The Brontosaurus, a massive Apatosaurine sauropod from the Late Jurassic period, has long captured the imagination, but determining its locomotion speed is a complex scientific challenge. Unlike living animals, a dinosaur’s movement must be inferred from its fossilized remains and the traces it left behind. The physical limitations imposed by its immense size suggest a slow-moving giant, requiring sophisticated biomechanical analysis. Paleontologists use skeletal reconstruction and trackway evidence to determine how fast this “thunder lizard” could move.
Anatomy and Mass: The Physical Parameters of Locomotion
The sheer scale of the Brontosaurus dictated its walking style and speed, acting as the primary constraint on its mobility. An adult Brontosaurus excelsus reached lengths of 21 to 23 meters, with a hip height of approximately 8.5 meters. Weight estimates typically fall within the range of 15 to 30 metric tonnes.
This tremendous mass was supported by four sturdy, graviportal limbs, similar to modern elephants but on a much larger scale. The legs were positioned in a pillar-like structure directly beneath the body, maximizing weight distribution and support. This robust, quadrupedal build was adapted for carrying immense weight, not for achieving rapid acceleration or high velocities.
How Paleontologists Calculate Sauropod Speed
To estimate a dinosaur’s walking speed, paleontologists turn to the fossilized record of its movement, known as ichnology, or the study of trace fossils. The most direct evidence comes from analyzing trackways, where a sequence of footprints provides a record of the animal’s pace and stride. The first step involves measuring the length of the stride, which is the distance between two successive imprints made by the same foot.
To incorporate the animal’s size into the calculation, researchers must estimate the trackmaker’s hip height, or leg length. A common method used for quadrupedal dinosaurs is to estimate hip height as four to five times the length of the rear footprint. These two values—stride length and hip height—are then used in a mathematical relationship developed by R. McNeill Alexander.
This relationship, often referred to as Alexander’s formula, calculates a dimensionless speed, or Froude number, by comparing the stride length to the animal’s hip height. A Froude number of 1.0 generally separates a walk from a run in modern animals. By incorporating the known gravitational constant, researchers convert the trackway measurements into an estimated speed in meters per second. This technique assumes dynamic similarity, allowing for a reasonable speed estimate for an extinct creature.
Cruising Pace Versus Maximum Speed Estimates
The speeds calculated from trackways of giant sauropods overwhelmingly suggest a habitually slow, deliberate gait. The sustained, energy-efficient pace of a Brontosaurus, known as its cruising speed, is generally estimated to have been quite low. This typical walking speed, derived from the most common trackway evidence, is often placed in the range of 1 to 2 miles per hour (about 1.6 to 3.2 kilometers per hour). This velocity is comparable to a slow human walk, reflecting an efficient movement for a herbivore that needed to conserve energy while foraging.
Trackways rarely show evidence of a true running gait for the largest sauropods, which would involve a period where all four feet were off the ground simultaneously. The theoretical maximum speed is highly debated and is a product of biomechanical modeling rather than direct evidence. Estimates for this maximum speed tend to be cautious, with some models suggesting a limit around 4.5 miles per hour (about 7 kilometers per hour). This higher estimate represents the absolute upper limit the animal could manage before its locomotion became physically impossible or hazardous. The lack of faster trackways suggests that any speed beyond a brisk walk was unsustainable for the Brontosaurus.
The Physical Limits of High-Speed Movement
The reason Brontosaurus was restricted to such low speeds lies in the fundamental physics of large bodies, specifically the square-cube law. This law explains that as an animal increases in size, its volume and mass increase much faster than the cross-sectional area of its bones. If the length of a bone doubles, its mass increases by a factor of eight, but its load-bearing area only increases by a factor of four.
The Brontosaurus was already near the upper limit of what its skeletal structure could support, meaning its bones were constantly under immense stress. Attempting to move at high speed would have exponentially increased the forces exerted on its limb bones. Furthermore, the risk of a fall was a profound deterrent to rapid movement. A stumble or fall at speed would have resulted in an impact force so immense that it would likely cause catastrophic skeletal injury or be instantly lethal. For the Brontosaurus, slow movement was a necessary adaptation for survival and structural integrity.