The idea of dinosaurs possessing a second brain is a persistent piece of popular culture, often attributed to the massive armored Stegosaurus. This concept suggests that the sheer size of some prehistoric creatures necessitated an additional control center beyond the small brain in their skull. While this notion captures the imagination, the scientific reality is that no dinosaur ever possessed two brains. Like all vertebrates, dinosaurs had a single, complex brain housed within their cranium, but the anatomical features that sparked this misconception are based on a real, though misunderstood, structure in their spinal column.
The Origin of the “Two Brains” Myth
The origins of the “second brain” idea date back to the late 19th century, when paleontology was establishing itself as a science. Early researchers, particularly Othniel Charles Marsh, were examining the fossilized remains of large, plant-eating dinosaurs. The discovery that the actual brain cavity of these immense creatures was surprisingly small—sometimes no larger than a walnut—led to confusion about how they coordinated their massive bodies.
Marsh observed a substantial enlargement in the neural canal, the passage for the spinal cord, located in the hip region of the Stegosaurus vertebrae. This cavity was considerably larger than the cranial cavity that housed the actual brain. In 1881, he described this expanded space as a “posterior braincase,” speculating it might contain an organ that helped coordinate the hind limbs and tail.
The speculation suggested this large, posterior cavity housed a neural mass that could act as a junction box, speeding up signals or allowing for “a posteriori” reasoning to manage the dinosaur’s bulk. Although Marsh and his contemporaries eventually abandoned the hypothesis, the dramatic image of a dinosaur with a second brain had already taken hold in popular media and scientific literature. The striking contrast between the tiny head brain and the enormous hip cavity ensured the myth’s longevity.
Dinosaur Neuroanatomy Versus the Neural Canal
To understand the reality behind the myth, it is necessary to distinguish between a brain and the structures housed within the vertebral column. A true brain is a complex organ of specialized neural tissue, protected by the cranium, serving as the central command center for the entire nervous system. The neural canal, by contrast, is the hollow space running through the vertebrae that protects the spinal cord and its associated nerve bundles.
The enlarged cavity discovered in the hip region is known as the sacral enlargement. This feature is present in many vertebrates, including humans and birds, where the spinal cord swells to accommodate an increased density of nerve cells. In large dinosaurs, particularly the sauropods and stegosaurs, this sacral enlargement was exceptionally pronounced, resulting in a vertebral canal that was significantly wider in the pelvis than in the skull.
This expanded area contained the sacral plexus, a dense network of peripheral nerves branching off the spinal cord. The sacral plexus is responsible for gathering sensory information and sending motor commands to the massive hind limbs and tail. Though this bundle of nerves was large, it lacked the complex, layered structure and integrative function of a true brain.
The Function of the Sacral Enlargement
The biological purpose of the sacral enlargement was to manage the intensive nervous system requirements of the dinosaur’s posterior body. The hind limbs and tail of large dinosaurs represented a substantial portion of their total mass, requiring a dedicated neural hub for coordination and movement. This large concentration of nerve cells facilitated rapid, localized reflexes that could operate without delay from the distant cranial brain.
The enlarged lumbosacral plexus allowed for a reflex arc, where sensory input from the hindquarters could be processed and motor responses initiated quickly at the spinal level. This capability was essential for coordinating the powerful strides of the rear legs and for the defensive or balance movements of a long, heavy tail. For example, a sudden threat detected near the tail could be met with an immediate, forceful tail swing.
Beyond nerve tissue, a prominent hypothesis suggests that the sacral enlargement may have housed a glycogen body. This organ is found in modern birds, the closest living relatives of dinosaurs, and consists of a mass of glycogen, a storage form of energy. Its presence in a similar location suggests that the dinosaurian sacral cavity may have served a metabolic role, possibly providing an energy source directly to the surrounding dense nervous tissue.