The question of whether Apatosaurus and Brontosaurus are the same dinosaur has a long history, but the current scientific answer is clear. For over a century, the scientific community considered Brontosaurus to be invalid, a junior synonym of Apatosaurus, meaning the two names referred to the same genus of sauropod dinosaur. However, a large-scale analysis published in 2015 successfully established Brontosaurus as a valid, separate genus. This means that two distinct types of “thunder lizard” walked the earth during the Late Jurassic period.
The Origins of the Names
The confusion began during the “Bone Wars,” a period in the late 19th century when American paleontologists intensely competed to discover and name new dinosaurs. Othniel Charles Marsh first described and named Apatosaurus ajax in 1877, based on incomplete fossil material found in Colorado. The name Apatosaurus means “deceptive lizard,” a reference to the unusual chevron bones in its tail that made the specimen misleading to classify initially. Just two years later, in 1879, Marsh named a second sauropod, Brontosaurus excelsus, from a more complete, but still headless, skeleton found in Wyoming. The name Brontosaurus, meaning “thunder lizard,” was evocative and quickly captured the public’s imagination.
The Century of Synonymy
The consensus that Brontosaurus was the same as Apatosaurus was established in 1903 by paleontologist Elmer Riggs. Riggs re-examined Marsh’s specimens and concluded that the anatomical differences between Apatosaurus ajax and Brontosaurus excelsus were not significant enough to justify two separate genera.
Under the rules of zoological nomenclature, the older name takes precedence, a principle known as priority. Since Apatosaurus was named in 1877, before Brontosaurus in 1879, the genus name Apatosaurus was retained. Consequently, Brontosaurus excelsus was officially reclassified as Apatosaurus excelsus, and Brontosaurus became an invalid junior synonym. This decision was widely adopted, and for the next hundred years, museums and textbooks popularized the idea that Brontosaurus did not exist as a unique dinosaur.
The Scientific Reinstatement
The long-held consensus was challenged by an extensive morphological analysis published in 2015 by Emanuel Tschopp and colleagues. This research involved a detailed, specimen-level comparison of 81 diplodocid sauropods, the family group that includes both Apatosaurus and Brontosaurus. The researchers employed modern statistical and cladistic analysis to rigorously determine evolutionary relationships based on shared physical characteristics. The study found sufficient, consistent anatomical differences between the specimens to warrant separation at the genus level. This degree of difference was comparable to that seen between other closely related genera within the Diplodocidae family, justifying the resurrection of Brontosaurus as a distinct and valid genus.
Key Distinctions Between the Genera
The separation of the two genera is based on multiple subtle but consistent differences in the skeleton, particularly in the neck and tail vertebrae. These small differences in the structure of the vertebral column reflect distinct evolutionary paths within the Diplodocidae family, confirming that Apatosaurus and Brontosaurus represent two unique lineages of long-necked dinosaurs.
Body Structure
Apatosaurus is generally considered a more robustly built dinosaur, with thicker limbs and a deeper chest cavity than Brontosaurus. The spines on the vertebrae in the thoracic region, or back, of Apatosaurus are notably taller and more curved, creating a larger arc.
Neck and Tail
Conversely, Brontosaurus possesses a relatively taller, less elongated neck structure compared to Apatosaurus. Furthermore, the shape of the caudal, or tail, vertebrae also shows variation between the two genera.