A new model has made it possible to estimate how the backbones of some of the first animals on land could and could not move using sturgeon as a material analogue. 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.
Researchers have overturned a long-standing hypothesis 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 we 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 out 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.
"If you can measure the size and shape of both the thing you are testing and the part of the animal you want to model mobility of, it all comes down to dividing out the sturgeon number and multiplying by the fossil number," says Korneisel.
"I undertook this study to find out how big of a difference this change in anatomy made for these animals" she continues, referring to the separation of the head from the shoulders, "even while performing the dissections on sturgeon and handling their vertebral columns, which are built a lot like those of early tetrapods, I felt that the tissue wasn't terribly stiff. So, after doing the math, the lack of significant head mobility for almost all of these animals was a huge surprise".
Korneisel emphasizes that all of these estimates are based on just the backbone, so actual mobility will have been less than the estimates presented here because of all of the tissues that surround the backbone like muscles, skin, and for many of these animals, bones embedded in the skin. And what about looking up or turning the head?
"Head elevation in these animals was no more than what you would expect in fish, and sometimes even less. We found that the kind of head twisting that we can do when we shake our heads was not feasible in any of these animals."
Where might this go in the future? Notochordal vertebral columns are rare today, but they were very common throughout life's history on earth. The researchers also collected data on the whole vertebral column of sturgeon, not just the front-most part, so mobility could be estimated for other parts of the body of early tetrapods or for a number of other extinct groups provided a good enough reconstruction of the skeleton can be made from the known fossils.
