Vincent, southern Australia, constrained by amino acid racemization dating of the benthic foraminifer Elphidium macelliforme

In contrast to the detection and resolution of Late Pleistocene interstadials (Marine Isotope Stages; MIS 5c, 5a and 3) in marine and ice core-based oxygen isotope records, accurately defining palaeosea-levels for these events from sea level indicators remains a challenge. Commonly, such investigations have been undertaken in tectonically active, subduction-related settings, on emergent marginal marine-coastal successions above present sea level, given that the higher ice volumes during these Marine Isotope Stages resulted in ice-equivalent sea levels significantly below present, particularly in far-field settings. Delineation of palaeosea-levels for this period, has accordingly involved an assumption about the long-term rate of coastal uplift. Core SV#23 collected from the tectonically highly stable, remote far-field setting of southern Gulf St Vincent, southern Australia, in a present water depth of 40 m obtained a 3.89 m undisturbed sedimentary record of Late Pleistocene to Holocene age. The basal 2.39 m of the sediment record is of Late Pleistocene interstadial age, as it overlies the Last Interglacial Glanville Formation (128–118 ka; MIS 5e) and occurs beneath the Holocene St Kilda Formation. The extent of aspartic acid and glutamic acid racemization in the benthic foraminifer Elphidium macelliforme sampled at 2 cm intervals continuously from the interstadial succession in Core SV#23, confirms the relative age interpreted from lithostratigraphy. A mean amino acid racemization (AAR) age of 75 ± 13 ka indicates a correlation with MIS 5a, and highlights the utility of E. macelliforme as a species for AAR geochronology. As the foraminifers more closely define the timing of sedimentation, the interstadial succession is regarded as beyond the range of radiocarbon dating (>50 kyr), as revealed in a comparison of radiocarbon ages of fossil molluscs with AAR ages for E. macelliforme from the same depth intervals within the interstadial succession. All of the radiocarbon ages for fossil marine molluscs from the interstadial succession represent minimum ages, resulting from contamination by trace levels of modern radiocarbon. The foraminiferal faunal assemblages within the interstadial succession reveal changes of water depth at the time of deposition, pointing to millennial-scale sea level variability. In particular, the ratio of Elphidium crispum to E. macelliforme indicates several oscillations in relative sea level within a bandwidth of 26 to 24 ± 4.6 m below present sea level (BPSL). Notwithstanding the large uncertainty associated with palaeosea-level estimation based on the Elphidium ratio, the inferred relative sea level record is in accord with the sea level records of Barbados and the Red Sea, and points to Sub-Milankovitch interstadial sea level variability as noted in Greenland and Antarctic ice cores.

Peesey Swamp—a Last Interglacial (MIS 5e) marine corridor across southern Yorke Peninsula, southern Australia

Peesey Swamp on southern Yorke Peninsula, southern Australia, is a north-northwesterly—south-southeasterly trending lowland depression, approximately 24 km long and 4 to 10 km wide. The richly fossiliferous mollusc and foraminiferal faunal assemblages of the last interglacial Glanville Formation in Peesey Swamp indicate that the paleoenvironment was a low- to medium-energy, sheltered sandflat to shallow-water seaway during the Last Interglacial Maximum (Marine Isotope Substage [MIS] 5e; 128–116 ka). The presence of the fossil bivalve molluscs Katelysia rhytiphora, Chlamys (Equichlamys) bifrons and Fulvia tenuicostata, in life position, indicate that the paleowater depth was ≤4 m in Peesey Swamp. The seaway cut across southern Yorke Peninsula, establishing a marine connection between southern Spencer Gulf and Investigator Strait during deposition of these taxa. Amino acid racemisation dating of the fossil marine molluscs Katelysia sp. and Fulvia tenuicostata confirms that the shelly assemblages at Peesey Swamp are correlative with the last interglacial Glanville Formation, a succession of richly fossiliferous, mixed quartz–skeletal carbonate sands documented from many sites along the southern Australian coastline. This study illustrates the significant changes in the geographical configuration of coastlines that may accompany relatively modest (<5 m) sea-level changes on low-gradient coastal landscapes. KEY POINTS Peesey Swamp on southern Yorke Peninsula was flooded by seawater forming a 4 to 10 km-wide seaway during the Last Interglacial Maximum (MIS 5e). Fossil mollusc and foraminiferal assemblages reveal deposition within shallow subtidal to intertidal sandflat environments indicating that the paleowater depth and height of sea-level in Peesey Swamp was ≤4 m. Amino acid racemisation dating of fossil marine molluscs confirms that the shelly assemblages at Peesey Swamp are of last interglacial (MIS 5e) age. The distribution of the foraminifer Marginopora vertebralis indicates that the eastward flow of the warm-water Leeuwin Current, during the Last Interglacial Maximum, was retarded by the biogeographical barriers of Kangaroo Island and Fleurieu Peninsula.

