Plesiosaur Drag: What Hydrodynamics Can Tell Us About an Ancient Marine Reptile

Did a Long Neck Make Plesiosaurs Slower?

Few prehistoric animals have a body plan as unusual as the plesiosaur. These extinct marine reptiles lived throughout the Mesozoic Era and were characterised by four powerful flippers and, in many species, remarkably long necks.

But an obvious question follows from this unusual anatomy: did having such a long neck make a plesiosaur harder to move through water?

Hydrodynamics, the study of how fluids such as water move around solid bodies provides a way to investigate how plesiosaurs interacted with the water around them. Modern computational fluid dynamics can even be used to create digital models of extinct animals and examine the forces acting on their bodies.

What Is Drag?

When an animal moves through water, the surrounding fluid creates resistance. This resistance is known as hydrodynamic drag.

Drag is influenced by factors including body shape, surface area, speed, and the way water flows around the animal. For a marine reptile, reducing unnecessary drag can potentially make forward swimming more efficient.

At first glance, the extremely long neck of some plesiosaurs might appear to be a major source of resistance. A longer structure means more surface area interacting with the surrounding water. But appearance alone does not tell us how an animal actually behaves in a fluid.

Testing the Plesiosaur Neck

A 2019 study published in the Journal of Vertebrate Paleontology investigated this question using computational fluid dynamics. Researchers created three dimensional digital models representing plesiosaurs with different neck lengths and thicknesses and simulated water flowing around them.

The models included different neck-to-body proportions, allowing the researchers to compare how neck morphology influenced hydrodynamic forces during forward motion.

The results were surprising. Within the conditions tested, increasing the length of the plesiosaur neck did not noticeably change the drag force experienced during forward swimming. The study examined simulated swimming speeds from approximately 1 to 10 metres per second.

This suggests that simply having a longer neck was not necessarily a major hydrodynamic disadvantage.

Why Neck Thickness Matters

Neck thickness produced a different result.

The researchers found that the thicker-necked models experienced less drag than the thinner-necked models. This demonstrates that hydrodynamic performance depends not simply on how long a structure is, but on its overall three-dimensional shape and how water flows around it.

This is an important point when reconstructing extinct marine animals. A long neck may look inherently inefficient from a modern perspective, but its actual interaction with water depends on its proportions, shape, orientation, and movement.

What Happens When the Neck Bends?

Forward swimming with a relatively straight neck is only one possible scenario. The study also examined what happened when plesiosaur necks were bent laterally. When the neck was curved sideways, more of its surface became exposed directly to the incoming flow. The result was an increase in drag, with the effect becoming particularly noticeable in the longest-necked models.

This raises an interesting distinction between straight line swimming and manoeuvring. A plesiosaur moving forward with its neck relatively aligned with the direction of travel could experience different hydrodynamic forces from an animal turning, bending its neck, or positioning its head at an angle.

Four Flippers and an Unusual Swimming Style

The neck was only one part of the plesiosaur's extraordinary anatomy. Plesiosaurs possessed two pairs of large flippers, an arrangement that has attracted considerable research into how these animals generated thrust and controlled their movement. Experimental work using reconstructed plesiosaur flippers has shown that the interaction between the fore and hind flippers could contribute substantially to propulsion and efficiency.

This means plesiosaur locomotion cannot be understood by looking at the neck in isolation.

The entire body, including the trunk, neck, and four flippers, interacted with the surrounding water. More recent computational research has also shown that plesiosaur limbs could contribute significantly to overall drag, illustrating how unusual their body plan was compared with other streamlined marine vertebrates.

Did Hydrodynamics Limit the Evolution of Long-Necked Plesiosaurs?

This is where the subject becomes especially interesting. If long necks had created a severe drag penalty during forward swimming, hydrodynamic resistance might have acted as a strong constraint on the evolution of increasingly elongated necks.

However, the 2019 CFD study found that neck elongation itself did not noticeably alter drag in its forward-swimming models. The authors therefore concluded that, under the conditions they tested, hydrodynamic effects were unlikely to have been the primary limiting factor behind the evolution of long necks in plesiosaurs.

That does not mean long-necked plesiosaurs were completely unaffected by hydrodynamics. Rather, it suggests that the relationship between neck length and swimming performance was more complicated than simply longer neck = more drag.

Reconstructing a Prehistoric Swimmer

Fossil bones tell us what a plesiosaur's skeleton looked like, but they cannot directly show us how water moved around the living animal.

This is where computational paleontology becomes valuable. By combining fossil anatomy with engineering principles, fluid dynamics, digital reconstruction, and computer simulations, researchers can test different hypotheses about how extinct animals moved.

These models do not provide a perfect recording of prehistoric behavior. Instead, they allow scientists to ask measurable physical questions: How much drag would a particular shape produce? How does changing neck length affect water flow? What happens when the neck bends? How might different body proportions influence swimming?

The Hydrodynamics of an Ancient Marine Reptile

The long neck of a plesiosaur may look like an obvious hydrodynamic problem, but physics tells a more interesting story. Research indicates that neck length alone did not necessarily create a substantial drag penalty during forward swimming under the conditions modeled. Neck thickness and orientation could have a greater influence on resistance, while bending the neck could increase drag.

The result is a fascinating example of how modern science can bring extinct animals back to life not literally, but through physics, biomechanics, and computer modeling. A plesiosaur fossil is a record of bones preserved in stone. Hydrodynamics allows those bones to become something more: evidence from which we can investigate how an extraordinary marine reptile may once have moved through the ancient oceans.