Digenean trematodes are parasites with a complex life cycle that often infest shell-bearing mollusks and produce distinct traces on the host skeleton that are recognizable in the fossil record. Here, three bivalve species (Transennella conradina, Abra segmentum, and Chamelea gallina) from Pleistocene and Holocene deposits of Florida and Italy were used to evaluate the hypothesis that trematode infestation affects shell morphology. The morphological effects of infestation were evaluated using geometric morphometrics and the pallial sinus index (PSI = pallial sinus length/shell length). For all three host species: (1) large size classes possess higher trematode prevalence (i.e., proportion of specimens possessing trematode-induced pits within a population) and higher per-specimen frequency of trematode-induced scars when compared with smaller size classes, suggesting ontogenetic accumulation of parasites; and (2) infested and non-infested specimens significantly differ in shell landmark-based morphology. Geometric morphometric analyses indicate that in two out of three species (Transennella conradina, Abra segmentum): (1) PSI and thin-plate spline analyses suggest significant pallial sinus reduction in infested specimens relative to non-infested; and (2) overall morphospace range, estimated by sample-standardized principal component (PC) hypervolume, was inflated with the inclusion of infested specimens. Consistent with previous studies, results indicate that trematode-induced morphological changes may influence the burrowing capabilities of the studied bivalves, affecting their ecological functioning and fitness. Changes in morphospace induced by trematode parasites hamper species delineation and confound morphometric and disparity patterns in the fossil record of infestation-prone species. Excluding fossil specimens with trematode traces can mitigate those confounding effects. Conversely, comparative morphometric analyses of infested and non-infested host specimens may allow us to investigate host responses to parasites over evolutionary timescales.

The Silica Shale brachiopod, Paraspirifer bownockeri, of Ohio and Michigan, USA, preserves a unique window into Middle Devonian communities, as individuals hosted numerous epibionts. Herein, we use qualitative and quantitative methods to test hypotheses regarding the palaeoecology of these brachiopod hosts and their epibionts. We analysed the number and type of epibionts on the dorsal valve, ventral valve, hinge, and commissure of over 200 specimens. Using a variety of statistical techniques, we tested hypotheses regarding the relationships of these epibionts both to their host and to one another. We also produced 3D models of these specimens to explore hypotheses as to how epibiont colonization impacted host morphology. Finally, we compare our results to previous work on Silica Shale brachiopods. Our data show a correlation between shell morphology and epibiont placement that may indicate preferential placement in the water column or epibiont influence on host morphology. We found an average of 1.96 epibionts per host, with several significant correlations between co-occurring pairs. By applying these new data to previously published life position models and epibiont placement data, we reevaluated the life position of these brachiopods and found that the most likely position for these brachiopods in life would have been with the commissure 30-45 degrees angled above the sea floor. Our interpretation of the results differs from the original work on these brachiopods with regards to the epibiont relationships to one another, colonization patterns, and life position but supports more recent work evaluating spirifer life position and morphology.

Parasites play an essential role in maintaining healthy ecosystems. However, as human activities continue to degrade the environment and contribute to climate change, it is essential to understand if and how parasite-host interactions has been affected. In this study, we examined the relationship between the bivalve species Chamelea gallina and trematodes. Trematodes are a group of parasites with no significant body fossil record. We analyzed the dynamics of the trematode population, including its diversity, prevalence, pit size, and pit aggregation, by studying 193 infested valves and 838 pits retrieved in fossil and dead assemblages of C. gallina from Adriatic Sea Basin (Italy). The samples were collected from Late Holocene marine succession (~3 ky cal B.P. with sub-centennial error) and thanatocoenosis from the modern seabed. Our analysis based on Gaussian finite mixture modeling shows no decrease in the number of trematode taxa infesting C. gallina over the past 3000 years. However, the prevalence of trematode infestation decreased significantly over time (one order of magnitude), along with the average number of pits, median size, and parasite pit aggregation. These changes suggest a severe decline (or collapse) of the trematode-C. gallina interaction in the Adriatic Sea during a time of increasing human influence on this land-locked basin.

