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We analyzed variations in taxonomic composition, abundance, and preservation of freshwater mollusks during the mid- to late Holocene in three shallow lakes of the southeastern Pampa plain. Mollusks were obtained from core samples (110–140 cm long) from the deepest part of the lakes, and sampled at 1 cm intervals. Six species were identified: Heleobia australis, Heleobia parchappii, Biomphalaria peregrina, Uncancylus concentricus, Succinea meridionalis, and Antillorbis nordestensis. Assemblages were dominated by H. parchappii, which displayed continuous and complete records in the three sections. With the exception of H. australis, fossil assemblages were represented by the same species that today inhabit Pampean lakes and preserved the same rank order of abundance as modern communities. A similar pattern of variation in mollusk assemblage composition was recognized in the three successions, which initiated prior to circa 6000 cal. year B.P, with assemblages composed of H. parchappii and S. meridionalis at low abundances. A broad expansion of H. parchappii occurred between circa 6000 and 5400 cal. year B.P., and the species rapidly dominated the whole malacofauna. At circa 5400 cal. year B.P., the freshwater B. peregrina was recorded for the first time, and its abundance gradually increased until circa 3600 cal. year B.P. At ∼3700 cal. year B.P., the estuarine H. australis was recorded. The arrival of the freshwater U. concentricus occurred at ∼1697 cal. year B.P. Finally, only H. parchappii and B. peregrina were represented in the topmost levels (younger than ∼736 cal. year B.P.). Changes in mollusk composition were mostly related to changes in water salinity, which brings a historical perspective to understanding the modern dominance of the euryhaline H. parchappii in these lakes. Shell preservation became favored when water bodies developed brackish conditions.
Multiple bioclaustrations identified as possible Chaetosalpinx are reported from a stromatoporoid of Rhuddanian age from Hiiumaa Island, Estonia. This is the second record of symbiotic worm endobionts from the end-Ordovician mass extinction recovery fauna of Estonia. The end-Ordovician mass extinction did not terminate complex ecological relationships between at least some worm endobionts and their hosts.
Land crabs are found in marine and terrestrial sediments and sedimentary rocks from the Pleistocene and Holocene throughout the Caribbean. Field surveys of two widespread and abundant species, Gecarcinus lateralis and Gecarcinus ruricola, were conducted three times a week for 4 weeks along the northern coast of San Salvador Island, the Bahamas, at three localities that varied in vegetation cover, available sediment, and wind and wave energy. A total of 1400 identifiable remains were found and scored for four taphonomic characters: fragmentation, edge modification, surface alteration, and color loss. The majority of specimens exhibited various levels of disarticulation, from complete limbs to only isolated podomeres. Overall, claws are overrepresented, and the distribution of recovered remains differed significantly (p < 0.0001) from that expected based on the anatomy of a single crab (eight legs, two claws, and one carapace per individual). Remains found in direct sunlight and open spaces consistently showed more surface alteration, edge modification, and more loss than those found within vegetation or on the beach. Fragmentation was not significantly different across localities (p = 0.045). This result suggests that the fossil record of land crabs may be biased as a result of their low preservation potential in terrestrial environments. The discovery of eight poorly preserved Holocene chelae on San Salvador Island contrasts starkly with the abundance and preservational quality of surficial remains. Rapid burial is likely required for their preservation but may not ensure the preservation of the cuticle.