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Understanding how the mechanisms that result in the development of new morphologies act is fundamental to exploring how evolutionoperates. Carcinosomatids are an unusual group of eurypterids which are characterized by the possession of a greatly expanded,disc-like preabdomen along with relatively short prosomal appendages bearing well-developed spines. The ontogeny of this highlydistinctive group is revealed by a species from the Silurian of Lesmahagow, Scotland. Specimens of Carcinosoma scorpioides ,originally considered to be three separate species, are here shown to represent at least three different ontogenetic stages rangingfrom early juveniles to adults. The earliest instars are markedly different from the adults, with a narrow preabdomen and elongatedprosomal appendages bearing moderately-sized spines, resulting in an overall appearance more reminiscent of the closely relatedmixopterids. The species first undergoes an expansion of the preabdomen while a relative reduction in the length of the prosomalappendages occurs in later instars. Other eurypterids exhibit a relative reduction in prosomal appendage length over ontogeny, withjuveniles having exceptionally long appendages, and also have a reduced prosomal armature which increases progressively duringgrowth. Eurypterids also show a positive allometric trend in preabdominal width early in their ontogeny. As such, Carcinosomascorpioides exhibits the same general developmental trends seen in other eurypterids, indicating that the ontogenetic trajectory ofeurypterids is generally conserved even amongst highly aberrant members of the clade. Furthermore, the preserved ontogeny of Carcinosoma scorpioides suggests that the unusual morphology of carcinosomatids developed due to peramorphic heterochronicprocesses whereby species develop exaggerated characteristics along the ontogenetic trajectory beyond the ancestral condition.Interestingly, despite these traits all being derived through peramorphy, mosaicism is evident in these heterochronic changes with thebody width increase occurring due to an increase in the rate of allometric change early on in ontogeny, while the reduced appendagelength and enlarged armature appears to be associated with an increase in body size and may be due to either a delay in reachingmaturity or an increase in the rate of growth later in ontogeny. 

Developmental sequence polymorphism (variation in the timing of developmental events) is a neglected form of variation, and whetherit is correlated with other forms of variation, what role it plays in shaping a species’ evolutionary trajectory, and the overall extent towhich it characterizes and can be measured in extinct species remain open questions. Intraspecific variation constitutes the rawmaterial with which natural selection shapes phenotypes over the course of evolutionary history. A complete understanding of themechanisms underpinning biological evolution therefore requires detailed knowledge of the different forms of variation by whichspecies are characterized. Intraspecific developmental variation is an important source of phenotypic variety because minordifferences in developmental pathways can result in significant differences among adult phenotypes. The fossil record provides representatives of past species, the most direct evidence we have of evolutionary change across geologictime. However, one limitation of studying fossils is that they are generally uncommon and sample sizes are often small. Intraspecificvariation of any kind is therefore rarely quantified for fossil species. Here, continuous growth and discrete developmental timing datafor the well-studied and abundant trilobite species Elrathia kingii are presented. 116 specimens representing a range of growth stageswere photographed, measured, and coded for the expression of discrete developmental transformations. The fossils are reposited atthe AMNH and were collected from a narrow stratigraphic interval in the Wheeler Formation of west central Utah. Linearmeasurements were plotted in multivariate space to construct growth series, a common practice in trilobite ontogenetic research. Inaddition to this, however, phenotypic characters associated with abrupt developmental transformations are documented for eachspecimen and coded into the growth series to illustrate the timing of these transformations relative to continuous changes in size andshape. The result is a series of plots in which overlap in developmental character states represents variation. This novel techniqueindicates that sequence polymorphism is present in E. kingii and future work will use this result as a starting point for the application ofOntogenetic Sequence Analysis. 

Trilobites are a well-preserved group of arthropods which have been documented from the Cambrian to the end of the Permian.Abnormalities, such as injuries or teratological (developmental) defects, have been observed and described in multiple individualsacross a wide range of species. Due to the rarity of such individuals, population scale investigations into the rate and possible causesof such abnormalities have been largely overlooked. Mississippian trilobites of the genus Kaskia were collected from two fossil sites,and individuals with segmentation abnormalities were observed to be prevalent at both localities. Comparison with the well-knowngenus Eldredgeops indicates that Kaskia exhibits a greater rate of abnormalities. One possible explanation for the prevalence ofabnormalities in the studied proetids is a genetic bottleneck that occurred as a result of the late Devonian mass extinction, which couldhave led to an increased risk of abnormal development. In order to make the claim that these abnormalities are biotic in origin, onemust rule out abiotic influence. A common environmental cause of abnormalities in extant marine arthropods is heavy metal pollution.To determine whether heavy metals may have acted as teratogens in these trilobites, representative individuals from both sites wereanalyzed for signs of metal incorporation into their exoskeleton using a Bruker M4 Tornado Plus micro XRF. No evidence of heavymetals was found, supporting the assertion that the segmentation defects are biotic in nature. These specimens are currently beinganalyzed for signs of diagenesis through petrographic analysis and SEM imaging, to ensure that the XRF readings reflectpaleoenvironmental conditions. 

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.