Fossils of Guiyu have been found in what is now Qujing, Yunnan, China, in late Silurian marine strata,[2] about 425 million years old.

The generic name Guiyu is a transliteration of the Mandarin 鬼魚 guǐyú "ghost fish" and the specific name oneiros is from Greek ὄνειρος "dream".

[4] The pectoral, pelvic and dorsal fins bore large spines, as seen in many placoderms and acanthodians (basal stem-chondrichthyans).

This theory had little support from phylogenetic analyses until the 2017 description of Ptyctolepis, a stem-sarcopterygian from the Early Devonian of China.

A cladogram from that study is shown below:[6] Janusiscus Chondrichthyes Dialipina ‘Ligulalepis’ Guiyu Sparalepis Achoania Psarolepis Meemannia Cheirolepis Other Actinopterygii Ptyctolepis Onychodonts Coelacanths Dipnomorpha Tetrapodomorpha