UNIVERSITY PARK, Pa. –– The first animals to venture out of the water and onto land wouldn’t have been able to turn their heads and look around much, according to a new study on the evolution of necks.
A model developed in collaboration with Penn State scientists using new fossil evidence and biomechanical studies in a modern fish suggests the earliest land animals may have had stiff necks, not the flexible necks that scientists have long believed. The results call into question a long-standing hypothesis about the evolution of a distinct neck in animals that first moved onto land some 380 million years ago, the researchers explained. The neck did not evolve to allow animals to move their heads more freely, but rather it likely evolved due to changes in the anatomy of these animals related to using their front limbs for locomotion, according to the new study.
The study, by an international team of scientists including Penn State Assistant Professor of Biology Tom Stewart, was published last week (Oct. 14) in the journal Nature .
“This is a really interesting time in Earth's history where some of the first forests are forming, and there's a really radical reorganization of the Earth,” Stewart said. “So, understanding exactly when our lineage made that transition from water to land, how it was achieved, and which anatomical systems were changing is an important part of that story. 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.”
Scientists have thought for decades that the necks of the earliest ancestors of four-limbed animals — tetrapods — became more mobile early on in their transition to land. The idea was that in shallow water or on land, these animals might need to be able to look down or from side-to-side to find food and avoid predators more than in deep water. However, Stewart said, reconstructing the function of the neck in these animals is difficult from the fossil record alone.
“The fish that were undergoing these changes had a vertebral column that was largely made up of soft tissue, which, unlike bone, is rarely preserved in the fossil record,” Stewart said. “In these cases, we can look to modern animals to serve as analogues for the past, like how living birds can teach us about now extinct dinosaurs.”
Fish don’t have necks; instead, their shoulder blades attach directly to their skull. If they need to look to the right or left, they must turn their entire bodies. But some fossil fish from the Devonian Period, between about 420 and 360 million years ago, have been found with a mix of traits — some purely aquatic and some appearing more like animals that live on land. For example, some have more robust fins that could potentially bear the animal’s weight and allow them to prop themselves up or push along the bottom through shallow water. These transitional fossil fish also, for the first time , start to have distinct necks.
“Tiktaalik is one of these fossil fish, and it was first described in 2006,” Stewart said. “It has been reasonably assumed for decades that this separation of head and shoulders 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, at least until now.”
Stewart explained that the fossils come from Ellesmere Island in Nunavut, Canada, which is the northern Arctic.
“The materials have been really important because Tiktaalik is one of the closest relatives to limbed vertebrates,” Stewart said. “It helps us to reconstruct a lot of intermediary steps in the transition to life on land, such as the move from suction feeding in water to biting on land or from swimming to walking.”
But the structure of early tetrapods’ backbones is unusual compared to most vertebrates today, making it hard to model how they moved, he said. They have a long rod of tissue called the notochord 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 — sometimes called a living fossil because of its primitive features — that has the same kind of backbone. By applying the material properties of sturgeon notochord to fossil animals, researchers have found that early tetrapod necks were surprisingly stiff and, even if their shoulders were separated from the head, the necks would not have allowed much mobility.
“We know that tetrapods did eventually gain higher neck mobility,” said Dana Korneisel, a postdoctoral researcher at Uppsala University in Sweden and first author of the paper. “Clearly it was advantageous eventually, but it was not an advantage available to these early tetrapods yet because of the limitations of their anatomy.”
The researchers tested the stiffness of the notochord of modern sturgeon and used the results to model the mobility of the neck in early tetrapods like Tiktaalik.
“I undertook this study to find out how big of a difference this change in anatomy made for these animals,” Korneisel said. “The vertebral columns of sturgeon are built a lot like those of early tetrapods, but they didn’t seem terribly stiff. So, after doing the modeling, the lack of significant head mobility for almost all of these animals was a huge surprise.”
The research team emphasized that their estimates are based on just the backbone, so actual mobility may be even less than approximated. The tissues that surround the backbone-like muscles, skin and, for many of these animals, bones embedded in the skin, may have further reduced the neck’s mobility.
“Head elevation in these animals was no more than what you would expect in fish and, sometimes, even less,” Korneisel said. “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.”
Notochordal vertebral columns are rare today, but they were common throughout life’s history on Earth.
“We want to understand how these early tetrapods were living and how that is reflected in changes to their bodies,” Stewart said. “Trying to reconstruct where an animal lived, how it moved, what its behavior was, is a really difficult thing. The cool part of this story is that it combines data from fossils and studies of living animals in a mechanical, functional perspective to reconstruct what the changes we see in the animal’s anatomy mean for their behavior.”
In addition to Stewart and Korneisel, the research team included Cassandra M. Donatelli at the University of Washington; Linfang Han, Marcos Martinez, Tetsuto Miyashita and Emily M. Standen at the University of Ottawa in Canada; and Andrew L. Traynor, Benjamin K. A. Otoo and Hillary C. Maddin at Carleton University in Canada.
The research was supported by The Steven Cohen Award for Student Research presented by the Society of Vertebrate Paleontology and the Natural Sciences and Engineering Research Council of Canada (RGPIN-06442). This content is the sole responsibility of the authors and does not necessarily represent the views of the funders.
Nature
Early tetrapod neck mobility modelled using an extant analogue
14-Oct-2026