The reasoning behind the old hypothesis seemed airtight. In fish, the shoulder blades are attached directly to the skull, anchoring the head to the body in a way that limits independent movement. In the earliest tetrapods, the shoulders became detached from the skull sometime in the Devonian, creating a new region of neck vertebrae between the two parts of the skeleton. It was reasonably assumed for decades that this anatomical separation must have allowed the head to move more freely, and the idea that improved neck mobility was an evolutionary driver for this change in the tetrapod body plan prevailed in textbooks and research papers alike.

The problem was that nobody could actually test the idea, because the backbones of early tetrapods are profoundly strange by modern standards. Unlike most vertebrates alive today, these animals retained a long rod of tissue called the notochord for their entire lives. The notochord runs through the whole vertebral column as a large tube of soft tissue, and the actual vertebrae are small elements arranged around it. Because this soft tissue behaves very differently from the joints between vertebrae in modern animals, standard biomechanical models of the spine simply did not apply, and the neck mobility hypothesis remained untestable for generations.

The breakthrough came from an unexpected relative. Sturgeon, though distantly related to tetrapods, possess the same kind of notochordal backbone that characterized the first land vertebrates. The research team realized that if they could measure the mechanical properties of sturgeon backbones directly, they could transfer those material properties to fossil animals and finally estimate how early tetrapod necks could and could not move. It was a way of turning a living fish into a material analogue for creatures that died out hundreds of millions of years ago.

The experimental work was painstaking. The researchers dissected out the backbones of sturgeon and subjected them to bending and twisting on a Universal Testing Machine, the standard instrument for characterizing how materials respond to force. They spent three days testing batches of three or four specimens at a time, first cutting out whole backbones and then isolating sections of the notochord itself so that both tissue types could be characterized separately. From these test data they calculated the material stiffness of the sturgeon tissues, a number that describes how much a material resists deformation under load.

Translating those measurements to fossils was, in principle, a matter of scaling. Lead author Dana Korneisel explained the logic plainly: if you can measure the size and shape of both the tissue you are testing and the part of the fossil animal whose mobility you want to model, the calculation comes down to dividing out the sturgeon number and multiplying by the fossil number. Geometry and material stiffness together determine how much a structure can bend, so with both quantities in hand, the range of motion of an extinct neck could finally be computed.

The results were startling, even to the person who did the math. Korneisel said she undertook the study to find out how big a difference the separation of the head from the shoulders actually made for these animals, but even while performing the dissections and handling the sturgeon vertebral columns, which are built much like those of early tetrapods, she felt the tissue was not terribly stiff. After applying the calculations, the lack of significant head mobility in almost all of the fossil animals came as a huge surprise. The early tetrapod necks were remarkably rigid, and even when modeled free of all surrounding bones, they would not have allowed much movement.

Moreover, the estimates represent an upper bound on what these animals could actually do. Korneisel emphasized that all of the figures are based on the backbone alone, so real mobility would have been lower than the published estimates because of everything that surrounds the spine: muscles, skin, and, in many of these animals, bones embedded in the skin that would have further restricted movement. As for the motions we take for granted, the findings were equally stark. Head elevation in these animals was no more than what would be expected in fish, and sometimes even less, and the kind of head twisting humans perform when shaking their heads was not feasible in any of the animals studied.

The implications ripple through the evolutionary tree. Tetrapods did eventually gain higher neck mobility, so the new results push that gain of function forward in time and up the evolutionary tree from where it was previously thought to occur. The detachment of the shoulders from the skull was clearly advantageous eventually, Korneisel noted, but it was not an advantage available to these early tetrapods because of the limitations of their anatomy. In other words, the anatomical change came first, and the functional payoff had to wait for later evolutionary innovations in the structure of the vertebral column itself.

The method also opens doors well beyond the neck. Notochordal vertebral columns are rare among animals today, but they were very common throughout the history of life on Earth, meaning the approach could be applied to many extinct groups beyond early tetrapods. The researchers also collected data on the whole vertebral column of the sturgeon, not just the front-most portion, so mobility could in principle be estimated for other parts of the early tetrapod body, or for entirely different fossil lineages, provided a good enough reconstruction of the skeleton can be made from the known fossils. What began as a test of a single hypothesis about the neck may end up reshaping how scientists model movement across vast stretches of vertebrate history, one carefully measured fish backbone at a time.

Subject of Research: Neck mobility in early tetrapods modeled using sturgeon notochordal backbones

Article Title: Early tetrapods had surprisingly little ability to move their necks

Article References: Early tetrapods had surprisingly little ability to move their necks. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: early tetrapods, neck mobility, notochord, sturgeon, Devonian Period, vertebral column, biomechanics, evolution, paleontology, Nature, Uppsala University, material analogue