Abstract
BackgroundArachnids are a highly successful group of land-dwelling arthropods. They are major contributors to modern terrestrial ecosystems, and have a deep evolutionary history. Whip spiders (Arachnida, Amblypygi), are one of the smaller arachnid orders with ca. 190 living species. Here we restudy one of the oldest fossil representatives of the group, Graeophonus anglicus Pocock, 1911 from the Late Carboniferous (Duckmantian, ca. 315 Ma) British Middle Coal Measures of the West Midlands, UK. Using X-ray microtomography, our principal aim was to resolve details of the limbs and mouthparts which would allow us to test whether this fossil belongs in the extant, relict family Paracharontidae; represented today by a single, blind species Paracharon caecus Hansen, 1921.ResultsTomography reveals several novel and significant character states for G. anglicus; most notably in the chelicerae, pedipalps and walking legs. These allowed it to be scored into a phylogenetic analysis together with the recently described Paracharonopsis cambayensis Engel & Grimaldi, 2014 from the Eocene (ca. 52 Ma) Cambay amber, and Kronocharon prendinii Engel & Grimaldi, 2014 from Cretaceous (ca. 99 Ma) Burmese amber. We recovered relationships of the form ((Graeophonus (Paracharonopsis + Paracharon)) + (Charinus (Stygophrynus (Kronocharon (Charon (Musicodamon + Paraphrynus)))))). This tree largely reflects Peter Weygoldt’s 1996 classification with its basic split into Paleoamblypygi and Euamblypygi lineages; we were able to score several of his characters for the first time in fossils. Our analysis draws into question the monophyly of the family Charontidae.ConclusionsOur data suggest that Graeophonus is a crown group amblypygid, and falls within a monophyletic Paleoamblypgi clade, but outside the family Paracharontidae (= Paracharonopsis + Paracharon). Our results also suggest a new placement for the Burmese amber genus Kronocharon, a node further down from its original position. Overall, we offer a broad phylogenetic framework for both the fossil and Recent whip spiders against which future discoveries can be tested.
Highlights
Arachnids are a highly successful group of land-dwelling arthropods
Previous interpretations picked up the median eye tubercle in G. anglicus, and the CT data confirmed the presence of lateral eye tubercles (Fig. 3a) which were considered equivocal in previous studies
G. anglicus is placed in the Carboniferous–Recent suborder Paleoamblypygi (Fig. 7), which can be defined based on the projecting anterior part of the carapace (Fig. 6)
Summary
Arachnids are a highly successful group of land-dwelling arthropods. They are major contributors to modern terrestrial ecosystems, and have a deep evolutionary history. Whip spiders (Arachnida: Amblypygi) are distinctive creatures (Fig. 1) with a long, slender, antenniform first pair of legs These whip-like appendages give the group its name, they are occasionally referred to as tailless whip scorpions because they resemble a related group of arachnids, the whip scorpions (Uropygi), albeit without the whip scorpion’s flagelliform telson. Based on the discovery of a better preserved specimen from Mazon Creek in Illinois ([11]; see [12]) Scudder later reinterpreted this find as a whip spider and created a new genus Graeophonus Scudder, 1890 to accommodate it as Graeophonus carbonarius. In the same monograph focussing on the British Middle Coal Measures Pocock [13] described Graeophonus anglicus Pocock, 1911 which—as the best preserved species—is the focus of the present study. A full overview of historical work on Palaeozoic taxa is provided by Dunlop et al [14]
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