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1. TESTING FOR FUNCTIONAL CONSTRAINTS AMONG TRENDS IN EURYPTERID APPENDICULAR DIVERSITY AND DISPARITY

   https://doi.org/10.1130/abs/2023AM-393625  

   Sanford, Riley; Lamsdell, James C. (October 2023, Geological Society of America Abstracts with Programs)

   Eurypterids were an extinct group of aquatic chelicerate arthropods which originated in the mid-Ordovician and persisted until the late Permian. Being character-rich organisms with a well-resolved and robust phylogeny, eurypterids are an excellent study group for exploring evolutionary mechanisms and processes, such as trends in diversity and disparity and drivers of morphological change. Here, we use eurypterids as a case study for exploring mosaic evolution and the role of functional constraints in limiting disparity through an emphasis on the morphological diversity of the prosomal appendages and differences in somatic variation in the prosoma and opisthosoma. A dataset comprising 122 characters coded for 39 taxa (selected for completeness) was compiled in order to explore patterns of complexity in eurypterid tagmata over the course of their evolutionary history. The matrix, while comprised of discrete characters, is explicitly distinct from the kind of matrices employed in phylogenetic analysis, with prosomal appendage armature and tergite pleural structures being coded somite by somite. In total, 62 characters are coded for the prosoma (16 of which relate to the prosomal carapace and 46 of which relate to the prosomal appendages) and 60 are coded for the opisthosoma (55 for the tergites and 5 for the telson). From this dataset Euclidean pairwise distances between all taxa were generated and subjected to ordination through principal coordinates analysis (PCO), generating a theoretical morphospace. We compare metrics for disparity(as summarized by the Sum of Ranges and Sum of Variance of occupied morphospace)and appendage differentiation (limb tagmatization as defined by Cisne 1974) to explore patterns in complexity and disparity across eurypterids. Prosomal and opisthosomal disparity was analysed together and separately, in order to test for mosaicism across the eurypterid tagmata, with eurypterids grouped according to either environmental occupation, the form of the chelicera, and the morphology of appendage VI. These analyses explore whether limb complexity correlates with different life habits, the impact of increasing cheliceral size on limb complexity, and whether the advent of swimming in the group resulted in a functional release for appendage specialization. 

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2. EARLY EVOLUTION OF SEA SCORPIONS: EURYPTERIDS (CHELICERATA: EURYPTERIDA) OF THE BIG HILL LAGERSTÄTTE

   https://doi.org/10.1130/abs/2024AM-401218  

   Bingham, Joseph; Lamsdell, James C; Meyer, Ron; Gunderson, Gerald (September 2024, Geological Society of America Abstracts with Programs)

   Eurypterids are a group of diverse chelicerates that first appeared during the Middle Ordovician and went extinct at the end of the Permian. Despite there being over 250 species of eurypterids known, the fossil record of eurypterids during the Ordovician is relatively sparce, comprising only 12 species representing the Megalograptidae, a ‘waeringopterid’ clade, Rhenopteridae, Onychopterellidae, Adelophthalmidae, and Eurypteroidea. As such, any new discoveries that elucidate on the early evolutionary history of the clade is noteworthy. Here, we report on early eurypterids from the Late Ordovician Big Hill Lagerstätte of Michigan’s Stonington Peninsula with preserved organic cuticle. Preliminary study of the available specimens indicates they may comprise two new species, each assignable to new genera within the Carcinosomatidae and Dolichopteridae, which would represent the oldest known occurrence of both clades. The new species may help elucidate on the morphological ground pattern of these clades and will be incorporated into existing phylogenetic frameworks. The eurypterids at Big Hill represent the oldest known eurypterid community for which more than a single species is known to co-occur. The Big Hill Lagerstätte is unique in preserving eurypterids, chasmataspidids, and xiphosurans all in association and as such affords an important window into the early evolution of euchelicerates. 

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3. Physicochemical controls on biogeographic variation of benthic foraminiferal test size and shape

   https://doi.org/10.1017/pab.2016.7  

   Keating-Bitonti, Caitlin R; Payne, Jonathan L (November 2016, Paleobiology)

