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Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography

Nature volume 537, pages 408411 (15 September 2016) | Download Citation

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Abstract

The transition from fish to tetrapod was arguably the most radical series of adaptive shifts in vertebrate evolutionary history. Data are accumulating rapidly for most aspects of these events1,2,3,4,5, but the life histories of the earliest tetrapods remain completely unknown, leaving a major gap in our understanding of these organisms as living animals. Symptomatic of this problem is the unspoken assumption that the largest known Devonian tetrapod fossils represent adult individuals. Here we present the first, to our knowledge, life history data for a Devonian tetrapod, from the Acanthostega mass-death deposit of Stensiö Bjerg, East Greenland6,7. Using propagation phase-contrast synchrotron microtomography (PPC-SRμCT)8 to visualize the histology of humeri (upper arm bones) and infer their growth histories, we show that even the largest individuals from this deposit are juveniles. A long early juvenile stage with unossified limb bones, during which individuals grew to almost final size, was followed by a slow-growing late juvenile stage with ossified limbs that lasted for at least six years in some individuals. The late onset of limb ossification suggests that the juveniles were exclusively aquatic, and the predominance of juveniles in the sample suggests segregated distributions of juveniles and adults at least at certain times. The absolute size at which limb ossification began differs greatly between individuals, suggesting the possibility of sexual dimorphism, adaptive strategies or competition-related size variation.

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Acknowledgements

Beamtime was allocated as inhouse beamtime and thanks to a proposal accepted by the ESRF (EC203, S.S.). This research was supported by an ERC grant (233111, P.E.A.) and a grant from the Vetenskapsrådet (2015-04335, S.S.). The authors thank J. Castanet, J.-S. Steyer, G. Clement, M. Coates, T. Smithson, A. R. Milner, H. Blom, D. Snitting, I. Adameyko, A. Soler, S. Martin and R. R. Schoch for discussions; G. Cuny and B. E. Kramer Lindow for access to the collections housed in the Natural History Museum of Denmark; and M. Lowe for access to the collections of the University Museum of Zoology, Cambridge.

Author information

Affiliations

  1. Science for Life Laboratory and Uppsala University, Subdepartment of Evolution and Development, Department of Organismal Biology, Evolutionary Biology Centre, Norbyvägen 18A, 752 36 Uppsala, Sweden

    • Sophie Sanchez
    •  & Per E. Ahlberg
  2. European Synchrotron Radiation Facility, 71 Avenue des Martyrs, CS-40220, 38043 Grenoble Cedex, France

    • Sophie Sanchez
    •  & Paul Tafforeau
  3. University Museum of Zoology, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK

    • Jennifer A. Clack

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Contributions

S.S., P.E.A. and P.T. conceived and designed the project. S.S. and P.T. performed the synchrotron experiments. The localities were excavated by J.A.C. and P.E.A. in 1987. P.T. processed and reconstructed the raw PPC-SRμCT scan data. S.S. segmented the scan data. S.S., P.E.A. and P.T. analysed the data. All authors discussed the interpretations. S.S. and P.E.A. developed the main text. S.S. made the figures and supplementary information. All authors provided a critical review of the manuscript and approved the final draft.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Sophie Sanchez.

Reviewer Information

Nature thanks J. Anderson, N. Fröbisch, R. Schoch and K. Stein for their contribution to the peer review of this work.

The synchrotron data will be made available through the ESRF palaeontology database (http://paleo.esrf.eu).

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https://doi.org/10.1038/nature19354

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