Morphology of Palaeospondylus shows affinity to tetrapod ancestors

Tatsuya Hirasawa (), Yuzhi Hu, Kentaro Uesugi, Masato Hoshino, Makoto Manabe and Shigeru Kuratani
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Tatsuya Hirasawa: Graduate School of Science, the University of Tokyo
Yuzhi Hu: Australian National University
Kentaro Uesugi: Japan Synchrotron Radiation Research Institute (JASRI)
Masato Hoshino: Japan Synchrotron Radiation Research Institute (JASRI)
Makoto Manabe: National Museum of Nature and Science
Shigeru Kuratani: Evolutionary Morphology Laboratory, RIKEN Cluster for Pioneering Research (CPR)

Nature, 2022, vol. 606, issue 7912, 109-112

Abstract: Abstract Palaeospondylus gunni, from the Middle Devonian period, is one of the most enigmatic fossil vertebrates, and its phylogenetic position has remained unclear since its discovery in Scotland in 1890 (ref. 1). The fossil’s strange set of morphological features has made comparisons with known vertebrate morphotype diversity difficult. Here we use synchrotron radiation X-ray micro-computed tomography to show that Palaeospondylus was a sarcopterygian, and most probably a stem-tetrapod. The skeleton of Palaeospondylus consisted solely of endoskeletal elements in which hypertrophied chondrocyte cell lacunae, osteoids and a small fraction of perichondral bones developed. Despite the complete lack of teeth and dermal bones, the neurocranium of Palaeospondylus resembles those of stem-tetrapod Eusthenopteron2 and Panderichthys3, and phylogenetic analyses place Palaeospondylus in between them. Because the unique features of Palaeospondylus, such as the cartilaginous skeleton and the absence of paired appendages, are present in the larva of crown tetrapods, our study highlights an unanticipated heterochronic evolution at the root of tetrapods.

Date: 2022
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DOI: 10.1038/s41586-022-04781-3

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