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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. 

Eurypterids—Palaeozoic marine and freshwater arthropods commonly known as sea scorpions—repeatedly evolved to remarkable sizes (over 0.5 m in length) and colonized continental aquatic habitats multiple times. We compiled data on the majority of eurypterid species and explored several previously proposed explanations for the evolution of giant size in the group, including the potential role of habitat, sea surface temperature and dissolved sea surface oxygen levels, using a phylogenetic comparative approach with a new tip-dated tree. There is no compelling evidence that the evolution of giant size was driven by temperature or oxygen levels, nor that it was coupled with the invasion of continental aquatic environments, latitude or local faunal diversity. Eurypterid body size evolution is best characterized by rapid bursts of change that occurred independently of habitat or environmental conditions. Intrinsic factors played a major role in determining the convergent origin of gigantism in eurypterids. 

Chasmataspidids are a group of Early Paleozoic (Middle Ordovician-Middle Devonian)chelicerates defined by an apparently unique opisthosomal tagmosis consisting of a microtergite, a three-segmented fused buckler, and a nine-segmented postabdomen. Although the number of known chasmataspidid species increased by half during the past decade, the group has not been a subject of detailed phylogenetic analysis, and its placement within Chelicerata is uncertain. Though recent analyses that include chasmataspidids support a monophyletic Chasmataspidida as sister to Sclerophorata (Eurypterida and Arachnida), few have sampled more than three of the 14 currently recognized species. Previous workers have suggested chasmataspidids may be a polyphyletic or paraphyletic group, or that chasmataspidids may resolve as the sister taxon to eurypterids, or even as a clade within Eurypterida. Without a broader sampling of chasmataspidids it is not possible to adequately test these various hypotheses, while a robust phylogenetic framework in necessary for understanding macroevolutionary and biogeographic trends within the group. Chasmataspidids also represent the earliest preserved euchelicerate in the fossil record, with Chasmataspis dated to approximately 478 million years ago, and as such its phylogenetic position in relation to other euchelicerates has implications for the divergence times of those clades. We present a new phylogenetic matrix comprising 81 characters coded for every currently described chasmataspidid species, analysis of which under maximum parsimony and Bayesian inference results in concordant phylogenetic topologies. Chasmataspidida resolves as in most recent analyses as a monophyletic clade sister to Sclerophorata, indicating that Xiphosura, Chasmataspidida, and Sclerophorata likely diverged in the Early Ordovician. The analysis also supports a taxonomic revision within Chasmataspidida; we propose dividing the clade into two superfamilies, with four constituent families. As part of this study the Silurian taxon Loganamaraspis was reevaluated and the morphology of appendage VI, previously considered to be retained as a walking limb, could not be ascertained. 

The infraorder Astacidea, comprising marine clawed lobsters and freshwater crayfish, include some of the most recognizable decapod crustaceans, many being harvested commercially for human consumption and aquaculture. While molecular analyses have elucidated relationships among extant lineages, the composition and placement of several fossil groups within Astacidea remain poorly resolved, with several conflicting phylogenetic hypotheses and taxonomic classifications being proposed in previous works. Among these controversial groups, Erymoidea have variably been placed in Astacidea or Glypheidea, a largely extinct infraorder of predominantly pseudochelate marine lobsters. Cladistic relationships of Stenochiroidea have also been problematic, having been regarded as ancestral to freshwater crayfish (Astacida) or extant marine lobsters (Nephropidae). Failure to reach a consensus regarding these groups can be at least partially attributed to the prevalence of morphological convergence and limited taxon sampling. To clarify evolutionary relationships among fossil and extant taxa, a Bayesian phylogenetic analysis of morphological and molecular data (mitochondrial genes: 12S, 16S and COI; nuclear genes: 18S, 28S and H3) was performed that included extensive taxon sampling of all currently recognized families of Astacidea as well as representatives of several potential sister groups. To overcome error introduced by homoplasy, relationships among extant taxa, as revealed by previous molecular analyses, were used to identify morphological characters with potentially robust phylogenetic signal. The resulting phylogeny places erymids within Glypheidea and supports a sister relationship between Astacidea and Glaessnericarididae. Stenochiroidea was found to be polyphyletic, with most genera forming a clade sister to Nephropidae; Pseudastacus is moved to Protastacidae, which resolves as the sister taxon to freshwater crayfish. The relationships among living and fossil taxa presented here provide new insight into the origins and evolutionary histories of the major lineages of marine clawed lobsters and freshwater crayfish. 

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. 

Horseshoe crabs (class Xiphosura) are a long-lived clade of aquatic chelicerate arthropods with a fossil record spanning approximately 480 million years. Though Xiphosura are often noted for their morphological stability, further investigation of evolutionary rate and paleoecological trends have revealed a remarkably dynamic clade, with both temporal and phylogenetic variability in evolutionary trends. Additionally, heterochrony has been revealed to be a strong driver behind xiphosuran evolution and the exploration of non-marine niches. Using combined geometric morphometric and evolutionary rate techniques, we further highlight the incongruency of the fossil record of xiphosurans with their designation as a “living fossil” or stabilomorph clade. Here, we compare the results of a geometric morphometric analysis with a discrete character evolutionary rate calculation performed using the R package Claddis. Both analyses incorporated 55 xiphosuran species, ranging temporally from the Ordovician Lunataspis aurora to all four modern species. Morphometric data was collected as 2Dlandmarks and semi-landmarks, with variable numbers of points due to varying levels of preservation amongst fossil specimens. These data were then used to produce a PCA for the visualization of morphospace. Both studies support a dynamic evolutionary history for Xiphosura. The discrete character analysis revealed peaks in discrete character evolution in the heterochronic non-marine clades, as well as an overall declining trend in evolutionary rate. Similarly, the clades with higher evolutionary rates occupy a wider portion of morphospace compared with the more morphologically stable clades. 

