Date Published:

Submitted to Sci

Abstract:

Successfully placing fossils in phylogenies is integral to understanding the tree of life. Crown-group Paleozoic mem- bers of the arachnid order Opiliones are indicative of ancient origins and one of the earliest arthropod terrestrialization events [1, 2]. Opiliones epitomize morphological stasis, and all known fossils have been placed within the four extant suborders [3–5]. Here we report a Carboniferous harvestman species, Hastocularis argus gen. nov., sp. nov., recon- structed with microtomography (microCT). Phylogenetic analysis recovers this species, and the Devonian Eophalan- gium sheari, as members of an extinct harvestman clade. We establish the suborder Tetrophthalmi subordo nov., which bore four eyes, to accommodate H. argus and E. sheari, the latter previously considered to be a phalangid [6–9]. Further- more, embryonic gene expression in the extant species Phalangium opilio demonstrates vestiges of lateral eye tubercles. These lateral eyes are lost in all crown-group Phalangida, but are observed in both our fossil and outgroup chelicerate orders. These data independently corroborate the diagnosis of two eye pairs in the fossil and demonstrate retention of eyes of separate evolutionary origins in modern harvestmen [10–12]. The discovery of Tetrophthalmi alters molecular divergence time estimates, supporting Carbonif- erous rather than Devonian diversification for extant subor- ders and directly impacting inferences of terrestrialization history and biogeography. Multidisciplinary approaches integrating fossil and neontological data increase confi- dence in phylogenies and elucidate evolutionary history.

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