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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.
Although the field of taphonomy has existed for >70 years, the majority of work has focused on siliciclastic settings at the expense of pure carbonates. The large variety of carbonate sediment types, together with the growing recognition that carbonate platforms are composed of facies mosaics that shift laterally in time and are frequently superimposed in the rock record, raises the question of whether quantitative paleoecological studies focused on the fossil record in carbonate environments are subject to significant biases with successive vertical facies changes. This study combines live-dead and surface-subsurface analyses on communities of benthic foraminifera from four localities around San Salvador Island, Bahamas, that represent a spectrum of energy settings, bioturbation intensity, and local anthropogenic impact, in order to test the fidelity with which community data are translated from life to death assemblage on the surface, and from death assemblage to the subsurface (i.e., subfossil record) in these different facies. Although the live-dead data are inconclusive, surface-subsurface analyses demonstrate that regardless of ambient water energy or local intensity of bioturbation, community indices of dominance, evenness, diversity, and equitability are translated with equivalent and minimal modification into subsurface sediments. In addition, surface-subsurface analyses at one site suggest that surface-subsurface agreement may be lower in sites affected by recent anthropogenic impact. Overall, the results of this study represent strong taphonomic vindication for quantitative paleoecological studies spanning facies changes in the carbonate fossil record.
Taphonomic processes can substantially affect grain composition and textural parameters of carbonate sediment, but can also produce taphonomic signatures that provide supplemental information on the environment of deposition. The aim of the present study is a microtaphofacies analysis of a tidal channel–metahaline lagoon system (North Branch of Pigeon Creek, San Salvador Island, Bahamas). Samples (39 in total) were collected in 13 sites in three zones located along the channel: in the inlet, in the middle part, and in the inner lagoon. Grain composition and taphonomic features—abrasion, boring, calcification, encrustation, fragmentation, and dissolution—were recorded using (1) examination of loose grains belonging to the sand-size fraction, and (2) point counting in thin sections produced from sediment samples. Proportions of different grain types were not correlated in thin sections and loose sediment samples from the same sites, with the exception of algae, which were systematically recorded in thin section in higher proportions. In examination of loose grains, abrasion, fragmentation, and calcification were the most useful taphonomic features, but not in thin sections; only fragmentation rates were correlated between the two proxies. Taphonomic information did not distinguish new facies in surface sediment, but was useful in refining environmental resolution and broadened the interpretation of the environment of deposition. Discrimination between sites based on compositional and textural characteristics of the sediment was improved more by increasing the number of replicates by site than by inclusion of taphonomic data. Application of recognized microtaphofacies to the fossil record is limited, because very little of grain diversity and taphonomic information can be recovered from analysis of thin sections.
This study explores spatial distribution, species diversity, and preservation potential of modern rocky intertidal gastropod communities from San Salvador Island, Bahamas. Using a hierarchical sampling approach, 17,703 intertidal gastropod specimens representing 9 genera and 15 species were recorded from 480 quadrats along 120 transects at 40 sites sampled at ten localities around the island. All localities are dominated by several species of littorinids and neritids. The rank-abundance structure was comparable across all localities, with the same species dominating at all but one locality. The hierarchical sampling scheme revealed a gradual increase in diversity across sampling levels, with most notable increase observed between site-level and locality-level diversity estimates. Habitat diversity estimates did not vary from bare and exposed surfaces to sheltered pits, crevices, and tide pools. Across energy regimes, there was no meaningful shift in habitat occurrences to more sheltered, protected habitats. Species occurrences in sheltered habitats should augment fossilization potential of those assemblages because, as shown in some paleontological studies, there is evidence of rocky intertidal environments from such settings as tide pools, pits, and crevices. Indeed, when data are restricted to more preservable, sheltered settings only, a good representation of total biodiversity and overall community structure of sampled faunal associations is still retained. Moreover, because the sampled communities appear remarkably homogeneous across and within localities, even a highly fragmentary fossil record (e.g., small area in one locality) would likely collect a substantial fraction of biodiversity and community structure of rocky intertidal communities of San Salvador Island.
Echinoderm remains in the sand-size fraction of tropical, shallow-water carbonates were investigated in order to correlate depositional environments, preservation potentials, distribution, and taphonomic signatures. Fifteen sediment bulk samples were collected from five different environments at Fernandez Bay, San Salvador Island, Bahamas. Studied environments include: (1) intertidal beach rock, (2) subtidal bedrock overgrown by algae, (3) a subtidal Sargassum meadow, (4) a subtidal patch reef, and (5) subtidal loose sands without vegetation. At least 50 echinoderm elements and or fragments were randomly sampled from the fractions of 125–250 µm, 250–500 µm, and 500–1000 µm, resulting in a total number of 2355 ossicles. The distribution and taphonomic signature of the remains were analyzed using exploratory data analyses. Echinoid remains decrease coincident with an increase in ophiuroid ossicles from shore to reef. Encrustation and bioerosion are rare in the sand-sized fraction, whereas fragmentation and abrasion are common traits observed in all environments. Within-environment variation is high, however, and only the analyses of further transects can assess the influence of variability along the shore. The combination of distribution patterns and taphonomic signatures, however, reveals trends to distinguish three depositional environments that are characterized by: (1) a high percentage of echinoid remains, and high fragmentation and abrasion levels; (2) high fragmentation and low abrasion values; and (3) a high percentage of ophiuroid remains, and intermediate values for both fragmentation and abrasion. These results can aid in future analyses and interpretation of ancient carbonate deposits.
Lithification with related diagenetic phenomena is an important step in a complex transition from living communities to fossil assemblages and a major taphonomic filter distorting the record of past biodiversity. Apart from direct diagenetic culling of fossils, cementation of fossiliferous deposits induces changes in sampling procedures used to extract paleontological data. This study explores the effects of this methodological shift on recorded fine-scale paleoecological patterns by using subfossil mollusk assemblages occurring in the unlithified and recently cemented storm-beach carbonate sands at Sand Dollar Beach, San Salvador Island, Bahamas, which experienced limited diagenetic alteration. Results show that consistent differences in relative abundance patterns of particular taxa can be observed between unlithified and lithified samples due to collection failure. Magnitude of this distortion is controlled in a large part by a degree of transport-related size sorting, with well-sorted assemblages dominated by small gastropods being more affected. This bias, however, is of limited importance and can be mitigated by selective exclusion of the smallest size classes (<5 mm) from the analysis. Moreover, unlithified and poorly lithified deposits record very similar rarefied richness estimates and patterns of diversity partitioning. This contrasts strongly with earlier estimates of lithification-related diversity loss, suggesting greater importance of diagenetic over methodological filters in creating lithification bias observed in the older rock record. Poorly lithified carbonate rocks—present in many late Neogene successions—may yield biodiversity data directly comparable to those recorded by unlithified sediments if careful collecting methods based on bulk samples are employed.