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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.