A generation ago, children were taught that fishes grew legs and invaded the land. The early tetrapods were finding new environments for food and/or security. However, the evidence-base was thin, and we can look back on this time and realise that children were being fed yet another just-so story. Research showed that the early tetrapods were not terrestrial animals at all, but were aquatic. So the story has changed, and the limbs are thought to have evolved to aid the animals moving through brackish or marine habitats that were crowded with plant life. This changing perspective is alluded to by Jennifer Clack and colleagues in the introduction to their latest paper:
"Early tetrapods, and the fish that gave rise to them, were originally interpreted as being terrestrially capable animals with load-bearing fins or limbs. Since the 1990s, however, new fossil discoveries and anatomical interpretations have demonstrated that the first limbed vertebrates were primarily aquatic in habit and that limbs evolved before the ability to 'walk' on land."
Some of the characters in the evolutionary story (source here)
Researchers have discussed different tetrapod locomotory styles and recognised a gap in knowledge. This has stimulated the recent research, which adopted the following methodology:
"To illuminate the evolution of early tetrapod locomotion, we conducted a computer-aided assessment of limb joint mobility in one of the best known Devonian tetrapods, Ichthyostega. To achieve this goal, we used micro-computed tomography (microCT) to scan suitable Ichthyostega specimens, created a digitally rendered three-dimensional skeletal model, and quantified maximum range of motion in the shoulder, elbow, hip and knee in three orthogonal planes of movement. To interpret joint mobility in a locomotor context, we compared the data of Ichthyostega with those of five morphologically and phylogenetically distinct modern tetrapod analogues with varying joint morphologies and locomotion behaviours. These include a salamander (Ambystoma tigrinum), crocodile (Crocodylus niloticus), platypus (Ornithorhynchus anatinus), seal (Halichoerus grypus) and otter (Lutra vulgaris)."
The findings confirm that the limbs could help the animal in water, but were not suitable for land movement:
"On the basis of our study of limb joint mobility, combined with rib and vertebral morphology, we conclude that Ichthyostega could not use 'normal' quadrupedal gaits. The ability to rotate the humerus and femur longitudinally and use symmetrical gaits (for example lateral sequence walking) must have evolved in other early tetrapod species. Given that a similar type of shoulder and/or hip joint morphology presents itself in some other early tetrapod species, limited limb joint mobility -particularly long-axis rotation - may have been more widespread."
These findings do call for some reassessment of the science and the evolutionary interpretations. BBC News reported on the find and cited comments from two of the researchers:
"Dr Pierce told BBC News: "We're almost bringing the animal back to life by doing this. What we've discovered is that some early tetrapods definitely did not have the ability to walk on land. We at this stage are not actually sure which animals - or group of animals - were the first to do this."
Co-author Prof Jenny Clack from the University of Cambridge added: "Our reconstruction demonstrates that the old idea, often seen in popular books and museum displays, of Ichthyostega looking and walking like a large salamander, with four sturdy legs, is incorrect.""
We are left with the enigmatic trackways from Poland (discussed here) that are thought to date earlier than the known aquatic tetrapods. It seems that there are no candidate fossil animals that could have made the tracks.
"Given our results, could an Ichthyostega-like early tetrapod have produced similar trackways to some of those recently described from the Middle Devonian? All available evidence from limb joint mobility and axial anatomy indicates that such animals could not have made symmetrical gait 'foot' prints. In particular, these early tetrapods probably lacked the necessary rotary motions in their limbs (and perhaps lateral flexion of the vertebral column) to push the body off the substrate and progress using alternating limb movements. Maybe as yet unknown tetrapod species (or known taxa that currently lack postcranial material) with different joint mobility and axial anatomy made these traces; available data cannot yet answer this conundrum."
So we can predict that a new tetrapod animal from the Middle Devonian awaits discovery. It is time the hype about Tiktaalik, and any of the other aquatic tetrapods, was dropped. They are not the evolutionary gems they are claimed to be. The fossils that are beginning to populate Romer's Gap are not filling in a gradualist story, as is discussed here. The fossil record typically provides evidence for discontinuity, not gradualism. Evolutionary theorists must be more serious about addressing these evidences (instead of calling them 'gaps in knowledge').
Three-dimensional limb joint mobility in the early tetrapod Ichthyostega
Stephanie E. Pierce, Jennifer A. Clack & John R. Hutchinson
Nature, 486, 523-526 (28 June 2012) | doi:10.1038/nature11124
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 move. 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 fishes, it seems that either early tetrapods evolved through an initial stage of restricted shoulder 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 trackways.
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