New Delhi: The first animals to develop necks nearly 380 million years ago may have been unable to turn their heads freely, challenging a long-standing theory about how vertebrates evolved to live on land.
Published in Nature on 14 October, the study suggests that necks initially evolved alongside changes in the skeleton that allowed animals to use their front limbs for movement, rather than to improve head mobility.
Scientists had previously believed that separating the skull from the shoulders helped early tetrapods, the ancestors of four-limbed vertebrates, move their heads independently of their bodies.
“It has been reasonably assumed for decades that this separation of head and shoulders must have allowed the head to move more freely,” Thomas Stewart, an assistant professor of biology at Penn State and a co-author of the study, told ScienceDaily.
Studying ancient necks using modern fish
Reconstructing how these animals moved has been difficult because their backbones were largely supported by a soft tissue structure called the notochord, which rarely survives fossilisation. Researchers addressed this problem by studying modern sturgeon, whose spinal columns have a similar arrangement.
They measured the stiffness of sturgeon backbones and used the results to model neck movement in six extinct vertebrates, including Tiktaalik, a 375-million-year-old fish closely related to early limbed land animals.
The models suggested that most early tetrapods could move their heads less than six degrees sideways and less than three degrees upwards. They were also incapable of rotating their heads.
Dana Korneisel, the study’s lead author and a postdoctoral researcher at Uppsala University, said the findings were unexpected.
“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,” she said in a statement.
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Necks may have evolved for movement
The findings suggest that separating the skull from the shoulders did not immediately improve head movement.
Instead, the researchers proposed that this anatomical change may have helped early vertebrates use their developing front limbs to support and propel their bodies.
Korneisel explained that the animals eventually gained greater neck flexibility, but this advantage came later.
“Clearly it was advantageous eventually, but it was not an advantage available to these early tetrapods yet because of the limitations of their anatomy,” she noted.
The researchers cautioned that their models estimated movement using the mechanical properties of the backbone. Muscles, skin and other tissues could have further restricted mobility.
Stewart said the research demonstrated how combining fossils with observations of living animals could help reconstruct the behaviour of extinct species: “What’s cool is that many of the changes that were occurring in the skeleton at this time, about 380 million years ago, are still reflected in our own bodies today.” he added.
(Edited by Janaki Pande)