Brontosaurus: the diplodocid genus restored

Brontosaurus was a large plant-eating diplodocid sauropod of the Late Jurassic. It lived about 155–152 million years ago in what is now the western United States, where its bones occur in the Morrison Formation. It had a massive trunk, a long neck and a very long tail, and was closely related to Apatosaurus.

The genus matters in the history of palaeontology because for more than a century it was treated as a younger synonym of Apatosaurus. A large diplodocid revision in 2015 again recognised Brontosaurus as a separate genus. That classification is widely used today, although the boundary between it and Apatosaurus remains a scientific hypothesis that new fossils and analyses could refine.

Quick facts

Name and discovery

The name combines Greek roots for “thunder” and “lizard”. It is often translated as “thunder lizard”. The name does not report a sound made by the animal. It reflects the impression that enormous fossil bones made on nineteenth-century researchers.

American palaeontologist Othniel Charles Marsh named the type species Brontosaurus excelsus in 1879. Its basis was skeleton YPM 1980, collected by William Harlow Reed at Como Bluff in Wyoming. The specimen was comparatively complete for its time, but it lacked a skull. The species name excelsus means elevated, lofty or distinguished.

In 1881 Marsh described another species, B. amplus. Later researchers considered its distinguishing features insufficient, and the name is now usually treated as a junior synonym of B. excelsus. The major change came in 1903, when Elmer Riggs concluded that Brontosaurus and Apatosaurus were too similar to be kept in separate genera. Because Apatosaurus had been published in 1877, its name had priority, and Brontosaurus excelsus became Apatosaurus excelsus.

That usage continued for most of the twentieth century and into the twenty-first. The word “brontosaur” remained familiar to the public, while scientific works generally treated it as outdated. In 2015, Emanuel Tschopp, Octávio Mateus and Roger Benson compared dozens of diplodocid skeletons using hundreds of anatomical characters. Their analysis found that specimens assigned to Brontosaurus formed a branch distinct enough from Apatosaurus to justify restoring the genus.

Classification and its limits

In modern dinosaur classification, Brontosaurus belongs to Sauropoda, Diplodocoidea, Diplodocidae and Apatosaurinae. Apatosaurus is also an apatosaurine. Both were more heavily built than many diplodocines such as Diplodocus and Barosaurus, but they differed in combinations of proportions and skeletal details.

The distinction does not rest on one striking feature. Researchers compare cervical and dorsal vertebrae, the shoulder blade and details of limb bones, alongside overall proportions. On average, Apatosaurus is reconstructed as heavier and broader through the neck, while Brontosaurus had a somewhat taller, narrower neck. Individual bones can vary with age, individual development and deformation after burial, so one difference alone is not decisive.

The status should therefore be stated precisely. Brontosaurus is not merely a colloquial name for Apatosaurus; current reviews commonly recognise it as a valid genus. Yet palaeontologists do not define a genus by a universal number of differences. The result depends on chosen characters, specimen completeness and the evolutionary tree recovered by an analysis. A new fossil with an intermediate combination could change the present arrangement. Profiles of named taxa are grouped in the dinosaur catalogue.

Species and historical names

The type and best-known species is Brontosaurus excelsus, anchored by YPM 1980. Material originally assigned to B. amplus is generally included with it. Many skeletons once labelled B. excelsus or Apatosaurus excelsus cannot be identified confidently to species after re-examination.

Brontosaurus parvus was described in 1902 as Elosaurus parvus. Its holotype, CM 566, belonged to an immature animal, complicating comparisons because some features changed during growth. Larger postcranial skeletons have also been referred to this species, but the exact sample depends on the phylogenetic scheme being used. Analyses place it among apatosaurines, while its position within Brontosaurus is less stable than that of the type species.

Brontosaurus yahnahpin was first named Apatosaurus yahnahpin in 1994. A separate genus, Eobrontosaurus, was proposed for it in 1998, and the 2015 revision transferred it to Brontosaurus. Its holotype TATE-001 is an informative partial postcranial skeleton. As with B. parvus, its exact position among the lower branches is less certain than its apatosaurine affinity.

Three species are therefore commonly recognised. B. amplus is treated as a synonym of B. excelsus; Elosaurus and Eobrontosaurus reflect earlier classification choices, rather than additional, undisputed brontosaur species.

What the fossils preserve

Knowledge of Brontosaurus comes chiefly from postcranial bones, those below and behind the skull. Different specimens preserve cervical, dorsal, sacral and caudal vertebrae, ribs, parts of the shoulder and pelvis, and bones of the forelimbs and hind limbs. The type YPM 1980 remains central for B. excelsus; YPM 1981 is important in the history of the former B. amplus.

