This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A new model using sturgeon as a material analog has made it possible to estimate how the backbones of some of the first animals on land could and could not move. These results disprove a long-standing hypothesis that changes in early tetrapod anatomy as they moved onto land gave them improved neck mobility.
The study, led by a researcher at Uppsala University, formerly of Carleton University in Canada, has been published in the journal Nature. The long-standing hypothesis was that our ancestors, the early tetrapods, gained increased neck mobility early on in their transition to land. In fish, the shoulder blades are attached to the skull.
In the early tetrapods, however, the shoulders became detached sometime in the Devonian period and they acquired a new region of neck vertebrae separating these two parts of the skeleton. It has been reasonably assumed for decades that this separation must have allowed the head to move more freely, so the hypothesis that neck mobility was an evolutionary driver for this change in the tetrapod body plan has prevailed. But the structure of early tetrapods' backbones is very unusual compared to most vertebrates today, making it hard to model how they moved.
They keep a long rod of tissue called the notochord for their whole lives that runs through the whole vertebral column, so their actual vertebrae are small and arranged around a large tube of soft tissue. Because this soft tissue behaves differently from joints between vertebrae that are typical today, scientists have not been able to test the neck mobility hypothesis. In this new study, though, researchers came up with a way of modeling early tetrapod neck mobility using a distantly related fish, the sturgeon, that has the same kind of backbone.
By applying the material properties of sturgeon notochordal backbones to fossil animals, researchers have found that early tetrapod necks were surprisingly stiff and, even free of surrounding bones, they would not have allowed much mobility. "We know that tetrapods did eventually gain higher neck mobility, so these results push that gain-of-function forward in time and up the evolutionary tree from where we previously thought it occurred. Clearly, it was advantageous eventually, but it was not an advantage available to these early tetrapods yet because of the limitations of their anatomy," says lead author Dana Korneisel.
To figure this out, the researchers first had to dissect the backbones of sturgeon to bend and twist them on a universal testing machine. They spent three days testing batches of three or four at a time, cutting out first the whole backbone and then sections of the notochord to test both tissues. From these test data, they calculated the material stiffness of the sturgeon tissues and then applied this stiffness to extinct animals.
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