Abstract
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.
Highlights
Palaeospondylus gunni Traquair, 1890 is commonly found in the Middle Devonian Achanarras fish beds, Achanarras Quarry2017 The Authors
Palaeospondylus is normally preserved in dorsal or ventral view; new X-ray tomographic data allow for a modified three-dimensional visualization in lateral view, allowing for a revised interpretation of the prominent L-shaped structure on the lateral face of the chondrocranium and the 3 element articulating to the posteroventral margin of this structure
A more complete examination of Palaeospondylus morphology is provided in the electronic supplementary material; phylogenetically relevant characters, with respect to the recent identification of Palaeospondylus as a hagfish, are described below
Summary
Palaeospondylus gunni Traquair, 1890 is commonly found in the Middle Devonian Achanarras fish beds, Achanarras Quarry2017 The Authors. (Scotland), part of a deeper-water lake fauna including jawless fishes ([1,2,3], but see [4] for possible 2 marine influence) and gnathostomes including placoderms, acanthodians and osteichthyans. Palaeospondylus has been assigned to almost every major jawless and jawed vertebrate group and identified as both larval and adult [6,7,8,9,10,11]. New X-ray tomographic scans of Palaeospondylus provide important new details of cranial anatomy, with respect to the otic capsule and vestibular system, allowing us to identify Palaeospondylus as a jawed vertebrate rather than a jawless hagfish. Along with a precerebral fontanelle, these suggest a chondrichthyan affinity for Palaeospondylus, with similarity to the stem chondrichthyans such as Pucapampella [13,14,15]
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