CM 566 is significant as a young B. parvus, because it shows some changes associated with growth. That same immaturity makes it a difficult taxonomic reference. TATE-001 preserves a substantial part of the skeleton of B. yahnahpin. Other specimens have been compared with these species, but not every old museum identification survives modern study.

No complete brontosaur has been found. Museum mounts fill gaps with casts and models and sometimes combine elements from more than one individual. The head deserves particular caution: no skull is securely associated with the type skeleton, so its shape is inferred from close apatosaurines and other diplodocid sauropods. There are also no confirmed skin impressions, colour evidence, calls, nests or parental-care traces attributable specifically to this genus.

Size and body

Adult Brontosaurus probably reached about 18–22 m in length. Large individuals are often estimated at roughly 12–18 tonnes, with some upper estimates approaching 20 tonnes. None of those values is a direct weighing. Results depend on which missing bones are reconstructed, the species identified, the volume assigned to soft tissues and the body density used in the model.

The trunk was massive and the limbs straight and column-like. The forelegs were somewhat shorter than the hind legs, so the back may have risen gently toward the pelvis. Its long neck was built from lightened vertebrae containing complex bony struts and air spaces. Split upper processes on many cervical vertebrae left room for ligaments and muscles. This construction combined strength with reduced skeletal mass.

The tail had a powerful base and narrowed toward a fine tip. It acted as a counterbalance to the neck and helped maintain balance during movement. Using it to strike is mechanically plausible, but the behaviour itself is not observed in the fossil record. The idea that it made a supersonic whip crack depends on models and is not demonstrated by bone damage or trace evidence.

As in related diplodocids, the hand was held nearly vertically and bore one large inner claw. Its function is unknown. Defence, support when raising the body, or interaction with the ground or plants have been discussed, but none can be assigned confidently to Brontosaurus.

Food, habitat and possible behaviour

Brontosaurus lived in landscapes represented by the Morrison Formation: river plains, floodplains, lakeside areas and drier terrain that varied across the region and through time. The climate was seasonal, so the familiar picture of one uniformly warm swamp is misleading. The broader setting is described in the Jurassic Period account.

Herbivory is well supported by its relationships and sauropod anatomy. Without a skull securely tied to the genus, however, we cannot assign every feature of a particular Apatosaurus tooth to Brontosaurus. A small head and front teeth probably helped strip or crop vegetation rather than chew it thoroughly. Ferns, horsetails, seed ferns, cycads and conifer branches were possible foods, but no direct stomach contents are known.

A long neck let the animal cover a broad feeding area without moving its whole body often. How high it normally held its head is disputed: the possible range of motion does not reveal its habitual posture. It should not be portrayed categorically as either a high-canopy specialist or an animal that fed only close to the ground.

The same deposits preserve many other named dinosaurs, including other large sauropods, stegosaurs and predatory theropods. Their coexistence allows questions about partitioning food, but does not show that all those animals lived in the same place at the same moment. Secure evidence is lacking for permanent brontosaur herds, migration, a particular speed, courtship combat or parental care.

Old mistakes and popular myths

The claim “Brontosaurus never existed” confuses the animal with its name. The bones have always been real. What changed was the decision to place them in a separate genus or in Apatosaurus. Since the 2015 revision, Brontosaurus has again been used scientifically, although its status remains a testable classification hypothesis.

Old mounted skeletons sometimes received a high, box-shaped skull resembling that of Camarasaurus. The type skeleton had no skull, and diplodocid anatomy was less well understood at the time. Modern restorations predict a small, low head of the general diplodocid type, but details of the actual brontosaur skull remain unknown.

Another outdated image shows a giant that had to stand in deep swamps because it supposedly could not bear its own weight on land. Its limbs and spine are consistent with terrestrial support. It may have entered water, as many large land vertebrates do, but it did not need water to support its body.

Also unsupported are a permanently dragging tail, a “second brain” in the pelvis and the necessity of swallowing stones to grind food. The enlargement of the spinal canal around the sacrum was not a second brain, and sauropod trackways generally do not show the continuous tail groove expected from a tail dragged on the ground.

Summary

Brontosaurus is a genus of apatosaurine diplodocids from the Late Jurassic of North America, recognised as valid in most current work. Its familiar outline is supported by numerous postcranial bones, while its skull and many details of its appearance are restored from close relatives.

The genus is also a useful example of how classification changes. Researchers commonly recognise B. excelsus, B. parvus and B. yahnahpin, while noting weaknesses in this arrangement: incomplete skeletons, the young age of the B. parvus holotype and the less stable placement of the two additional species.