Data from: A Palaeozoic stem-group to mite harvestmen revealed through integration of phylogenetics and development

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. Opiliones epitomize morphological
stasis, and all known fossils have been placed within the four extant
suborders. 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. 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.
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.