An asynchronous Mesozoic marine revolution: the Cenozoic intensification of predation on echinoids

Predation traces found on fossilized prey remains can be used to quantify the evolutionary history of biotic interactions. Fossil mollusc shells bearing these types of traces provided key evidence for the rise of predation during the Mesozoic marine revolution (MMR), an event thought to have reorganized global marine ecosystems. However, predation pressure on prey groups other than molluscs has not been explored adequately. Consequently, the ubiquity, tempo and synchronicity of the MMR cannot be thoroughly assessed. Here, we expand the evolutionary record of biotic interactions by compiling and analysing a new comprehensively collected database on drilling predation in Meso-Cenozoic echinoids. Trends in drilling frequency reveal an Eocene rise in drilling predation that postdated echinoid infaunalization and the rise in mollusc-targeted drilling (an iconic MMR event) by approximately 100 Myr. The temporal lag between echinoid infaunalization and the rise in drilling frequencies suggests that the Eocene upsurge in predation did not elicit a coevolutionary or escalatory response. This is consistent with rarity of fossil samples that record high frequency of drilling predation and scarcity of fossil prey recording failed predation events. These results suggest that predation intensification associated with the MMR was asynchronous across marine invertebrate taxa and represented a long and complex process that consisted of multiple uncoordinated steps probably with variable coevolutionary responses.

Late Paleogene paleotopographic evolution of the northern Cordilleran orogenic front: Implications for demise of the orogen

The paleotopographic history of the North American Cordilleran orogen holds the key to understanding mechanisms of orogenesis and subsequent orogenic collapse. It has been suggested that the orogenic front in western Montana (USA) and Alberta (Canada) was more than 4 km high during Late Cretaceous−early Eocene contractional deformation and during the initial phase of extension in the middle Eocene; however, the late Eocene−Oligocene topographic evolution during continued extensional collapse remains poorly constrained. Here we extend the paleotopographic record in the Kishenehn Basin in northwestern Montana and southeastern British Columbia (Canada) to the late Oligocene by studying δ18O values of fossil mollusks and cement and paleosol carbonates. The molluscan taxa changed from three sympatric groups with preferred habitats ranging from tropical wet, semi-arid subtropical, and temperate during the middle and late Eocene, to mainly a single group associated with temperate environment during the Oligocene, reflecting a decline in molluscan biodiversity induced by climate cooling across the Eocene−Oligocene transition. Reconstructed δ18O values of alpine snowmelt and basinal precipitation decreased by 1.4‰ and 3.8‰, respectively, from the middle to late Eocene, reflecting climate cooling and ∼1 km surface uplift of the basin floor. The reconstructed alpine snowmelt δ18O values then increased by 2.9‰ in the Oligocene suggesting a ∼0.5 km drop in elevation of the orogenic front. Collectively, the results of our new and previously published δ18O data chronicle the paleotopographic response to the change from flat-slab subduction to slab rollback over a 45 m.y. period. These data suggest that the orogenic front was characterized by high elevation (&amp;gt;4 km) in the ancestral Lewis-Clark-Livingston ranges during latest Cretaceous−early Eocene (ca. 75−52 Ma) contraction. The initial phase of extension related to the Kishenehn Basin created a lowland basin with a surface elevation of only ∼1.5 km during the early middle Eocene (ca. 46−44 Ma) whereas the ranges remained &amp;gt;4 km high. The high range elevations were sustained for at least 12 m.y. in the middle to late Eocene concurrent with extension, while the basin floor elevation was uplifted to ∼2.5 km by the latest Eocene (ca. 36−34 Ma). Basin aggradation can explain at most half of the 1 km basin floor uplift. The remaining amount (at least 0.5 km) and sustained high range elevation suggest that range denudation and crustal extension was compensated by the isostatic and thermal effects of slab rollback and/or passage of a slab window and infusion of hot asthenosphere beneath the continent. The range elevation in the orogenic front decreased ∼0.5 km by the late Oligocene (ca. 28 Ma), associated with a decrease in rock uplift rate associated with extension. A post-Oligocene elevation drop of ∼1 km resulted in both the ranges and basin floor reaching modern topography in the Kishenehn Basin drainage, likely due to the regional effect of Neogene Basin and Range extension. This study, along with the previous investigation of the Kishenehn Basin by Fan et al. (2017), are the first studies that systematically investigate paleorelief of the orogenic belt by reconstructing paleoelevations of the mountains and the basin at the same time. The results highlight that the Cordilleran orogenic front of northern Montana and southern British Columbia sustained its high elevation edifice for at least 12 m.y. after the start of extension. We suggest that initial crustal extension did not result in orogenic demise because of concurrent thermal and isostatic uplift.

