How Big Were Brachiosaurus? Size, Weight, and Height

Brachiosaurus was one of the longest and tallest animals ever to walk on land, a long-necked plant-eater that lived in western North America about 150 million years ago. It was roughly 18 to 22 meters (59 to 72 ft) long and stood about 9 to 13 meters (30 to 43 ft) tall, with most modern weight estimates for the best-known skeleton between about 28 and 47 metric tons. Its exact size is less settled than museum signs suggest, because the main skeleton is incomplete and scientists estimate weight in several different ways. That skeleton was probably not fully grown, so adults may have been larger, while older figures of 70 tons or more are now considered too high. And the giant mounted skeleton in Berlin, the one most people picture as Brachiosaurus, turned out to be a separate animal from Africa.1Journal of Vertebrate Paleontology. A Re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and Its Generic Separation from Giraffatitan brancai (Janensch 1914)

Brachiosaurus Size at a Glance

The height range is wide because height depends mostly on how upright the neck was held, not on the length of any bone. A neck held forward gives the lower figure, and a raised neck gives the higher one. Whether the neck really went up that far is debated.

Length is the steadier number, since the neck’s pose doesn’t change it. It still has a spread, because some of the tail and neck had to be inferred from related animals.

The weight span needs careful reading too. The 28 to 47 ton range is not a margin of error around one measurement. It reflects different modeling choices applied to the same skeleton, so no single figure inside it is more correct than another. The paired figures in the list are the same weights in two units, metric tons and short tons, not two separate estimates. The upper arm bone is a real object, and bone measurements are firmer than weight or height figures, which depend on assumptions.

All of these numbers come from one animal. The original skeleton, found in Colorado in 1900 and named in 1903, is the specimen the species is defined by. That animal was probably not fully grown, because two shoulder bones had not yet fused together, which happens with maturity.6Journal of Vertebrate Paleontology. A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914)

Why Weight Estimates Vary So Much

One approach looks at bone thickness. In living land animals, the thickness around the middle of the upper arm and thigh bones closely tracks body weight.7Journal of Zoology. Long‐bone circumference and weight in mammals, birds and dinosaurs Measuring those bones in a fossil gives a weight estimate even when the rest of the skeleton is missing.8Journal of Zoology. Long‐bone circumference and weight in mammals, birds and dinosaurs The catch is that this method gives lighter weights for giant sauropods like Brachiosaurus than earlier estimates did.9Journal of Zoology. Long‐bone circumference and weight in mammals, birds and dinosaurs

Another approach laser-scans a mounted skeleton and stretches a virtual skin tightly over it to measure its volume.10Discover Magazine. How to weigh dinosaurs with lasers This gave the Berlin skeleton about 23.2 tons.11Discover Magazine. How to weigh dinosaurs with lasers But a tight skin models a starved, unrealistically thin animal, so the true weight was higher.12Discover Magazine. How to weigh dinosaurs with lasers That figure is best read as a floor, because real animals have muscle, fat and skin that stand out from the bone. Combining a floor from volume modeling with a bone-thickness estimate is why modern figures are given as ranges instead of one exact weight.

Older figures ran much higher. One older estimate for the Berlin skeleton reached about 63 tons.13Pensoft (Mitt. Mus. Nat. kd. Berl., Geowiss. Reihe). Posture and Mechanics of the Forelimbs of Brachiosaurus brancai (Dinosauria: Sauropoda) Another, based on inner-ear canal size, suggested around 75 tons.14PubMed Central. Biology of the sauropod dinosaurs: the evolution of gigantism Yet the inner-ear canals pointed to different weights depending on which canal was measured, which shows how uncertain such methods are.15PubMed Central. Biology of the sauropod dinosaurs: the evolution of gigantism

Older methods often built scale models, submerged them in water to measure volume, padded the body generously with muscle and fat, and didn’t account for the air spaces inside the skeleton. The totals ran high as a result.

Those air spaces matter. Like birds, sauropods had air sacs that hollowed out parts of their backbones.16Paleobiology. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs Their necks weighed less than solid bone would have, which helped very long necks evolve.17Paleobiology. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs Estimates that assume solid, dense bodies tend to overshoot.

Skin, fat and muscle don’t fossilize, so modelers must decide how thick to make the body around the skeleton. On an animal this big, small changes in that thickness add many tons.

So the 28 to 47 ton range, which reflects modern methods, is the safest figure to quote. No method weighs muscle and organs directly, so every number is a careful guess.

The Shape That Made It So Tall

Brachiosaurus had upper arm and thigh bones of about the same length.18Wikipedia. Brachiosaurus Its forearm and hand bones, though, were long, so the whole front leg was longer than the back leg.19Wikipedia. Brachiosaurus Its back sloped upward, with shoulders much higher than hips and the neck leaving the body at a steep angle.20Wikipedia. Brachiosaurus It also carried a larger share of its weight on its front legs than its African cousin did.21Journal of Vertebrate Paleontology. A Re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and Its Generic Separation from Giraffatitan brancai (Janensch 1914)

That sets it apart from other long-necked dinosaurs. Diplodocus had shorter front legs and a near-level back, built more like a suspension bridge. Brachiosaurus was built more like a giraffe, sloping up toward the head. The name Brachiosaurus means “arm lizard,” after those long front limbs. The raised shoulders gave extra height before the neck even began, so the head could reach upward without an impossibly long neck.

