Vertebral architecture in the earliest stem tetrapods
- Stephanie E. Pierce,
- Per E. Ahlberg,
- John R. Hutchinson,
- Julia L. Molnar,
- Sophie Sanchez,
- Paul Tafforeau
- & Jennifer A. Clack
Nature, (2013) doi:10.1038/nature11825
Published online 13 January 2013
The construction of the vertebral column has been used as a key anatomical character in defining and diagnosing early tetrapod groups1. Rhachitomous vertebrae2—in which there is a dorsally placed neural arch and spine, an anteroventrally placed intercentrum and paired, posterodorsally placed pleurocentra—have long been considered the ancestral morphology for tetrapods1, 3, 4, 5, 6. Nonetheless, very little is known about vertebral anatomy in the earliest stem tetrapods, because most specimens remain trapped in surrounding matrix, obscuring important anatomical features7, 8, 9.
Here we describe the three-dimensional vertebral architecture of the Late Devonian stem tetrapod Ichthyostega using propagation phase-contrast X-ray synchrotron microtomography. Our scans reveal a diverse array of new morphological, and associated developmental and functional, characteristics, including a possible posterior-to-anterior vertebral ossification sequence and the first evolutionary appearance of ossified sternal elements. One of the most intriguing features relates to the positional relationships between the vertebral elements, with the pleurocentra being unexpectedly sutured or fused to the intercentra that directly succeed them, indicating a ‘reverse’ rhachitomous design10. Comparison of Ichthyostega with two other stem tetrapods, Acanthostega7 and Pederpes8, shows that reverse rhachitomous vertebrae may be the ancestral condition for limbed vertebrates. This study fundamentally revises our current understanding11 of vertebral column evolution in the earliest tetrapods and raises questions about the presumed vertebral architecture of tetrapodomorph fish12, 13 and later, more crownward, tetrapods.
Three-dimensional limb joint mobility in the early tetrapod Ichthyostega
Published online 23 May 2012
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.
- Scientists reassemble the backbone of life with a particle acceleratorynchrotron X-rays (scienceblog.com)
- Early Land-Dwelling Animals Moved About Like Seals, Probably Didn’t Balance Balls on Their Noses (geekosystem.com)
- First land animals shuffled like seals (abc.net.au)
- Astonishing 3D images reveal the first four-legged land animals in amazing detail – and overturn a century of research (thisismoney.co.uk)
- First Land Animals Shuffled Like Seals (news.discovery.com)
- Paleontologists Use 3-D Models to Rewrite Evolution (blogs.discovermagazine.com)
- Inference of the Protokaryotypes of Amniotes and Tetrapods and the Evolutionary Processes of Microchromosomes from Comparative Gene Mapping (plosone.org)
- Anatomy ‘wrong’ in early animals (bbc.co.uk)
- If you go down to the Triassic today… (ichthyosaurs.wordpress.com)