Temperatures of Late Cretaceous (Campanian) methane-derived authigenic carbonates from the Western Interior Seaway, South Dakota, USA, using clumped isotopes

Abstract Methane seep deposits, comprising large, carbonate-rich mounds formed from hydrocarbon seepage, were widely distributed in the Late Cretaceous Western Interior Seaway (WIS) of North America. Well-preserved, methane-derived authigenic carbonates (MDACs) from these deposits have been shown to retain petrological, paleontological, and geochemical imprints of their ancient depositional setting, all of which are important for understanding the dynamics and evolution of the shallow, epeiric WIS. To better characterize the environmental conditions of WIS seeps, we applied clumped isotope paleothermometry to magnesium calcite MDAC samples from five seep localities in the upper Campanian Pierre Shale, South Dakota, USA. We measured 21 subsamples, including 18 micritic carbonates and demonstrated apparent clumped isotope equilibrium between MDACs and WIS bottom waters. Extreme 13C depletion in most samples (δ13C ranging to −45.44‰) indicates they were precipitated with oxidized methane as a major source of dissolved inorganic carbon, which itself implies a close association with ancient methanotrophic metabolism. The average clumped isotope paleotemperature from the micritic carbonates is 23 ± 7 °C (1σ standard deviation), which agrees with bottom water paleotemperatures inferred from δ18O measurements of MDACs and well-preserved mollusk shells at similar localities in the WIS. The calculated average δ18Ow value for these samples is −0.5 ± 1.7‰ (1σ SD), which is indistinguishable from previously reported calculation on Campanian seawater δ18Ow from fossil mollusk shells, but elevated above younger fossils collected from other locations in the WIS. Our conclusions are inconsistent with previously hypothesized disequilibrium for WIS MDAC clumped isotope and therefore we propose that fossil MDAC deposits may be used as paleotemperature archives.

Depositional environments and taphonomy of the bone-bearing beds of the Lower Cretaceous Kuwajima Formation, Tetori Group, Japan

