Phylogenetic analysis suggests that this eye arrangement is the ancestral condition for harvestmen, placing Tetrophthalmi and Cyphophthalmi in a basal position within Opiliones.

A reanalysis of harvestman phylogeny based on 78 genes and an updated understanding of morphology revealed that Tetrophthalmi are derived members of the living suborder Eupnoi (true daddy-longlegs).

The team determined H. argus and E. sheari shared three features – two pairs of eyes, an open gonostome (a part of the reproductive system), and external male genitalia – which distinguish them from all other harvestmen.

[5][4] Garwood et al. also argue that a Carboniferous harvestmen diversification is more consistent with changes observed in other terrestrial arthropods, which have been linked to high oxygen levels during that period.

[1] The restoration of Eophangium sheari, the oldest known Opiliones fossil, to membership in Eupnoi is consistent with the original description of this species as a true daddy-longlegs.