Abstract
BackgroundThe 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.ResultsHere 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.ConclusionThese 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 materialThe online version of this article (doi:10.1186/s40851-016-0057-0) contains supplementary material, which is available to authorized users.
Highlights
The taxonomic position of the Middle Devonian fish-like animal Palaeospondylus has remained enigmatic, due mainly to the inability to identify homologous cranial elements
The dental plate of the oral apparatus is located below the neurocranium, associated with the rostral part of the lingual plate that supports the muscles of the cyclostome ‘tongue’ [22]
The history of studies on Palaeospondylus [1,2,3,4,5,6,7,8,9,10] constitutes a heuristic search for homologous body parts, a task that we suggest may be completed with the present study, in which it was compared with the hagfish embryo
Summary
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 most of the major vertebrate taxa over a century of heuristic taxonomic research, despite the lack of conclusive morphological evidence. 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]; none of these assignments has been supported by conclusive evidence [11,12,13] This problem has been attributed to an inability to homologize its skeletal elements [14], and previous hypotheses have never successfully explained its anatomical configuration. Analyses of the embryonic development of the hagfish resolved the disparity in cranial morphology of vertebrates [22, 24], and for the first time, made it possible to compare the cranial skeleton of Palaeospondylus
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