Thebone-bearing beds of the Lower Cretaceous Kuwajima Formation (Tetori Group) are described. Three facies of bone-bearing beds (Facies I: carbonaceous sandstones; Facies II: dark grey fine-grained silty sandstones; Facies III: dark greenish-grey mudstones) are present in inter-channel deposits that originated on a floodplain. The grain size of the sediments, and plant and molluscan fossils occurring in each bone-bearing bed, indicate that Facies I was deposited in a peat marsh, Facies II in a shallow lake, and Facies III in a vegetated swamp. Isolated small bones and teeth are the most abundant vertebrate fossils. Common elements in Facies II are aquatic vertebrates such as fishes and turtles. Facies III is characterized by the occurrence of terrestrial lizards, tritylodontid synapsids and mammals. Vertebrate fossil assemblages in Facies II and III are not mixed with each other even though they both represent parautochthonous assemblages. In contrast, Facies I is allochthonous, and is composed mostly of reworked sediments from Facies II. Depositional environments of the bone-bearing beds are strongly correlated with the composition of their fossil assemblages, indicating that different facies preserve the original faunal differences that existed between the shallow lake and vegetated swamp environments.

Early Pliocene gastropod diversity from the central eastern Pacific Ocean, Mexico (Guerrero) and their paleogeographical interpretation

We report on an early Pliocene marine gastropod assemblage from Guerrero State, Mexico. Sixty-three gastropod species were identified and used to interpret their paleogeographic significance. From this assemblage, 15 species are reported as fossils for the first time, and 10 have not been previously reported older than early Pleistocene. The genus Conasprella is reported in the eastern Pacific for the first time. Bursa rugosa, Conus patricius, and Eupleura muriciformis are paciphile species, with a distribution on both sides of the Isthmus of Panama prior to the closure of the Central American Seaway, but distributed during the recent only in the Pacific. In addition, Architectonica nobilis and Distortio decussata were present in both the Atlantic and Pacific oceans during the Pliocene. These gastropods help to bridge a huge gap, between Nicaragua and southern California, USA, in fossil molluscan occurrences from the eastern Pacific part of the Tertiary Caribbean faunal province. They help document the early Pliocene Tertiary Caribbean faunal province in southern Mexico in the eastern Pacific before the final closure of the Isthmus of Panama during the late Pliocene.

Inferring octopodoid and gastropod behavior from their Plio-Pleistocene cowrie prey (Gastropoda: Cypraeidae)

Predation is an evolutionary force shaping sea floor communities, with the record of drilling predation being particularly useful to study predatory behavior on short and long timescales. Most predatory drill holes are caused by gastropods, but octopods within Octopodoidea also produce characteristic drill holes, yet remain severely understudied in deep time. Consequently, studies comparing octopodoid versus gastropod drill holes from the same prey group are exceedingly rare. Cypraeid gastropod (cowries) are commonly drilled by octopodoids today, but nothing is known about drill holes produced by octopodoids and gastropod in fossil cypraeids. We studied 2406 Plio-Pleistocene fossil cowries from southern Florida, USA. Drilling by octopodoids was common (13.4% of specimens), whereas gastropod drill holes were rare (0.2%). There is strong site selectivity: most octopodoid drill holes are found in the ventral-left-posterior and ventral-left-middle shell regions. These parts are not the thinnest regions of the shell, but represent the location where the columellar muscle anchors the soft tissue to the shell interior. We suggest that the octopodoids sought to paralyze and relax this muscle via the drill hole to minimize the time to consumption. Specimens with multiple octopodoid drill holes (24.1% of drilled specimens) have a higher proportion of drill holes located outside the ventral-left-posterior and ventral-left-middle regions. Thus, drilling outside these regions appears less effective. The few gastropod drill holes occur on the dorsal side. A meta-analysis shows that octopodoids produce significantly more drill holes per drilled molluscan shell than do gastropod drillers. We found mixed evidence for size selectivity: whereas the size of drilled specimens does not significantly differ from non-drilled specimens, drill-hole size is significantly larger in larger cowries, which may imply they were drilled by larger octopodoid individuals. We suggest that the molluscan fossil record should be leveraged to further study the evolution of octopodoid predation and behavior.

