"When I was in Taiwan I was able to obtain and examine several diverse lineages of flatfishes and comment on the ability of flatfishes to protrude their eyes. This ability enabled by an accessory organ, the "recessus orbitalis" has been considered to unite flatfishes as a group and has been assumed to be present across flatfish lineages.
We present the first description of the recessus orbitalis in over 100 years with new dissections. We find that the recessus orbitalis may be observed across numerous flatfish families and provide direct evidence for its presence in seven families not previously documented.
The reccesus orbitalis; however, is absent from the most primitive flatfish family, Psettodidae, comprising an entire suborder of flatfishes and represented by three species. Ancestral character estimation indicates that the recessus orbitalis was most likely not present in the common ancestor of all flatfishes, and it is a derived character state of the Pleuronectoidei flatfish suborder, not flatfishes as a whole."
Read the paper The distribution of the recessus orbitalis across flatfishes (order: Pleuronectiformes) @ Wiley Online Library
Also read: Why Taiwan? Investigating the Biogeography of Flatfishes from Their Evolutionary Hotspot @ Research & Reflections