The Mazon Creek Lagerstätte (Moscovian Stage, late Carboniferous Period; Illinois, USA) captures a diverse view of ecosystems in delta-influenced coastal settings through exceptional preservation of soft tissues in siderite concretions. The generally accepted paradigm of the Mazon Creek biota has been that of an inferred paleoenvironmental divide between what have been termed the Braidwood and Essex assemblages, wherein the former represents a freshwater ecosystem with terrestrial input and the latter a marine-influenced prodelta setting with abundant cnidarians, bivalves, worm phyla, and diverse arthropods. Here, we revisit the paleoecology of the Mazon Creek biota by analyzing data from nearly 300,000 concretions from more than 270 locations with complementary multivariate ordinations. Our results show the Braidwood assemblage as a legitimate shoreward community and provide evidence for further subdivision of the Essex assemblage into two distinct subassemblages, termed here the Will-Essex and Kankakee-Essex. The Will-Essex represents a benthos dominated by clams and trace fossils along the transition between nearshore and offshore deposits. The Kankakee-Essex is dominated by cnidarians, presenting an ecosystem approaching the geographic margin of this taphonomic window. These new insights also allow a refined taphonomic model, wherein recalcitrant tissues of Braidwood organisms were subject to rapid burial rates, while organisms of the Essex assemblage typically had more labile tissues and were subject to slower burial rates. Consequently, we hypothesize that the Braidwood fossils should record more complete preservation than the Essex, which was exposed for longer periods of aerobic decomposition. This is supported by a higher proportion of non-fossiliferous concretions in the Essex than in the Braidwood.

Interactions between the parasitic larvae of digenean trematodes (mainly gymnophallids) and bivalves often produce characteristic pit-like malformations on shells. Tracking these traces in past and modern marine death assemblages has provided valuable insights into parasite-host responses to natural and anthropogenic environmental changes. Despite major breakthroughs, empirical explorations of parasite-host dynamics in the geological record are primarily based on trace occurrence data, overlooking the ecological information embedded in the spatial distribution of these traces (e.g. infective behavior, association with specific host anatomy, spatial relationships of traces with different size classes). Spatial Point Pattern Analysis of Traces (SPPAT), increasingly used to study predatory traces on mollusks, offers a promising approach to address this gap. However, its application to trematode-host interactions requires careful consideration of assumptions and caveats, such as the minimum number of traces required to accurately capture parasite-host dynamics and the reliability of point patterns constructed from data across multiple host skeletons. Here, we present a spatially explicit framework for extracting information from spatial patterns of trematode-induced pits on bivalve shells using SPPAT. We address methodological questions involved in assembling a point pattern of traces from multiple host specimens, and discuss critical issues related to drawing inferences from pooled point data. Our approach is illustrated using Late-Holocene samples of Chamelea gallina from the Northern Adriatic Sea (Italy), a species of commercial importance and a key model in climate change research. Our findings indicate that trematode-induced traces on C. gallina are non-randomly distributed, forming aggregated patterns. Notably, we detect no significant spatial differences between the two size classes of traces retrieved. This study extends the methodological toolkit for analyzing parasite-host interactions and highlights the potential of spatial trace patterns to enhance our understanding of their ecological and temporal dynamics.

Paired petrography and acid maceration has shown that preferential silicification of shelly faunas can bias recovery based on taxon and body size. Here, silicified fossils from the Upper Ordovician Edinburg Formation, Strasburg Junction, Virginia, USA, were analyzed using X-ray tomographic microscopy (μCT) in conjunction with recovered residues from acid maceration of the same materials to further examine sources of potential bias. Results reveal that very small (<~1 mm) fossils are poorly resolved in μCT when scanning at lower resolutions (~30 µm), underestimating abundance of taxa including ostracods and bryozoans. Acid maceration, meanwhile, fails to recover poorly silicified fossils prone to disarticulation and/or fragmentation during digestion. Tests for patterns of breakage, however, indicate no significant size or taxonomic bias during extraction. Comparisons of individual fossils from 3-D fossil renders and maceration residues reveal patterns of fragmentation that are taxon-specific and allow the differentiation of biostratinomic and preparational breakage. Multivariate ordinations and cluster analyses of μCT and residue data in general produce concordant results but indicate that the variation in taxonomic composition of our samples is compromised by the resolvability of small size classes in μCT imaging, limiting the utility of this method for addressing paleoecological questions in these specific samples. We suggest that comparability of results will depend strongly on the sample size, taphonomic history, textural, and compositional characteristics of the samples in question, as well as μCT scan parameters. Additionally, applying these methods to different deposits will test the general applicability of the conclusions drawn on the relative strengths and weaknesses of the methods.

