[Latest Research] What Was the Function of Spinosaurus’s Giant “Sail”? Exploring the Mysteries of Thermoregulation and Underwater Locomotion

Among all the dinosaurs discovered to date, “Spinosaurus” is the most unique and has rewritten many of the established rules regarding theropods.
While its features—such as its elongated skull, crocodile-like teeth, and a body adapted to life by the water—are endless, what captures the imagination most is the giant “sail” on its back, reaching up to about 1.8 meters in height.

Why did one of the largest carnivorous dinosaurs in history, which lived in North Africa during the mid-Cretaceous period (about 112 to 93 million years ago) and is said to have reached lengths of 15 to 18 meters, evolve such a flamboyant structure?
In this article, we will explain in detail the true functions (and the functions it was not suited for) of the Spinosaurus sail, which are becoming clear through the latest research involving biomechanical modeling.

Why Was the “Thermoregulation (Radiator)” Theory Rejected?

One of the long-believed hypotheses regarding dinosaur back sails was that they “assisted in thermoregulation, absorbing or releasing heat.”
This was based on an analogy with the synapsid “Dimetrodon” from about 300 million years ago.

However, this hypothesis was overturned by a paper published in the academic journal “Geological Magazine” in 2016.

Few Traces of Blood Vessels

The neural spines that made up the sail were highly ossified and showed almost no traces of blood vessels.

Lack of Blood Flow

Animals that rely on heat exchange at the body surface possess a massive blood flow just beneath the epidermis, but Spinosaurus fossils lacked this feature.

Because a dense network of blood vessels is essential to function as a radiator or heat pump, the theory that the sail was purely a thermoregulatory organ is currently rejected.

The Most Plausible Interpretation: Its Role as a “Display (Ornamentation)”

So far, the most widely supported theory for the sail’s function is that it served as a visual signal to other dinosaurs, namely an “ornamental trait.”

Even today, prominent visual structures are widely seen, such as the flashy plumage of birds, the giant antlers of deer, and the throat fans (dewlaps) of lizards.
These typically evolve for sexual selection or social signaling.

Appealing to the Opposite Sex and Intimidating Rivals

Because it was visible from a distance whether on land or walking in the water, it was useful for conveying attractiveness to potential mates or assessing the strength of rivals.

Avoiding the Risk of Injury

By showing off its dominance, it may have been used in “ritual combat” to settle disputes without the costs of direct physical conflict.

It is argued that structures like sails develop significantly in species with pronounced differences in appearance between males and females, or in species that exhibit complex social behaviors.

A “Rudder” Underwater? The Semi-Aquatic Lifestyle and Hydrodynamics

To solve the mystery of Spinosaurus, one must look at its entire body, not just the sail.
A 2014 paper in “Science” argued that they were not purely terrestrial, but “semi-aquatic” predators living by the water and underwater.

Fossil Traits Indicating a Semi-Aquatic Nature

High-Density Bones

Assisted in buoyancy control, much like modern penguins.

Webbed or Paddle-Like Feet

Specialized for moving on muddy ground or in shallow waters.

A Tall, Flexible Tail

Adapted for side-to-side undulating propulsion.

An Elongated Snout

Ideal for preying on fish.

Given these traits, it is thought that the sail may have had a “hydrodynamic function” in addition to being an ornament.

Posture Control During Lunges (2021 Study)

A 2021 paper (in the academic journal “Life”) compared the Spinosaurus sail to the drop keel of a boat or the “dorsal fin of a sailfish.”
It argued that when lunging toward fish in the shallows of rivers and lakes, the sail helped maintain balance by reducing rolling and pitching.

Drag During Swimming and Walk Stabilization (2022 Study)

On the other hand, a 2022 paper (in the academic journal “eLife”) suggested through biomechanical modeling analysis that “it did not possess high swimming capabilities like whales or marine reptiles.”
This is because if the entire sail was underwater, it would actually create drag and reduce swimming efficiency.
However, even if its hydrodynamic functions were limited, it is highly likely that it helped “ensure stability” when entering or exiting the water, or when making sharp turns while hunting by walking underwater.

Conclusion: A “One-of-a-Kind Adaptation” Born of Evolution

Current researchers generally agree on the view that the reason for the Spinosaurus sail’s existence “cannot be reduced to just a single function.”

• “Ornamentation (visual signaling)” to mates and rivals
• “Stabilization (hydrodynamic function)” to assist hunting and moving in shallow waters

By fulfilling these multiple complementary roles, it was able to become a one-of-a-kind large predator dominating the rivers and lakes during the Cretaceous period.
Modern biomechanics and computer modeling will likely continue to uncover the process by which evolution integrated form and function, transcending categories like “ornamentation” and “aquatic adaptation.”