''Palaeospondylus'' ("early vertebra") is a

prehistoric fish __NOTOC__ Prehistoric fish are early fish that are known only from fossil records. They are the earliest known vertebrates, and include the first and extinct fish that lived through the Cambrian to the Quaternary. The study of prehistoric fish is ...

, a fossil

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

. Its fossils were originally described from the Achanarras slate quarry in

Caithness Caithness (; ; ) is a Shires of Scotland, historic county, registration county and Lieutenancy areas of Scotland, lieutenancy area of Scotland. There are two towns, being Wick, Caithness, Wick, which was the county town, and Thurso. The count ...

, Scotland, and a second species has been discovered in Australia. The Scottish

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

as preserved is carbonised, and indicates an eel-shaped animal up to 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. Palaeospondylus

The

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

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 Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...

. Previously, it had been classified as a larval

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

, unarmored

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

, an

agnatha Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...

n, an early stem

hagfish Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...

, and a Chimaera. A 2017 study suggested that it was a stem chondrichthyan. In 2022, researchers reported, based on studies using synchrotron radiation X-ray micro-computed tomography, that the

neurocranium In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the cal ...

of ''Palaeospondylus'' was similar to those of the stem-tetrapods '' Eusthenopteron'' and '' Panderichthys'', and concluded that ''Palaeospondylus'' was between those two phylogenetically. Brownstein (2023) criticized this study, suggesting it would be basal gnathostomes instead. Hirasawa and Kuratani, who are authors in 2022 study, replied to that and reviewed phylogeny again, resulted it would be closer to '' Acanthostega'' instead. The 2024 study, using braincase data, ruled out the tetrapod hypothesis.

Species

* ''Palaeospondylus australis'', early Devonian, Australia * ''Palaeospondylus gunni'', middle Devonian, Scotland

References

* {{Taxonbar, from=Q3889332 Prehistoric vertebrate genera Devonian animals of Europe Devonian animals of Australia Prehistoric vertebrates of Europe Fossils of Scotland Caithness History of the Scottish Highlands Devonian genus extinctions

Species

See also

References