Our study uses data from Holocene core samples and modern death assemblages to understand how human-induced environmental change in the northern Adriatic Sea (Italy) may have affected parasite-host dynamics in the economically important bivalve Chamelea gallina. Thirty-one radiocarbon dates confirm temporal distinctness between the periods before and after the onset of significant human influence and confirm that trematode prevalence has decreased by an order of magnitude over the past similar to 2 k.y. The median number of parasiteinduced pits per bivalve host and parasite aggregation has also decreased significantly, signaling a substantial decrease in the effective population size of digenean trematodes. Gaussian finite mixture modeling of pit size does not support the hypothesis of parasite extinction. Combined, these results indicate the (potentially ongoing) collapse of parasite-host interactions in C. gallina in concert with human influence on the Adriatic and its transition to an "urban sea."

Trace fossils preserved with fossil worm-shaped remains suggest the presence of free-living flatworms during the Ordovician at the latest and their occurrence in terrestrial environments during the Permian. The presence of hooks associated with acanthodian and placoderm fishes indicates the existence of parasitic monopisthocotyleans, with a simple life cycle, during the Devonian. The presence of eggs in shark coprolites suggests the occurrence of eucestode tapeworms, with complex life cycles, during the Permian, possibly even earlier in the Carboniferous. Fossil evidence for trematode flatworms, also with complex life cycles, is more recent, including diverse findings associated with bivalves, lizards, and coprolites of archosaurs in terrestrial environments between 126 and 76 Ma in the Cretaceous. Convincing evidence for gymnophallid trematodes in marine environments appears in the Eocene, with an earlier occurrence in Cretaceous freshwater environments. This chronological pattern of first appearance (Turbellarians > Monopisthocotylea > Cestoda > Trematoda) is surprisingly concordant with some recent molecular phylogenetic analyses. Further evidence to test these hypotheses could be obtained by conducting systematic screenings for resistant remains of platyhelminths such as hooks and eggs as well as characteristic traces such as trails or shell concretions preserved with their producers. Additional study and scrutiny are particularly needed for trace fossils attributed to free-living flatworms that are not associated with their producers. We make recommendations on how different constraints on flatworm evolution can be interpreted and used in future studies.

Supplemental data, methods, results, and R script.

Northern Arizona University, Flagstaff, Arizona, USA, recently installed a MIni CArbon DAting System (MICADAS) with a gas interface system (GIS) for determining the C-14 content of CO2 gas released by the acid dissolution of biogenic carbonates. We compare 48 paired graphite, GIS, and direct carbonate C-14 determinations of individual mollusk shells and echinoid tests. GIS sample sizes ranged between 0.5 and 1.5 mg and span 0.1 to 45.1 ka BP (n = 42). A reduced major axis regression shows a strong relationship between GIS and graphite percent Modern Carbon (pMC) values (m = 1.011; 95% CI [0.997-1.023], R-2 = 0.999) that is superior to the relationship between the direct carbonate and graphite values (m = 0.978; 95% CI [0.959-0.999], R-2 = 0.997). Sixty percent of GIS pMC values are within +/- 0.5 pMC of their graphite counterparts, compared to 26% of direct carbonate pMC values. The precision of GIS analyses is approximately +/- 70 C-14 yrs to 6.5 ka BP and decreases to approximately +/- 130 C-14 yrs at 12.5 ka BP. This precision is on par with direct carbonate and is approximately five times larger than for graphite. Six Plio-Pleistocene mollusk and echinoid samples yield finite ages when analyzed as direct carbonate but yield non-finite ages when analyzed as graphite or as GIS. Our results show that GIS C-14 dating of biogenic carbonates is preferable to direct carbonate C-14 dating and is an efficient alternative to standard graphite C-14 dating when the precision of graphite C-14 dating is not required.

The Eocene Pipestone Springs Main Pocket (Renova Formation, Jefferson County, Montana, United States of America) is a locality renowned for its diverse Chadronian (late Eocene; ∼38–33.9 million years ago) mammalian fauna and abundant coprolites. Two distinct coprolite size classes were previously identified in the trace fossil assemblage from which we selected representatives to investigate feeding behaviors and dietary selection of the producers. A subset of the selected coprolites was analyzed based on their compositional and taphonomic attributes using non-destructive x-ray tomographic microscopy in combination with more traditional methods including thin-section petrography, scanning electron microscopy, and energy dispersive spectroscopy. Among the features extracted in the tomographic data were skeletal fragments, including those showing evidence of bone-crushing; delicate hair molds; encrusted lithic fragments; and several irregular pores and cracks throughout the coprolites. Segmentation and volumetric renders permit quantitative assessment of the relative proportions of inclusions, revealing porosity as a primary volumetric element aside from the matrix and bone inclusions. There was no significant difference in the total volume of bone extracted between coprolite size class, though the smaller coprolites preserved a relatively higher volumetric proportion of undigested skeletal material. This multi-visualization approach provides a means to observe and evaluate differences in the coprolite gross morphology and inclusions across the two size classes, thereby offering valuable insights into the broader paleoecology of the Pipestone Springs Main Pocket coprolite producers and holding promise for comparable paleo-dietary studies of other coprolite-rich deposits.

