Search for: All records

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. 

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. 

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. 

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. 

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. 

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. 

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.