Current descendants of the spiderlike arachnid have one functional pair and one vestigial pair.
From ScienceDaily:
The primitive fossilised harvestman, named Hastocularis argus, was found in eastern France and had not only median eyes — those found near the centre of the body — but lateral eyes on the side of the body as well.
…
“Arachnids can have both median and lateral eyes, but modern harvestmen only possess a single set of median eyes — and no lateral ones. These findings represent a significant leap in our understanding of the evolution of this group.”
The team supported their results by examining the expression of an ‘eye stalk’ gene in living harvestmen and found that in a modern harvestman embryo this gene shows hints of a now-lost lateral eye.
One wonders if a present day harvestman species will crash the party sporting functional lateral eyes. Something similar happened to the “blind” cave fish).Eyes went vestigial in total darkness, but hybridization started to bring them back.
We covered the original fossil find earlier in “300 mya Harvester: The fossil record is proving to be less and less Darwinian as we examine the details.” and “For daddy longlegs [harvestman], evolution never happened, it seems”
Here’s the abstract of the open access paper:
Successfully placing fossils in phylogenies is integral to understanding the tree of life. Crown-group Paleozoic members 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 argusgen. nov., sp. nov., reconstructed with microtomography (microCT). Phylogenetic analysis recovers this species, and the Devonian Eophalangium 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 ]. Furthermore, 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 Carboniferous rather than Devonian diversification for extant suborders and directly impacting inferences of terrestrialization history and biogeography. Multidisciplinary approaches integrating fossil and neontological data increase confidence in phylogenies and elucidate evolutionary history. – Russell J. Garwood, Prashant P. Sharma, Jason A. Dunlop, Gonzalo Giribet. A Paleozoic Stem Group to Mite Harvestmen Revealed through Integration of Phylogenetics and Development. Current Biology, April 2014 DOI: 10.1016/j.cub.2014.03.039
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