How is parasitism likely to respond to anthropogenic global change? Digenean trematode prevalence among bivalve mollusk hosts in multiple coastal environments has been linked to sea-level rise on centennial and millennial time scales. Previous efforts have ruled out the influence of changing diversity, community structure, taphonomy, and salinity (fossil-based proxy) on this pattern but, until recently, we have not been able to address the role of other abiotic environmental factors. Here we present the results of stable isotope analyses (δ18O and δ13C) of the shallow marine bivalve Chamelea gallina from the Holocene and modern northern Adriatic (Italy) and trace element analysis of the estuarine bivalves Potamocorbula amurensis and Corbicula formosana from the Holocene Pearl River (China) delta using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Generalized Linear Models (GLM) of 1,297 δ18O and δ13C analyses from 57 C. gallina valves derived from 11 modern death assemblages and four Holocene core samples reveal that elevated trematode prevalence is associated with relatively negative δ18O values, relatively positive δ13C values, and a high correlation between δ18O and δ13C values. We interpret this to mean that trematode prevalence is higher during warm temperatures with minimal freshwater influence. GLMs of 3,295 LA-ICP-MS spot analyses on 48 valves from the two estuarine species (a separate GLM for each taxon), derived from 12 cored samples from Pearl River deposits, reveal a strong association between trematode prevalence and elevated Ba/Ca ratios and low species richness, which we interpret as high parasitic infestation of an oligotypic community in hypoxia-dominated environments. Taken together, the results suggest that parasitic patterns are linked to sea-level rise and geochemical insights point toward case-specific causal factors that are going to be more widespread due to anthropogenic climate change.

Gymnophallid trematodes are complex life cycle parasites that induce characteristic pits in their bivalve intermediate hosts, which serve as their sole fossil record. Previous work demonstrated that trematode prevalence increases with rising sea levels, but little has been done to investigate patterns of trematode pit size in relation to Holocene sea-level and environmental changes. Here we focus on trematode pits preserved in the bivalve Chamelea gallina from five late Holocene core samples (2510-3140 y BP) and eight modern death assemblages from the Po-Adriatic system (northern Italy). Using ImageJ, we measured 838 total pits, with 715 from the core samples and 123 from the death assemblages. The geometric mean of the primary and secondary axis was used as a proxy for trace size, which ranges from 0.117 mm to 1.708 mm. The median size of Holocene pits (0.497 mm) is significantly larger (pWilcoxon = 1.73e-05) than the median size of modern pits (0.396 mm), indicating a decrease in trematode metacercariae body size over this time. There was no significant relationship between trematode pit size and host bivalve body size and only whole, well-preserved Holocene valves were analyzed to minimize the influence of taphonomy. We interpret a change in C. gallina-trematode parasite-host interactions over the last 3 ky, which could be the result of several scenarios. Modern pits could be created by different taxa, which have smaller body sizes, or the pits could be formed by the same parasitic taxa which now may have a decreased body size, perhaps due to stress. These results, coupled with a survey of modern metacercariae sizes, suggest that the trematode pit size record can provide relevant information on parasite paleoecology and, perhaps, identity. Such information will enable more nuanced analyses of parasite-host response to environmental change in the past with an eye to the future.

ABSTRACT Here we describe an epibiont association between conulariids and holdfast producers, with attachment scars resembling those of the tubular epibiont, Sphenothallus, from the Silurian (late Telychian Series) Brandon Bridge Formation, Wisconsin. The conulariid population represents the most abundant sessile organisms in the Waukesha Biota and consists of two species, Conularia niagarensisHall, 1852 and Metaconularia cf. manni (Roy, 1935). Attachment scars present on the conulariid test offer a unique glimpse into the paleoecology of this Silurian benthic assemblage. However, body fossils of the attached epibiont are scarce and have not been observed attached or near conulariid specimens. This study evaluates the identity and paleoecological relationship between the conulariids and their enigmatic epibionts. Statistical analyses of attachment trace size, frequency, and distribution on the conulariid test gives insight to the nature of their symbiotic relationship. Our results did not find any significant support for a parasitic relationship. However, commensalism cannot be ruled out and serves as an alternative explanation for the relationship between these two organisms.