Three-dimensional limb joint mobility in the early tetrapod Ichthyostega

Journal name:
Nature
Volume:
486,
Pages:
523–526
Date published:
DOI:
doi:10.1038/nature11124
Received
Accepted
Published online

The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs—particularly the specialization of joints and the structures that guide their motions—fundamentally changed the ways in which early tetrapods could move1, 2, 3, 4. Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes5, 6, 7, it seems that either early tetrapods evolved through an initial stage of restricted shoulder8, 9 and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways10.

At a glance

Figures

  1. Three-dimensional reconstruction of Ichthyostega from [mgr]CT scan data.
    Figure 1: Three-dimensional reconstruction of Ichthyostega from μCT scan data.

    a, Anterolateral view. b, Dorsal view. c, Lateral view. d, Ventral view. The forelimbs and hindlimbs are shown in their resting pose from which ranges of motion were calculated. A list of the specimens used to create the model and the procedure followed for model construction can be found in Supplementary Information, as can a comparison with the most recent two-dimensional reconstruction presented in ref. 4. Scale bar, 10cm.

  2. Maximum ranges of mobility in the limb joints of Ichthyostega and five modern tetrapod analogues.
    Figure 2: Maximum ranges of mobility in the limb joints of Ichthyostega and five modern tetrapod analogues.

    a, Hip joint. b, Shoulder joint. Mobility was examined in three orthogonal planes of movement including flexion/extension, adduction/abduction and pronation/supination (or long-axis rotation). The most obvious difference between Ichthyostega and the modern tetrapods analysed is a distinct lack of long-axis rotation. A validation test of the method used to calculate range of mobility can been found in Supplementary Information.

  3. Partitioned range of mobility in the hip joint and shoulder joint of Ichthyostega.
    Figure 3: Partitioned range of mobility in the hip joint and shoulder joint of Ichthyostega.

    a, Hip joint mobility partitioned into positive and negative angular movements. b, Movement of the hip in flexion/extension. c, Movement of the hip in adduction/abduction. d, Shoulder joint mobility partitioned into positive and negative angular movements. e, Movement of the shoulder in flexion/extension. f, Movement of the shoulder in adduction/abduction. Asterisks indicate the resting pose. Specific details of the method used to calculate joint range of mobility are provided in Supplementary Information, as are animations of maximum range of mobility in the shoulder and hip of Ichthyostega. Scale bars, 10cm.

References

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Author information

Affiliations

  1. Department of Veterinary Basic Sciences and Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, Hatfield AL9 7TA, UK

    • Stephanie E. Pierce &
    • John R. Hutchinson
  2. University Museum of Zoology, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK

    • Stephanie E. Pierce &
    • Jennifer A. Clack

Contributions

All authors contributed to project concept and design. S.E.P. collected and analysed the data and wrote the manuscript, including main text, figures and Supplementary Information. J.A.C. and J.R.H. provided a critical review of all aspects of manuscript development. All authors approved the final draft.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

PDF files

  1. Supplementary Information (1.8M)

    This proof contains Supplementary Text, which includes details of how the 3D model of Ichthyostega was constructed, Supplementary References, Supplementary Figures 1-6 and Supplementary Tables 1-6.

Movies

  1. Supplementary Movie 1 (17.2M)

    This movie shows the 3D whole body reconstruction of Ichthyostega spinning 360 degrees in yaw and roll.

  2. Supplementary Movie 2 (8.3M)

    This movie shows maximum range of motion in the shoulder joint of Ichthyostega during flexion/extension, adduction/abduction and pronation/supination.

  3. Supplementary Movie 3 (7.6M)

    This movie shows maximum range of motion in the hip joint of Ichthyostega during flexion/extension, adduction/abduction and pronation/supination.

Additional data