Fossil molluscs from borehole Hollum (Ameland, the Netherlands) constrain three successive Quaternary interglacial marine intervals in the southern North Sea Basin

Abstract When dealing with stratigraphic successions in marginal basin settings, the geological record is often fragmented due to erosion and reworking processes. The North Sea Basin is an example: it has a fragmented Quaternary record; in particular, Middle Pleistocene intervals are poorly known. As a result, we have little insight into climate, marine environmental conditions and biodiversity in this period. Here we describe and discuss a succession of three interglacial marine mollusc-bearing intervals in a borehole from Ameland in the northern Netherlands (borehole B01H0189 near Hollum). These intervals are attributed to marine isotope stages MIS7, MIS5e and MIS1. The Holocene Celtic type of faunas (interval 0–26.24 m below surface (b.s.)) and Eemian Lusitanian type of faunas (26.24–30.40 m b.s.) are well-known from previous research. The newly reported MIS7 Oostermeer fauna (32.80–39.00 m b.s.) represents mostly full marine settings between storm wave base and fair-weather wave base. In composition and diversity, the MIS7 and MIS1 faunas strongly resemble and differ from the MIS5e fauna. This is the first well-documented record of three stacked marine interglacial assemblages from the southern North Sea Basin at one location. This new record enables us to make complete marine faunal characterisations of successive interglacial periods. Key implications for southern North Sea stratigraphy and palaeogeography are the resemblance of marine faunas and conditions in MIS7 and MIS1, the presence of a relatively warm latest MIS6 freshwater interval and confirmation and characterisation of the warm Eemian interval north of the classical type area.

Palaeoenvironmental conditions of the Palaeolithic–Neolithic transition in the Fergana Valley (Central Asia) – New data inferred from fossil mollusсs in Obishir-V rockshelter (Kyrgyzstan)

In the Fergana Valley and the surrounding territories, the number of archaeological sites increased during the Early Holocene. The coincidence of this cultural event, represented by the Obishirian archaeological unit with the onset of the Holocene, suggests that this growing intensity of human activity was driven by climate change, in particular warming and/or wetting. To test this hypothesis, we used fossil terrestrial molluscs from Obishir-V, the eponymic Obishirian site in southern Kyrgyzstan, as a proxy of palaeoenvironmental conditions. We studied pre-Obishirian loess-like deposits from the final Pleistocene and the Early–Middle Holocene sequence of Obishirian-bearing deposits. We analysed taxonomic composition of the mollusc assemblages and used it to infer palaeoecological characteristics, namely preferences in temperature and humidity as well as in differing habitats. Our results showed that the palaeolandscapes and the vegetation during the accumulation of the studied deposits remained almost unchanged. The mollusc assemblages reveal constant dry and warm climatic conditions and stable steppe-like vegetation. Our observations of rather stable climatic conditions in the Fergana Valley during the Pleistocene–Holocene turnover suggests that climate was only a minor driver of the rapid demographic change in Central Asia during those times. Some other factors beyond the changes of humidity must have played an important role. On the basis of our data derived from fossil mollusc assemblages at Obishir-V, we propose to consider the hypothesis that the factor responsible for the intense human occupation of the Fergana Valley during the Early Holocene had not an environmental, but rather a cultural background, such as the neolithization process, such as introduction of domestic livestock.

Out of the oven and into the fire: Unexpected preservation of the seasonal δ18O cycle following heating experiments on shell carbonate

