Taphonomy of a Panopea Ménard de la Groye, 1807 shell bed from the Pisco Formation (Miocene, Peru)

Invertebrate taphonomy can provide significant information about the post-mortem processes that affected the fossil record. In the East Pisco Basin of southern Peru, a Panopea Ménard de la Groye, 1807 shell bed was found in the upper Miocene strata of the Pisco Formation, hinting at a peculiar biostratinomic and diagenetic history. This bed contains abundant invertebrate fossil molds cemented by dolomite. The specimens of the deep infaunal bivalve, Panopea sp., occur together with bivalves representative of shallow infaunal species (Trachycardium sp. and Dosinia ponderosa [Gray, 1838]) and balanid barnacles, which are sessile encrusters. The Panopea specimens host compound molds evidencing an abundant encrusting fauna, including serpulids, ?foraminifera, bryozoans, and barnacles that colonized the inner surfaces of the valves before their final burial. We hypothesize that short-term, storm-related processes exhumed the living bivalves, resulting in a sedimentological concentration of relatively well-preserved shells. After the death of the exhumed bivalves, the inner surfaces of the articulated Panopea shells, representing hard-substratal, sheltered environments on an otherwise unstable sandy seafloor (i.e., “benthic islands”), were colonized by different encrusting organisms. Following the final burial, dolomite precipitated, cementing the sediment infill of the valves. Lastly, a decrease of pH occurred at the sulfate reduction-methanogenesis boundary, inducing the dissolution of the shell carbonate.

Radiocarbon dating supports bivalve-fish age coupling along a bathymetric gradient in high-resolution paleoenvironmental studies

Studies of paleocommunities and trophic webs assume that multispecies assemblages consist of species that coexisted in the same habitat over the duration of time averaging. However, even species with similar durability can differ in age within a single fossil assemblage. Here, we tested whether skeletal remains of different phyla and trophic guilds, the most abundant infaunal bivalve shells and nektobenthic fish otoliths, differed in radiocarbon age in surficial sediments along a depth gradient from 10 to 40 m on the warm-temperate Israeli shelf, and we modeled their dynamics of taphonomic loss. We found that, in spite of the higher potential of fishes for out-of-habitat transport after death, differences in age structure within depths were smaller by almost an order of magnitude than differences between depths. Shell and otolith assemblages underwent depth-specific burial pathways independent of taxon identity, generating death assemblages with comparable time averaging, and supporting the assumption of temporal and spatial co-occurrence of mollusks and fishes.

The BivalveGlycymeris pilosaas an Archive of14C in the Mediterranean Sea

ABSTRACTThis study combines radiocarbon (14C) analysis and sclerochronology research, an approach that to the best of our knowledge, has not yet been applied using bivalves from the Mediterranean Sea. We analyzed shells from the North Adriatic Sea: live- and dead-collected specimens of the infaunal bivalveGlycymeris pilosaand two dead-collected specimens ofGlycymerissp. According to crossdating results, growth increment time series obtained from acetate peels of the dead-collectedG. pilosa(S3FP11) indicate the potential for creating longer chronologies from live and dead-collected specimens. The greatest longevity was seen in the dead-collectedGlycymerissp. specimen S3F3, estimated to be ~130 years (started growing AD 1678–1742 and died AD 1826–1860), indicating the potential to extendGlycymerisgrowth increment chronologies to past centuries. The highest ∆14C values obtained corresponded to the calendar year 1974. The14C record obtained fromG. pilosacorrelates well with the modeled surface ocean (mixed-layer) bomb pulse curve (Reimer et al. 2009). Based on the results obtained from the shell growth increment assigned to AD 1950, the reservoir age and reservoir correction (ΔR) are 264±23 years and –6±32 years, respectively.

Anchors and snorkels: heterochrony, development and form in functionally constrained fossil crassatellid bivalves

New growth rate estimates for nine species from three genera of New Zealand Crassatellidae (Mollusca: Bivalvia), combined with existing morphometric ontogenetic descriptions, allow identification of heterochronic processes in the evolution of these genera. Both paedomorphosis (progenesis and neoteny) and peramorphosis (hypermorphosis and acceleration) have occurred within the clade. Overall, morphological variability and response to environmental pressure in this nonsiphonate group is restricted by the interplay of anatomical and life habit constraints. Stability in the substrate, predator avoidance, sluggish burrowing speed, and inability to escape by deep burial are suggested as key drivers of, or constraints on, morphological change. Two groups of shell characters are identified: heavy, armored “anchors” and elongate “snorkels,” which combine juvenile and adult traits in shells of different sizes and ages, produced by heterochronic variation in developmental timing. Anchors and snorkels both represent different “solutions” to the problems of life as a nonsiphonate, infaunal bivalve.

Mixed assemblages of drilling predators and the problem of identity in the fossil record: A case study using the muricid gastropod Ecphora

Drillholes made by naticid and muricid gastropods are frequently used in evolutionary and ecological studies because they provide direct, preservable evidence of predation. The muricid Ecphora is common in many Neogene Atlantic Coastal Plain assemblages in the United States, but is frequently ignored in studies of naticid predation. We used a combination of Pliocene fossil, modern beach, and experimentally derived samples to evaluate the hypothesis that Ecphora was an important source of drillholes in infaunal bivalve prey shared with naticids. We focused on the large, thick-shelled venerid, Mercenaria, which is commonly drilled by naticids today. Laboratory experiments, modern beach samples, and the published literature confirm that naticids preferentially drill near the umbo (significant clumping of holes), show a significant correlation between prey size and predator size (estimated by outer borehole diameter), and prefer Mercenaria <50 mm antero-posterior width when other prey are present. Fossil samples containing Ecphora (with or without other large muricids) show no drillhole site stereotypy (no significant clumping, greater variability in placement), no significant predator: prey size correlation, drilled prey shells larger than the largest modern naticids could produce in an experimental setting, and drillholes larger in diameter than those estimated for the largest Pliocene naticids, thus supporting our hypothesis. Substantial overlap in the placement of holes drilled by naticids and muricids, however, made identifying predators from drillhole position problematic. The lack of overlapping ranges of prey shell thickness between fossil and other samples precluded the use of drillhole morphology to establish predator identity (e.g., ratio of inner borehole diameter to outer borehole diameter, drillhole angle). Whereas the difficulty in determining predator identity from drillholes limits the types of analyses that can be reliably performed in mixed-predator assemblages, recognizing Ecphora as a prominent drilling predator creates the opportunity to investigate previously unrecognized questions.