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The fossil record documents only a small fraction of all organisms that have ever inhabited Earth. Fossilization is a rare process, and most organisms never become fossils. Moreover, the fossil record only exhibits fossils that have been discovered. Nevertheless, sedimentary rock fossils of long-lived, abundant, hard-bodied organisms dominate the fossil record. These fossils offer valuable information, such as an organism's physical form, behavior, and age. Studying the fossil record helps...
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Palaeospondylus as a primitive hagfish.

Tatsuya Hirasawa1, Yasuhiro Oisi2, Shigeru Kuratani1

  • 1Evolutionary Morphology Laboratory, RIKEN, 2-2-3 Minatojima-minami, Chuo-ku, Kobe, Hyogo 650-0047 Japan.

Zoological Letters
|September 10, 2016
PubMed
Summary

Palaeospondylus, a Devonian fish-like animal, is likely a stem hagfish. New comparative analysis reveals key similarities in cranial and skeletal features between Palaeospondylus and hagfish embryos.

Keywords:
CyclostomesDevonianFossil taxaHagfishHomologyPalaeospondylus

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Area of Science:

  • Paleontology
  • Vertebrate Zoology
  • Evolutionary Biology

Background:

  • The taxonomic classification of Palaeospondylus, a Middle Devonian vertebrate, has been debated for over a century due to unclear cranial element homology.
  • Previous research has placed Palaeospondylus within various major vertebrate taxa without conclusive morphological evidence.

Purpose of the Study:

  • To resolve the enigmatic taxonomic position of Palaeospondylus.
  • To investigate the potential affinity of Palaeospondylus with extant vertebrate groups through comparative morphology.

Main Methods:

  • Comparative morphological analysis of hagfish embryos and Palaeospondylus specimens.
  • Detailed examination of chondrocranial elements, including the nasal capsule, neurocranium, and velar bar.

Main Results:

  • Identified significant resemblances between Palaeospondylus and hagfish embryos in chondrocranial elements and craniofacial development.
  • Reclassified the ventral skeletal complex of Palaeospondylus as a cyclostome-specific lingual apparatus.
  • The vertebral column of Palaeospondylus, previously a point of contention, is now consistent with hagfish anatomy.

Conclusions:

  • Morphological evidence strongly supports the classification of Palaeospondylus within the hagfish lineage.
  • Palaeospondylus is proposed to be a stem hagfish, representing an early diverging member of this group.