Anatomically modern Carboniferous harvestmen demonstrate early cladogenesis and stasis in Opiliones

Abstract

Harvestmen, the third most-diverse arachnid order, are an ancient group found on all continental landmasses, except Antarctica. However, a terrestrial mode of life and leathery, poorly mineralized exoskeleton makes preservation unlikely, and their fossil record is limited. The few Palaeozoic species discovered to date appear surprisingly modern, but are too poorly preserved to allow unequivocal taxonomic placement. Here, we use high-resolution X-ray micro-tomography to describe two new harvestmen from the Carboniferous (~305 myr) of France. The resulting computer models allow the first phylogenetic analysis of any Palaeozoic opiliones, explicitly resolving both specimens as members of different extant lineages, and providing corroboration for molecular estimates of an early Palaeozoic radiation within the order. Furthermore, remarkable similarities between these fossils and extant harvestmen implies extensive morphological stasis in the order. Compared with other arachnids-and terrestrial arthropods generally-harvestmen are amongst the first groups to evolve fully modern body plans.

Carboniferous araneomorph spiders reinterpreted as long-bodied harvestmen

“…It should be stressed that only the tetrophthalmid, one of the eupnoids, and one of the dyspnoids have had their placement tested cladistically. All three taxa derive from exceptionally preserved fossils from Montceau-les-Mines in France and were reconstructed using computed tomography to create three-dimensional models of the nodule-hosted specimens (Garwood et al 2011. These studies demonstrate that a long-legged eupnoid and a spiny dyspnoid harvestman, both with essentially modern body plans, coexisted alongside the extinct, four-eyed (tetrophthalmid) lineage which had a combination of characters quite unlike that of any living species.…”

“…Indeed, recent and ongoing studies of Coal Measures Opiliones have begun to reveal a wide range of both modern and extinct body plans (Garwood et al 2011and references therein). The longbodied morphology documented here adds to the recorded Palaeozoic diversity (and disparity) of Coal Measures harvestmen.…”

A revised dated phylogeny of the arachnid order Opiliones

“…Consecutive efforts have improved upon previous counterparts, mostly through incorporation of newly discovered fossils and updated hypotheses concerning their placement (Garwood et al, 2011; Garwood et al, 2014). As an example, the oldest fossil harvestman, E. sheari , has previously been treated as crown-Eupnoi (e.g., Sharma and Giribet, 2011; Giribet et al, 2012a) until the first formal phylogenetic analysis including this species placed it as a stem-Cyphophthalmi with Hastocularis argus , the only known four-eyed harvestman (Garwood et al, 2014).…”

<span lang="EN-US"><span style="font-family: 'Times New Roman'; font-size: small;"><strong>Order Opiliones Sundevall, 1833. <em>In</em>: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) </strong></span></span>

“…9. In spite of the original placement of † Halitherses Giribet & Dunlop, 2005 in the Troguloidea, later work (Garwood et. al.…”

“…7. I gathered here not only the Phalangioidea incertae sedis, but also the Eupnoi incertae sedis of Dunlop et al (2004), Dunlop & Anderson (2005), Huang et al (2009) and Garwood et al (2011). 8.…”