• DOI:10.1073/pnas.1920733117
• Corpus ID: 215408474

Aquatic stem group myriapods close a gap between molecular divergence dates and the terrestrial fossil record

@article{Edgecombe2020AquaticSG,
  title={Aquatic stem group myriapods close a gap between molecular divergence dates and the terrestrial fossil record},
  author={Gregory D. Edgecombe and Christine Strullu‐Derrien and Tomasz G{\'o}ral and Alexander J. Hetherington and Christine Thompson and Markus Koch},
  journal={Proceedings of the National Academy of Sciences},
  year={2020},
  volume={117},
  pages={8966 - 8972}
}

• G. Edgecombe, C. Strullu‐Derrien, +3 authors M. Koch
• Published 6 April 2020
• Environmental Science, Geography
• Proceedings of the National Academy of Sciences

Significance Most arthropod diversity is now found on land, with hexapods (insects), arachnids, and myriapods being major terrestrial radiations. Molecular dating consistently predicts that these groups have earlier evolutionary origins than are recorded by fossils. A reason for this difference between molecular- and fossil-based age estimates is that few candidates for marine or freshwater stem groups of these terrestrial lineages have been identified. A Devonian euthycarcinoid arthropod preserves details of the head shared by myriapods, adding support to the theory that they are each other’s closest relative. The fossil record of euthycarcinoids in the Cambrian, including trackways made on tidal flats, clarifies the marine-to-terrestrial transition in the myriapod lineage and reconciles molecular and fossil-based estimates for the timing of myriapod origins. Identifying marine or freshwater fossils that belong to the stem groups of the major terrestrial arthropod radiations is a longstanding challenge. Molecular dating and fossils of their pancrustacean sister group predict that myriapods originated in the Cambrian, much earlier than their oldest known fossils, but uncertainty about stem group Myriapoda confounds efforts to resolve the timing of the group’s terrestrialization. Among a small set of candidates for membership in the stem group of Myriapoda, the Cambrian to Triassic euthycarcinoids have repeatedly been singled out. The only known Devonian euthycarcinoid, Heterocrania rhyniensis from the Rhynie and Windyfield cherts hot spring complex in Scotland, reveals details of head structures that constrain the evolutionary position of euthycarcinoids. The head capsule houses an anterior cuticular tentorium, a feature uniquely shared by myriapods and hexapods. Confocal microscopy recovers myriapod-like characters of the preoral chamber, such as a prominent hypopharynx supported by tentorial bars and superlinguae between the mandibles and hypopharynx, reinforcing an alliance between euthycarcinoids and myriapods recovered in recent phylogenetic analysis. The Cambrian occurrence of the earliest euthycarcinoids supplies the oldest compelling evidence for an aquatic stem group for either Myriapoda or Hexapoda, previously a lacuna in the body fossil record of these otherwise terrestrial lineages until the Silurian and Devonian, respectively. The trace fossil record of euthycarcinoids in the Cambrian and Ordovician reveals amphibious locomotion in tidal environments and fills a gap between molecular estimates for myriapod origins in the Cambrian and a post-Ordovician crown group fossil record.