Oxygen isotope values of shell carbonate sampled along the growth trajectory of fossil mollusks provide one of the few types of proxy data available for reconstructing seasonal temperature variation in Earth's past. Understanding how diagenesis affects the preservation of seasonal cycles is key to their interpretation in a paleoclimate context. An assumption common in the literature, albeit tenuous, is that the alteration of an aragonite shell to calcite will homogenize intra-shell stable isotope variation, thus attenuating or erasing any seasonal cycle and resetting bulk shell values. Demonstration of regular variation in δ18O values across growth bands in a still-aragonite mollusk is therefore generally interpreted to reflect retention of the primary environmental signal. However, this implied assumption remains untested, and experimental studies have demonstrated that heating can decrease shell δ18O values with negligible mineralogical transformation. Here we investigate how dry heat impacts both the geochemistry and mineralogy of shell aragonite by roasting time-equivalent shell sections of a modern bivalve, Spisula solidissima, at 200 °C for intervals ranging from 2 hours to 2 weeks, and comparing seasonal oxygen isotope variations recovered from the same one-year period of shell accretion with those from untreated shell. Oxygen isotope values from heated shell sections decline by as much as 1.5‰ across all parts of the ~2.5‰ annual cycle, and degree of 18O depletion decreases logarithmically with time heated. Neither Raman spectroscopy nor X-ray diffraction indicate mineralogic change in association with heating. Scanning electron microscopy reveals that heating results in some intra-crystal pitting and subtle smoothing of crystal edges, but heated shell lacks any textural evidence for inversion to calcite, suggesting that fine-scale isotopic exchange may take place with shell-bound water vapor and/or organic matter. The surprising magnitude of this shift in the absence of mineralogic transformation highlights the potential impact of diagenetic processes on the preservation of paleotemperature signals, seasonal or otherwise, in accretionary biogenic carbonates, and suggests a need for more stringent guidelines in assessing the preservation of these materials during the employment of common sclerochronological methodologies.

Climatic structures and intensities of the last two glacials documented by terrestrial molluscs from Chinese loess sequences

Knowledge of the structures and intensities of past glaciations is crucial for understanding Quaternary palaeoclimatic evolution. Terrestrial molluscs are a reliable proxy of past environmental conditions and can provide specific climatic insights into glacials. Here we compare the climatic evolution of the last two glacials, based on mollusc fossils from the L2 and L1 loess units (equivalent to Marine Isotope Stages (MIS) 6 and 4–2, respectively) of the Xifeng and Luochuan sections of the Chinese Loess Plateau (CLP). The results show that both glacials had a tripartite structure but with different intensities, as indicated by variations in the abundance of the dominant cold‐aridiphilous (CA) mollusc species, Vallonia tenera and Pupilla aeoli. The first stage, from ~191 to 184 ka for MIS 6, and from ~71 to 57 ka for MIS 4, was characterized by a cold, arid climate, as indicated by the abundance of V. tenera and P. aeoli, and the limited occurrence of thermo‐humidiphilous (TH) species. The second stage, from ~184 to 155 ka for MIS 6, and from ~57 to 29 ka for MIS 3, was characterized by a relatively mild, humid climate, indicated by a decrease in V. tenera and P. aeoli, and an increase in TH species. The last stage, from ~155 to 130 ka for MIS 6, and from ~29 to 11.7 ka for MIS 2, experienced the coldest and driest conditions, as indicated by the dominance of V. tenera and P. aeoli in late MIS 6 and by the reduced occurrence of the grassland biomass, necessary for the growth of CA species, in MIS 2. The climate during MIS 2 was much colder and drier than that during late MIS 6. We suggest that the structures of the glacials were controlled mainly by insolation driven by changing astronomical configurations, amongst which obliquity likely played the major role. The different glacial intensities across the CLP may also have been related to variations in Northern Hemisphere ice sheets and the temperature of the tropical oceans.

Spatial scaling of beta diversity in the shallow-marine fossil record

AbstractBeta diversity quantifies the spatial structuring of ecological communities and is a fundamental partition of biodiversity, central to understanding many macroecological phenomena in modern biology and paleobiology. Despite its common application in ecology, studies of beta diversity in the fossil record are relatively limited at regional spatial scales that are important for understanding macroevolutionary processes. The spatial scaling of beta diversity in the fossil record is poorly understood, but has significant implications due to temporal variation in the spatial distribution of fossil collections and the large spatiotemporal scales typically employed. Here we test the spatial scaling of several common measures of beta diversity using the Cenozoic shallow-marine molluscan fossil record of New Zealand and derive a spatially standardized time series of beta diversity. To measure spatial scaling, we use and compare grid-cell occupancy based on an equal-area grid and summed minimum spanning tree length, both based on reconstructed paleocoordinates of fossil collections. We find that beta diversity is spatially dependent at local to regional scales, regardless of the metric or spatial scaling utilized, and that spatial standardization significantly changes apparent temporal trends of beta diversity and, therefore, inferences about processes driving diversity change.