   Abstract The sizes and shapes of marine organisms often vary systematically across latitude and water depth, but the environmental factors that mediate these gradients in morphology remain incompletely understood. A key challenge is isolating the individual contributions of many, often correlated, environmental variables of potential biological significance. Benthic foraminifera, a diverse group of rhizarian protists that inhabit nearly all marine environments, provide an unparalleled opportunity to test statistically among the various potential controls on size and volume–to–surface area ratio. Here, we use 7035 occurrences of 541 species of Rotallid foraminifera across 946 localities spanning more than 60 degrees of latitude and 1600 m of water depth around the North American continental margin to assess the relative influences of temperature, oxygen availability, carbonate saturation, and particulate organic carbon flux on their test volume and volume–to–surface area ratio. For the North American data set as a whole, the best model includes temperature and dissolved oxygen concentration as predictors. This model also applies to data from the Pacific continental margin in isolation, but only temperature is included in the best model for the Atlantic. Because these findings are consistent with predictions from the first principles of cell physiology, we interpret these statistical associations as the expressions of physiological selective pressures on test size and shape from the physical environment. Regarding existing records of temporal variation in foraminiferal test size across geological time in light of these findings suggests that the importance of temperature variation on the evolution of test volume and volume–to–surface area ratio may be underappreciated. In particular, warming may have played as important a role as reduced oxygen availability in causing test size reduction during past episodes of environmental crisis and is expected to inflict metabolic stress on benthic foraminifera over the next century due to anthropogenic climate change. 

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4. Environmental correlates of molluscan predator–prey body size in the northern Gulf of Mexico

   https://doi.org/10.1017/pab.2023.22  

   Calderaro, Luke A.; Harnik, Paul G.; Rillo, Marina C. (February 2024, Paleobiology)

   The Mississippi River watershed drains 40% of the continental United States, and the tremendous primary productivity in the adjacent north-central Gulf of Mexico has created one of the most extensive dead zones on Earth. In contrast, smaller watersheds deliver fewer nutrients to the northeastern gulf, and consequently, productivity is limited and hypoxia is uncommon. How has variation in primary productivity, oxygen availability, and sea-surface temperature affected coastal food webs? Here, we investigate environmental controls on the size of molluscan predators and prey in the northern Gulf of Mexico using Holocene death assemblages. Linear mixed models indicate that bivalve size and the frequency of drilling predation are affected by dissolved oxygen concentrations; drilling frequency declines with declining oxygen, whereas bivalve size increases. In contrast, sea-surface temperature is positively associated with the size of molluscan predators and prey. Net primary productivity contributes relatively little to predator or prey size, and predator-to-prey size ratios do not vary consistently with environmental conditions across the northern gulf. Larger bivalves in areas of oxygen limitation may be due to decreased predation pressure and, consequently, greater prey longevity. The larger size of bivalves and predatory gastropods in warmer waters may reflect enhanced growth under these conditions, provided dissolved oxygen concentrations exceed a minimum threshold. Holocene death assemblages can be used to test long-standing hypotheses regarding environmental controls on predator−prey body-size distributions through geologic time and provide baselines for assessing the ongoing effects of anthropogenic eutrophication and warming on coastal food webs. 

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5. Comparative mitogenomic analysis of subterranean and surface amphipods (Crustacea, Amphipoda) with special reference to the family Crangonyctidae

   https://doi.org/10.1186/s12864-024-10111-w  

   Benito, Joseph B; Porter, Megan L; Niemiller, Matthew L (December 2024, BMC Genomics)

   Abstract Mitochondrial genomes play important roles in studying genome evolution, phylogenetic analyses, and species identification. Amphipods (Class Malacostraca, Order Amphipoda) are one of the most ecologically diverse crustacean groups occurring in a diverse array of aquatic and terrestrial environments globally, from freshwater streams and lakes to groundwater aquifers and the deep sea, but we have a limited understanding of how habitat influences the molecular evolution of mitochondrial energy metabolism. Subterranean amphipods likely experience different evolutionary pressures on energy management compared to surface-dwelling taxa that generally encounter higher levels of predation and energy resources and live in more variable environments. In this study, we compared the mitogenomes, including the 13 protein-coding genes involved in the oxidative phosphorylation (OXPHOS) pathway, of surface and subterranean amphipods to uncover potentially different molecular signals of energy metabolism between surface and subterranean environments in this diverse crustacean group. We compared base composition, codon usage, gene order rearrangement, conducted comparative mitogenomic and phylogenomic analyses, and examined evolutionary signals of 35 amphipod mitogenomes representing 13 families, with an emphasis on Crangonyctidae. Mitogenome size, AT content, GC-skew, gene order, uncommon start codons, location of putative control region (CR), length ofrrnLand intergenic spacers differed between surface and subterranean amphipods. Among crangonyctid amphipods, the spring-dwellingCrangonyx forbesiexhibited a unique gene order, a longnad5locus, longerrrnLandrrnSloci, and unconventional start codons. Evidence of directional selection was detected in several protein-encoding genes of the OXPHOS pathway in the mitogenomes of surface amphipods, while a signal of purifying selection was more prominent in subterranean species, which is consistent with the hypothesis that the mitogenome of surface-adapted species has evolved in response to a more energy demanding environment compared to subterranean amphipods. Overall, gene order, locations of non-coding regions, and base-substitution rates points to habitat as an important factor influencing the evolution of amphipod mitogenomes. 

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