The Berlin Skeleton Is a Different Dinosaur

The tall skeleton in Berlin’s natural history museum comes from Tendaguru in Tanzania. It has been placed in its own genus, Giraffatitan, because of many differences from the American Brachiosaurus.22PubMed Central. Biology of the sauropod dinosaurs: the evolution of gigantism The American animal had a longer, deeper body and probably a longer, taller tail.23Journal of Vertebrate Paleontology. A Re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and Its Generic Separation from Giraffatitan brancai (Janensch 1914) It was overall a larger animal than the main Giraffatitan specimen.24Journal of Vertebrate Paleontology. A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914)

The two are close cousins and looked broadly similar, so the Berlin skeleton was first assigned to the same genus and carried the name Brachiosaurus for decades. Only later, when researchers compared the bones in detail, did the differences justify a new genus.

This confuses size figures online. Many older books and sites quote Berlin skeleton measurements as Brachiosaurus, and because the Berlin mount is far more complete, it was used to fill gaps in the American animal’s picture. Some familiar record figures, such as the height of the Berlin mount, describe Giraffatitan and not the American animal. When reading a size, look for whether the source says Colorado, Tanzania, or the Berlin skeleton, so you know which animal is meant.

Could Some Have Been Even Bigger

Possibly. The longest known sauropod upper arm bones are about 2.13 meters, from Brachiosaurus and Giraffatitan finds.25Sauropod Vertebra Picture of the Week. At last it can be told: we found a big Brachiosaurus in the Salt Wash A Brachiosaurus upper arm bone from Utah measures 2.01 meters.26Sauropod Vertebra Picture of the Week. At last it can be told: we found a big Brachiosaurus in the Salt Wash Another Colorado find also reaches about 2.13 meters.27Sauropod Vertebra Picture of the Week. At last it can be told: we found a big Brachiosaurus in the Salt Wash Several individuals, then, reached at least the size of the original, which itself was still growing. Several bones near the same size suggest big size was not a fluke of the original skeleton.

A single bone is hard to scale up, though. It shows an animal was big but not exactly how heavy, since proportions differ between individuals and ages. Weight also depends on how deep and wide the body was, which a limb bone doesn’t record. Claims of bigger individuals based on single bones are best treated as hints about what is possible.

Did It Really Hold Its Neck Straight Up

Modeling of the Berlin animal’s neck found that strain on its joints stayed even along most of the neck when held upright.28Mitteilungen aus dem Museum für Naturkunde in Berlin. Geowissenschaftliche Reihe. Neck posture and overall body design in sauropods With the neck tilted forward or level, that strain rose three to four times.29Mitteilungen aus dem Museum für Naturkunde in Berlin. Geowissenschaftliche Reihe. Neck posture and overall body design in sauropods That study concluded those lower postures were not used often.30Mitteilungen aus dem Museum für Naturkunde in Berlin. Geowissenschaftliche Reihe. Neck posture and overall body design in sauropods

But the strain study asks only whether the bones and joints could comfortably hold a posture, not whether the heart could supply the head in that posture. A neck that is mechanically comfortable upright can still be unworkable for the circulatory system, so the two kinds of study answer different questions.

The heart side is demanding. Pushing blood up to a head many meters above the heart needs very high pressure.31Nature. Haemodynamics in the Sauropoda One estimate found a vertical neck would have used about half the animal’s energy just to pump blood.32PubMed Central. Raising the sauropod neck: it costs more to get less That would leave little for moving, growing and finding food.

Changing head height adds another burden. Quick changes in blood pressure would be needed whenever the animal lowered or raised its head while feeding.33Nature. Haemodynamics in the Sauropoda

Giraffes show that high pressure is possible. They have unusually high blood pressure to reach a head about 2 meters above the heart, and special valves that stop blood rushing to the head when drinking. But Brachiosaurus’s head could sit several times higher. A middle answer is possible: it may have raised its neck to feed but rested it lower at other times.

What It Ate and Where Its Size Came From

Brachiosaurus was likely one of the only Jurassic sauropods clearly able to feed high in conifer-like trees.34The SauropodsEvolution and Paleobiology. Digital Reconstructions of Sauropod Dinosaurs and Implications for Feeding Reaching high foliage gave it food that smaller plant-eaters couldn’t use, which may have reduced direct competition. Yet chemical traces in teeth suggest neighboring sauropods split food mainly by plant type rather than by height.35University of Maryland Department of Geology. A Stable Isotopic Investigation of Resource Partitioning Among Neosauropod Dinosaurs of the Upper Jurassic Morrison Formation So height was an advantage but not the only way of sharing.

Energy was a limit. One calculation found a fully warm-blooded body above about 55 tons could not have eaten enough low-quality Jurassic plants.36Paleobiology. The improbable endotherm: the energetics of the sauropod dinosaur Brachiosaurus Its metabolism was probably slower than that of a mammal of its size. A body that ran cooler needs less food per ton, so a giant could survive on plants low in nutrition. That doesn’t mean it was sluggish; its energy budget sat between that of a reptile and a mammal.

Growth was fast. Bone structure shows Tendaguru sauropods grew about as fast as large modern mammals.37Fossil Record. Life history of Tendaguru sauropods as inferred from long bone histology They kept growing after reaching breeding age, but did stop eventually.38Fossil Record. Life history of Tendaguru sauropods as inferred from long bone histology The African brachiosaur could breed at about 40% of its maximum size,39Paleobiology. Longbone histology of the Tendaguru sauropods: implications for growth and biology and a related Tendaguru sauropod reached full size at 26 or more years.40Paleobiology. Longbone histology of the Tendaguru sauropods: implications for growth and biology Full size still took decades, so a breeding-age skeleton can be well short of adult size.