Palaeospondylus

Palaeospondylus ("early vertebrae") is a prehistoric fish, a fossil vertebrate. Its fossils were originally described from the Achanarras slate quarry in Caithness, Scotland, and a second species has been discovered in Australia.^[1]

Quick Facts Scientific classification, Type species ...

Palaeospondylus Temporal range: Middle Devonian PreꞒ Ꞓ O S D C P T J K Pg N
Fossil on display at the Cincinnati Museum Center
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Agnatha?
Genus:	Palaeospondylus Traquair, 1890
Type species
Palaeospondylus gunni Traquair, 1890

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The Scottish fossil as preserved is carbonised, and indicates an eel-shaped animal up to 6 centimetres (2 in) in length. The skull, which must have consisted of hardened cartilage, exhibits pairs of nasal and auditory capsules, with a gill apparatus below its hinder part, and ambiguous indications of ordinary jaws.^{[citation needed]}

The phylogeny of this fossil has puzzled scientists since its discovery in 1890, and many taxonomies have been suggested. In 2004, researchers proposed that Palaeospondylus was a larval lungfish.^[2] Previously, it had been classified as a larval tetrapod, unarmored placoderm, an agnathan, an early stem hagfish, and a Chimaera.^[3][4] A 2017 study suggested that it was a stem chondrichthyan.^[5]

In 2022, researchers reported, based on studies using synchrotron radiation X-ray micro-computed tomography, that the neurocranium of Palaeospondylus was similar to those of the stem-tetrapods Eusthenopteron and Panderichthys, and concluded that Palaeospondylus was between those two phylogenetically.^[6] Brownstein (2023) criticized this study, suggesting it would be basal gnathostomes instead.^[7] Hirasawa and Kuratani, who are authors in 2022 study, replied to that and reviewed phylogeny again, resulted it would be closer to Acanthostega instead.^[8] The 2024 study, using braincase data, ruled out the tetrapod hypothesis.^[1]