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1. Mapping Past Distributions of Marine Mollusks Using Shell Death Assemblages

   https://doi.org/10.58782/flmnh.cpmy8278  

   Herbert, Gregory S; Kramer, Andrew M; Geiger, Stephen P; Jimenez_Bustos, Ana; Sanders, Stephanie A; Seiden, Nicole; Rogers, Jaime A (February 2023, Bulletin of the Florida Museum of Natural History)

   Marine species assessments rely heavily on baseline surveys conducted after the 1960s, long after many anthropogenic pressures began, which could lead to misinformed management decisions and poor conservation outcomes. In this study, we collaborated with Florida Fish and Wildlife to conduct stock assessments for mollusks of the west Florida shelf that incorporate shell death assemblages. One of our first assessments was of the Florida Fighting Conch, Strombus alatus, an abundant gastropod that is also under consideration as a replacement fishery for the threatened Queen Conch. Live and dead shells were collected from >300 dredge tows between 2008-2018 covering the entire west Florida shelf. Shells were age-partitioned by 14C- and AAR-calibrated taphonomic criteria. Counts were converted to densities per m2. Inverse distance weighting interpolation of S. alatus death assemblages reveals multiple population centers along the coast and a rapid decrease in density with depth from 25-120 m. In contrast, live conchs were absent in our dredge samples from shelf depths deeper than 40 m. These differences are confirmed by single-visit occupancy methods that account for variation in detectability across the samples. Live-dead differences in spatial distribution are probably influenced by time averaging in death assemblages, which increases detectability of conchs in deeper habitats, where they may be too rare to be sampled alive. However, extirpation of offshore populations was also indicated by independent natural history collection occurrence records, which show numerous live-collected conchs from 1940-1980 but none afterwards, despite an increase in sampling effort. These results suggest that live-dead comparisons can reveal biodiversity loss at the scale of large marine ecosystems. 

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2. Spatial distribution, diversity, and taphonomy of clypeasteroid and spatangoid echinoids of the central Florida Keys

   https://doi.org/10.7717/peerj.14245  

   Grun, Tobias B.; Kowalewski, Michał (January 2022, PeerJ)

   Background Irregular echinoids are ecosystem engineers with diverse functional services. Documenting present-day distribution of those widespread organisms is important for understanding their ecological significance and enhancing our ability to interpret their rich fossil record. Methods This study summarizes SCUBA surveys of clypeasteroid and spatangoid echinoids conducted in 2020 and 2021 along the central part of the Florida Keys. The survey included observations on both live and dead specimens, their distribution, habitat preferences, abundance, and live-dead comparison. Results Echinoids were found at 17 out of 27 examined sites (63%) and occurred across a wide range of habitats including coastal seagrass meadows, subtidal sand and seagrass settings of the Hawk Channel, backreef sands, and fine muddy sands of deeper forereef habitats. The encountered species, both dead and alive, included Clypeaster rosaceus (four sites), Clypeaster subdepressus (five sites), Encope michelini (three sites), Leodia sexiesperforata (eight sites), Meoma ventricosa (nine sites), and Plagiobrissus grandis (four sites). All sites were dominated by one species, but some sites included up to five echinoid species. Live-dead fidelity was high, including a good agreement in species composition of living and dead assemblages, congruence in species rank abundance, and overlapping spatial distribution patterns. This high fidelity may either reflect long-term persistence of local echinoid populations or fragility of echinoid tests that could prevent post-mortem transport and the formation of time-averaged death assemblages. Regardless of causative factors, the live-dead comparisons suggest that irregular echinoid assemblages, from settings that are comparable to the study area, may provide a fossil record with a high spatial and compositional fidelity. The survey of live fauna is consistent with past regional surveys in terms of identity of observed species, their rank abundance, and their spatial distribution patterns. The results suggest that despite increasingly frequent hurricanes, active seasonal fisheries, massive tourism, and urban development, irregular echinoids continue to thrive across a wide range of habitats where they provide diverse ecosystem services by oxygenating sediments, recycling organic matter, supporting commensal organisms, and providing food to predators. Results reported here document the present-day status of local echinoid populations and should serve as a useful reference point for assessing future regional changes in echinoid distribution and abundance. 