Horseshoe crabs (Chelicerata: Xiphosura) are generally considered to exhibit a highly conserved morphology throughout their evolutionary history and are one of the archetypal ‘living fossil’ groups. This narrative has been challenged in recent years, with numerous lines of evidence indicate that horseshoe crabs have been an evolutionarily dynamic lineage, exhibiting several shifts into non-marine environments and associated peaks in rates of evolutionary change. Nevertheless, marine forms are still characterized by a relatively limited morphological variability for most of their evolutionary history, as evidenced by a consistent developmental trajectory shared between species over 250million years. Attempts to ascertain when horseshoe crabs adopted this ontogenetic trajectory are hindered by the sparse early Paleozoic record of the group; only two species, both assigned to the genus Lunataspis, have been described from the Ordovician, and no Silurian species are known. A new, highly aberrant horseshoe crab from the Late Ordovician Big Hill Lagerstätte, Michigan, provides evidence of early morphological experimentation within the group, indicating that even marine lineages were variable early on in their evolutionary history. The new species represents a distinct genus characterized by a greatly elongated prosomal carapace and is represented by two available specimens (with a third held in a private collection), all of which preserve the same highly unusual carapace shape, indicating the unusual morphology to be a genuine characteristic of the species. Geometric morphometric analysis places the new species in an unoccupied region of morphospace distinct to that of other horseshoe crabs, confirming early morphological experimentation within the clade. Interestingly, while the prosoma is markedly different to any other horseshoe crab species known, the thoracetron is similar to that of Lunataspis. Taken in combination with the known ontogeny of Lunataspis borealis, which exhibits the characteristic xiphosurid development of the thoracetron but a more eurypterid-like ontogenetic trajectory of the prosoma, the new species indicates that developmental canalization occurred within the horseshoe crab lineage, with the thoracetron canalizing prior to the prosoma. 

A key aspect of geoscience education initiatives is creating engaging programs that inspire future generations to care about the past, present, and future of our planet. Here, we present a lesson plan designed for 6-12 grade students that uses horseshoe crab (Xiphosura) paleobiology as a tool to teach students about paleoecology, phylogenetics and the scientific process. Framed as a criminal investigation, students are placed in groups and briefed as “fossil detectives”, who are tasked with identifying horseshoe crabs and determining their evolutionary and ecological affinities. Students are provided with a guidebook, evidence bags, and a phylogenetic poster with missing blanks for five horseshoe crabs, ranging in age from Ordovician to modern. Students use the fossil evidence bags of associated biota and guidebooks to determine the locality, age, identity, and paleoenvironmental affinity of each xiphosuran suspect. With this newfound data, paired with morphological observations, students then place each of the five horseshoe crab suspects within a time-scaled phylogeny poster. Afterwards, students are prompted to use logical reasoning skills to determine the minimum number of times horseshoe crabs have explored non-marine environments and which common ancestors likely made this transition on the phylogenetic tree. A pre- and post-test are also being developed to measure the outcomes of this lesson plan. 

Natural History collections contain primary data spanning the history of life on Earth. Much of these data remain understudied and therefore has not been integrated into our current understanding of paleontology. One such collection is the eurypterid collection at the University of Michigan Museum of Paleontology (UMMP). Last summer this material was digitized and preliminary morphometrics work was conducted. Here, we leverage the work we completed to include material from other previously published studies on eurypterid morphometrics. Specifically, we are interested in evaluating landmark placement between studies and comparing results of the two studies separately and combined. 130 specimens from the UMMP possess intact prosoma and were landmarked in the StereoMorph package in R. Four fixed landmarks and two sets of sliding landmarks along curves of the prosoma were employed. The data were analyzed using a Principal Components Analysis(PCA) and results were visualized in R using ggplot2. Previous work utilized more landmarks, which were unobtainable with the UMMP dataset. So for a viable comparison, approximately 115 specimens (those used in Bicknell and Amati 2021) from the New York State Museum and Yale Peabody were landmarked using our smaller landmark set. This allows us to examine the efficacy of different amounts and types of landmarks (fixed versus curves) and the resulting distribution in morphospace. The resulting morphospace shows a broad occupation of the genus Eurypterus, which supports previous studies. Additionally, there is a difference in distance between groups in the combined morphospace compared to previous work. This is likely due to the variation in landmarks used to capture specific aspects of the prosoma. This case study in landmark variation provides evidence that landmark selection, research question, and reproducibility should be carefully considered. Furthermore, targeted digitization of museum collections will increase mobilization of primary datasets.