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Palaeospondylus gunni  Traquair, 1890 is an enigmatic Devonian vertebrate whose taxonomic affinities have been debated since it was first described. Most recently,  Palaeospondylus  has been identified as a stem-group hagfish (Myxinoidea). However, one character questioning this assignment is the presence of three semicircular canals in the otic region of the cartilaginous skull, a feature of jawed vertebrates. Additionally, new tomographic data reveal that the following characters of crown-group gnathostomes (chondrichthyans + osteichthyans) are present in  Palaeospondylus : a longer telencephalic region of the braincase, separation of otic and occipital regions by the otico-occipital fissure, and vertebral centra. As well, a precerebral fontanelle and postorbital articulation of the palatoquadrate are characteristic of certain chondrichthyans. Similarities in the structure of the postorbital process to taxa such as  Pucapampella , and possible presence of the ventral cranial fissure, both support a resolution of  Pa. gunni  as a stem chondrichthyan. The internally mineralized cartilaginous skeleton in  Palaeospondylus  may represent a stage in the loss of bone characteristic of the Chondrichthyes.

Questioning hagfish affinities of the enigmatic Devonian vertebratePalaeospondylus