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3. The Plio-Pleistocene Pinecrest Beds: Problems, progress, and potential

   Hendricks, JR; Anderson, BM; Portal, RW; Lee, HG; Allmon, WD (July 2024, Museum of Paleontology, The University of Michigan; University of Michigan Papers on Paleontology)

   The Plio-Pleistocene Pinecrest beds (Tamiami Fm.) of southern Florida rank amongst the most species-rich assemblages known from the Cenozoic macrofossil record. The tropical to subtropical fauna of the Pinecrest beds includes hundreds of mollusk species and subspecies (perhaps over 1,000), as well as diverse corals, bryozoans, and vertebrates. Some elements of the Pinecrest fauna are extant, but most species went extinct during one or more extinction pulses early in the Pleistocene. There was significant species turnover, but perhaps limited overall change in biodiversity relative to the present. Characterization of biotic change during this interval is important for understanding the evolution of the modern molluscan fauna of the southeastern United States but is unfortunately stymied by two major confounding factors. First, there are no natural exposures of the Pinecrest, and detailed sections have only been exposed in now-flooded quarries near Sarasota, Florida. Most samples from these and other quarries and canal cuts come from spoil piles, which often mix shells from multiple time intervals and habitats, limiting stratigraphic and ecological resolution for occurrence records. Second, the mollusks of the Pinecrest have not been comprehensively treated systematically and some elements of the macrofauna have likely been taxonomically over-split, leading to confusion and wariness among researchers about using published records in analyses. Conversely, the micromollusk fauna (<5.5 mm) has been understudied and many new species await description. Refining our understanding of this major regional turnover event is dependent upon a stable taxonomic foundation and supporting specimen occurrence data. Current estimates of diversity from literature and museum datasets do not closely align. Literature from the mid 1990s suggest over 550 species of gastropods and 250 species of bivalves in the Pinecrest beds, but numerous species have been described subsequently, the majority in the gray literature. In contrast to published tallies, over 1,000 and 400 names have been applied, respectively, to Pinecrest gastropods and bivalves in the collections the Florida Museum of Natural History, representing 140 families. Over 280 micromollusk species may also be present in the Pinecrest fauna. We have begun a project to comprehensively refine and substantiate these estimates of biodiversity to better characterize the Plio-Pleistocene turnover event(s) that led to the establishment of the modern fauna. Funding source: This research is supported by NSF DEB 2225014. 

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4. Age variability and decadal time-averaging in oyster reef death assemblages

   https://doi.org/10.1130/G50778.1  

   Durham, Stephen R.; Dietl, Gregory P.; Hua, Quan; Handley, John C.; Kaufman, Darrell; Clark, Cheryl P. (September 2023, Geology)

   Using paleoecological data to inform resource management decisions is challenging without an understanding of the ages and degrees of time-averaging in molluscan death assemblage (DA) samples. We illustrate this challenge by documenting the spatial and stratigraphic variability in age and time-averaging of oyster reef DAs. By radiocarbon dating a total of 630 oyster shells from samples at two burial depths on 31 oyster reefs around Florida, southeastern United States, we found that (1) spatial and stratigraphic variability in DA sample ages and time-averaging is of similar magnitude, and (2) the shallow oyster reef DAs are among the youngest and highest-resolution molluscan DAs documented to date, with most having decadal-scale time-averaging estimates, and sometimes less. This information increases the potential utility of the DAs for habitat management because DA data can be placed in a more specific temporal context relative to real-time monitoring data. More broadly, the results highlight the potential to obtain decadal-scale resolution from oyster bioherms in the fossil record. 

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5. Multiple phyla, one time resolution? Similar time averaging in benthic foraminifera, mollusk, echinoid, crustacean, and otolith fossil assemblages

   https://doi.org/10.1130/G49970.1  

   Nawrot, Rafał; Berensmeier, Michaela; Gallmetzer, Ivo; Haselmair, Alexandra; Tomašových, Adam; Zuschin, Martin (May 2022, Geology)

   Abstract Time averaging of fossil assemblages determines temporal precision of paleoecological and geochronological inferences. Taxonomic differences in intrinsic skeletal durability are expected to produce temporal mismatch between co-occurring species, but the importance of this effect is difficult to assess due to lack of direct estimates of time averaging for many higher taxa. Moreover, burial below the taphonomic active zone and early diagenetic processes may alleviate taxonomic differences in disintegration rates in subsurface sediments. We compared time averaging across five phyla of major carbonate producers co-occurring in a sediment core from the northern Adriatic Sea shelf. We dated individual bivalve shells, foraminiferal tests, tests and isolated plates of irregular and regular echinoids, crab claws, and fish otoliths. In spite of different skeletal architecture, mineralogy, and life habit, all taxa showed very similar time averaging varying from ~1800 to ~3600 yr (interquartile age ranges). Thus, remains of echinoids and crustaceans—two groups with multi-elemental skeletons assumed to have low preservation potential—can still undergo extensive age mixing comparable to that of the co-occurring mollusk shells. The median ages of taxa differed by as much as ~3700 yr, reflecting species-specific timing of seafloor colonization during the Holocene transgression. Our results are congruent with sequestration models invoking taphonomic processes that minimize durability differences among taxa. These processes together with temporal variability in skeletal production can overrule the effects of durability in determining temporal resolution of multi-taxic fossil assemblages. 

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