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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.


Proposed phylogenetic position of Palaeospondylus. The molecular-based estimation of the lamprey-hagfish divergence was adopted from [63]
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Fig5: Proposed phylogenetic position of Palaeospondylus. The molecular-based estimation of the lamprey-hagfish divergence was adopted from [63]

Mentions: Given the potential extensive taphonomic bias and the lack of data about soft tissue anatomy unlike circumstances of other species [56–58], it is difficult to build a character matrix to conduct a cladistic analysis of Palaeospondylus. Nevertheless, the above comparison strongly suggests that Palaeospondylus and the hagfish share a cranial skeletal configuration that is distinguishable from those of the lamprey and crown gnathostomes. On the other hand, some features seen in adult individuals of the known hagfish species are less conspicuous in Palaeospondylus. In particular, in the extant hagfishes, as well as in the Late Carboniferous hagfish Myxinikela siroka [59], the position of the nasal capsule, which initially develops at the rostral end of the cranium becomes relatively caudal in the cranium of adult individuals, whereas in Palaeospondylus, the nasal capsule remained at the rostral end. Based on this synapomorphy between extant hagfishes and Myxinikela, we suggest that the phylogenetic position of Palaeospondylus is best explained as a stem hagfish lineage basal to Myxinikela (Fig. 5).Fig. 5


Palaeospondylus as a primitive hagfish
Proposed phylogenetic position of Palaeospondylus. The molecular-based estimation of the lamprey-hagfish divergence was adopted from [63]
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig5: Proposed phylogenetic position of Palaeospondylus. The molecular-based estimation of the lamprey-hagfish divergence was adopted from [63]
Mentions: Given the potential extensive taphonomic bias and the lack of data about soft tissue anatomy unlike circumstances of other species [56–58], it is difficult to build a character matrix to conduct a cladistic analysis of Palaeospondylus. Nevertheless, the above comparison strongly suggests that Palaeospondylus and the hagfish share a cranial skeletal configuration that is distinguishable from those of the lamprey and crown gnathostomes. On the other hand, some features seen in adult individuals of the known hagfish species are less conspicuous in Palaeospondylus. In particular, in the extant hagfishes, as well as in the Late Carboniferous hagfish Myxinikela siroka [59], the position of the nasal capsule, which initially develops at the rostral end of the cranium becomes relatively caudal in the cranium of adult individuals, whereas in Palaeospondylus, the nasal capsule remained at the rostral end. Based on this synapomorphy between extant hagfishes and Myxinikela, we suggest that the phylogenetic position of Palaeospondylus is best explained as a stem hagfish lineage basal to Myxinikela (Fig. 5).Fig. 5

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.