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Palaeospondylus as a primitive hagfish

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ABSTRACT

Background: The taxonomic position of the Middle Devonian fish-like animal Palaeospondylus has remained enigmatic, due mainly to the inability to identify homologous cranial elements. This animal has been classified into nearly all of the major vertebrate taxa over a century of heuristic taxonomic research, despite the lack of conclusive morphological evidence.

Results: Here we report the first comparative morphological analysis of hagfish embryos and Palaeospondylus, and a hitherto overlooked resemblance in the chondrocranial elements of these animals; i.e., congruence in the arrangement of the nasal capsule, neurocranium and mandibular arch-derived velar bar. The large ventral skeletal complex of Palaeospondylus is identified as a cyclostome-specific lingual apparatus. Importantly, the overall morphological pattern of the Palaeospondylus cranium coincides well with the cyclostome pattern of craniofacial development, which is not shared with that of crown gnathostomes. Previously, the presence of the vertebral column in Palaeospondylus made its assignment problematic, but the recent identification of this vertebral element in hagfish is consistent with an affinity between this group and Palaeospondylus.

Conclusion: These lines of evidence support the hagfish affinity of Palaeospondylus. Moreover, based on the less specialized features in its cranial morphology, we conclude that Palaeospondylus is likely a stem hagfish.

Electronic supplementary material: The online version of this article (doi:10.1186/s40851-016-0057-0) contains supplementary material, which is available to authorized users.

No MeSH data available.


Cranial skeletons of Palaeospondylus. a,b Restoration of Palaeospondylus gunni cranial skeleton in dorsal (a) and ventral (b) views. amp, ampyx; cp, caudal plate; gam, gammation; hem, hemidome; otc, otic capsule; ros, rostralia; rp, rostral plate; tau, tauidion; Ve, V-shaped element
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Fig1: Cranial skeletons of Palaeospondylus. a,b Restoration of Palaeospondylus gunni cranial skeleton in dorsal (a) and ventral (b) views. amp, ampyx; cp, caudal plate; gam, gammation; hem, hemidome; otc, otic capsule; ros, rostralia; rp, rostral plate; tau, tauidion; Ve, V-shaped element

Mentions: Palaeospondylus gunni [1] from the Middle Devonian of Scotland (Fig. 1, Additional file 1: Figure S1, Additional file 2: Figure S2) has long been an enigmatic fossil in vertebrate palaeontology [1–11]. Over the past 125 years, attempts have been made to classify this fossil as a cyclostome [1, 6, 8], frog tadpole [2], lungfish larva [3, 7, 10], holocephalan [4], elasmobranch [5], placoderm [9], and even a secondarily boneless osteichthyan [11]; however, none of these assignments has been supported by conclusive evidence [11–13]. This problem has been attributed to an inability to homologize its skeletal elements [14], and previous hypotheses have never successfully explained its anatomical configuration. Indeed, the arrangement of skeletal elements of Palaeospondylus has never yet been integrally compared to any of the actual ontogenetic stages of certain taxa.Fig. 1


Palaeospondylus as a primitive hagfish
Cranial skeletons of Palaeospondylus. a,b Restoration of Palaeospondylus gunni cranial skeleton in dorsal (a) and ventral (b) views. amp, ampyx; cp, caudal plate; gam, gammation; hem, hemidome; otc, otic capsule; ros, rostralia; rp, rostral plate; tau, tauidion; Ve, V-shaped element
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5015246&req=5

Fig1: Cranial skeletons of Palaeospondylus. a,b Restoration of Palaeospondylus gunni cranial skeleton in dorsal (a) and ventral (b) views. amp, ampyx; cp, caudal plate; gam, gammation; hem, hemidome; otc, otic capsule; ros, rostralia; rp, rostral plate; tau, tauidion; Ve, V-shaped element
Mentions: Palaeospondylus gunni [1] from the Middle Devonian of Scotland (Fig. 1, Additional file 1: Figure S1, Additional file 2: Figure S2) has long been an enigmatic fossil in vertebrate palaeontology [1–11]. Over the past 125 years, attempts have been made to classify this fossil as a cyclostome [1, 6, 8], frog tadpole [2], lungfish larva [3, 7, 10], holocephalan [4], elasmobranch [5], placoderm [9], and even a secondarily boneless osteichthyan [11]; however, none of these assignments has been supported by conclusive evidence [11–13]. This problem has been attributed to an inability to homologize its skeletal elements [14], and previous hypotheses have never successfully explained its anatomical configuration. Indeed, the arrangement of skeletal elements of Palaeospondylus has never yet been integrally compared to any of the actual ontogenetic stages of certain taxa.Fig. 1

View Article: PubMed Central - PubMed

ABSTRACT

Background: The taxonomic position of the Middle Devonian fish-like animal Palaeospondylus has remained enigmatic, due mainly to the inability to identify homologous cranial elements. This animal has been classified into nearly all of the major vertebrate taxa over a century of heuristic taxonomic research, despite the lack of conclusive morphological evidence.

Results: Here we report the first comparative morphological analysis of hagfish embryos and Palaeospondylus, and a hitherto overlooked resemblance in the chondrocranial elements of these animals; i.e., congruence in the arrangement of the nasal capsule, neurocranium and mandibular arch-derived velar bar. The large ventral skeletal complex of Palaeospondylus is identified as a cyclostome-specific lingual apparatus. Importantly, the overall morphological pattern of the Palaeospondylus cranium coincides well with the cyclostome pattern of craniofacial development, which is not shared with that of crown gnathostomes. Previously, the presence of the vertebral column in Palaeospondylus made its assignment problematic, but the recent identification of this vertebral element in hagfish is consistent with an affinity between this group and Palaeospondylus.

Conclusion: These lines of evidence support the hagfish affinity of Palaeospondylus. Moreover, based on the less specialized features in its cranial morphology, we conclude that Palaeospondylus is likely a stem hagfish.

Electronic supplementary material: The online version of this article (doi:10.1186/s40851-016-0057-0) contains supplementary material, which is available to authorized users.

No MeSH data available.