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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.
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EARLY EVOLUTION OF SEA SCORPIONS: EURYPTERIDS (CHELICERATA: EURYPTERIDA) OF THE BIG HILL LAGERSTÄTTE
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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- Award ID(s):
- 1943082
- PAR ID:
- 10582984
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2024AM-401218
