Abstract
Understanding the temporal context of terrestrialization in chelicerates depends on whether terrestrial groups, the traditional Arachnida, have a single origin and whether or not horseshoe crabs are primitively or secondarily marine. Molecular dating on a phylogenomic tree that recovers arachnid monophyly, constrained by 27 rigorously vetted fossil calibrations, estimates that Arachnida originated during the Cambrian or Ordovician. After the common ancestor colonized the land, the main lineages appear to have rapidly radiated in the Cambrian–Ordovician boundary interval, coinciding with high rates of molecular evolution. The highest rates of arachnid diversification are detected between the Permian and Early Cretaceous. A pattern of ancient divergence estimates for terrestrial arthropod groups in the Cambrian while the oldest fossils are Silurian (seen in both myriapods and arachnids) is mirrored in the molecular and fossil records of land plants. We suggest the discrepancy between molecular and fossil evidence for terrestrialization is likely driven by the extreme sparseness of terrestrial sediments in the rock record before the late Silurian.
Highlights
Arachnids are an important group of terrestrial arthropods, including the familiar ticks, mites, spiders, and scorpions, together with pseudoscorpions, camel spiders, vinegaroons, whip spiders, and a few other groups
The supposed aquatic mode of life of various fossil scorpions has been questioned on both morphological and geological grounds (Dunlop et al, 2008; Kühl et al, 2012). Another challenge to a single terrestrialization event in arachnids came from analyses of phylogenomic datasets, which have often recovered the marine Xiphosura to be nested within Arachnida (Ballesteros and Sharma, 2019)
In this tree Chelicerata and Euchelicerata are monophyletic, with the horseshoe crabs retrieved as the sister group of monophyletic Arachnida
Summary
Arachnids are an important group of terrestrial arthropods, including the familiar ticks, mites, spiders, and scorpions, together with pseudoscorpions, camel spiders, vinegaroons, whip spiders, and a few other groups. These phylogenetic hypotheses and interpretations of fossil ecology have been seen as requiring independent events of terrestrialization Some of these views have been overturned by strong molecular (Regier et al, 2010; Sharma et al, 2014; Sharma and Wheeler, 2014; Leite et al, 2018; Ballesteros and Sharma, 2019; Lozano-Fernandez et al, 2019) and morphological (Garwood and Dunlop, 2014; Klußmann-Fricke and Wirkner, 2016) evidence for scorpions being nested within the Arachnida as the sister group of the other arachnids with book lungs, the Tetrapulmonata. The supposed aquatic mode of life of various fossil scorpions has been questioned on both morphological and geological grounds (Dunlop et al, 2008; Kühl et al, 2012) Another challenge to a single terrestrialization event in arachnids came from analyses of phylogenomic datasets, which have often recovered the marine Xiphosura to be nested within Arachnida (Ballesteros and Sharma, 2019). This remains a contentious issue, as other phylogenomic analyses have yielded trees in which Arachnida is monophyletic (Lozano-Fernandez et al, 2019)
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