Exhibit Spotlight: Brachiosaurus altithorax
Easy. Breezy. Beautiful. Brachiosaurus.
Steven Spielberg did well in using this iconic giant to make the first full appearance of a dinosaur in Jurassic Park one of the most memorable scenes in cinema. It might no longer count as the largest dinosaur ever found, but its distinctive profile and still impressive size never fail to impress. Stand next to the Brachiosaurus sculpture in our park and you'll see what I mean. As arresting as this animal can be physically, the science and stories behind it prove just as fascinating:
The name Brachiosaurus means “arm lizard” and refers to the animal’s odd backward construction relative to other sauropods. Most sauropods' back legs grew longer than their arms, but Brachiosaurus turns that pattern around. As a result, the spinal column tilts upward, angling the neck higher than other sauropods. The Brachiosaurus skeleton mounted at the Museum of Ancient Life in Lehi, UT, had to have its neck remounted in a downward-bent position because the engineers couldn’t fit the skeleton into a space with a 60 foot high ceiling! This thing isn’t just huge, it’s also plainly tall. Tooth wear, neck articulation, and other lines of evidence converge to show this animal’s design made it a browser, not unlike a giraffe in some ways.
It’s funny how some of the most popular dinosaurs are based on very little fossil evidence. Since Elmer Riggs described the first Brachiosaurus altithorax fossils in 1903, no one has yet found a better skeleton. The first one didn’t have that much going for it, though: it definitely established the animal’s bizarre proportions and enormous size, but that’s about it. Unfortunately, this isn’t unusual for giant sauropods; their bones don't stand up well to fossilization and erosion processes, possibly due to their sheer size. Several discoveries of Brachiosaurus skeletal parts suggest that more bones from those individuals did become fossils, but erosion had torn them apart before anyone discovered them. The bigger the dinosaur skeleton, the more chances nature has to break it down.
Most of Brachiosaurus’ popular image comes from an African cousin once considered a different species of the same genus, called Brachiosaurus brancai. More recent analyses have split this genus in two: the African skeletons, now called Giraffatitan brancai, had more slender proportions, shorter tails, shorter and shallower torsos, and possibly taller heads with more prominent profiles. Comparing their skeletons in detail makes Brachiosaurus look longer and lower than Giraffatitan, even though Brachiosaurus grew somewhat larger.
I say "possibly taller heads" because scientists aren't sure if we have recovered a Brachiosaurus skull yet. One of O.C. Marsh's expeditions in Colorado uncovered a fossil sauropod skull in 1883. Informally, he regarded it as a Brontosaurus skull but never got around to making the case officially. In 1998, Ken Carpenter and Virginia Tidwell compared the skull to other sauropod species of the time and argued that Brachiosaurus made the best candidate. However, they could not rule out the possibility that the skull represents a new species. Time will tell if their hypothesis is correct, but it may take a while: Brachiosaurus fossils of any kind are extremely rare, and sauropod skulls are even rarer.
Brachiosaurus fossils occur in the famous Morrison formation along with other giants of the Jurassic like Apatosaurus, Brontosaurus, and Camarasaurus. Considering that the Morrison formation preserves floodplains more than forests, this might explain why fossils of gigantic browsers don’t show up too much. What fossils we do get may have tumbled down rivers or represent animals that got lost. The heyday of the Morrison might also represent the Brachiosaurus’ decline, since they only occur in the lower, older levels of the formation. Then again, giraffes manage a browsing diet just fine on the African savannah, so Brachiosaurus may have just filled a pretty exclusive niche. All these factors may combine to explain Brachiosaurus' relative rarity.
Let's take a closer look at that dramatic scene in Jurassic Park where the Brachiosaurus rears up on its hind legs. Could Brachiosaurus pull off that kind of movement? Would it make sense for Brachiosaurus to even try? Well, for one thing, consider how many calories it would take for the animal to do that, then consider how many calories that wee sprig of leaves it bites off might provide. Does the benefit outweigh the cost? And how much height does the animal actually gain from standing on its hind legs in that scene? Like, 2 feet? In 2011, Heinrich Mallison tested this very scene together with other plausible models of a rearing Brachiosaurus and found that while several different kinds of sauropod were indeed physically capable of performing this stunt, Brachiosaurus would have been dangerously unstable on its hind legs. No motive, and no means, so not realistic, but visually impressive nonetheless.
Our own Brachiosaurus sculpture, shown above, got a facelift last summer. Through some miscommunication or other, the full size original sculpture ended up with its eyes where its nose should go. While its remodeled head and spiffy new paint job have improved its overall look, it still follows the old convention of using the brancai species as a model. We opted not to re-label it as Giraffatitan because it doesn't exactly represent that genus either. It represents old ideas about Brachiosaurus, and continuing to call it by that name gives us opportunities to tell the story of how scientists have studied it over the past century. In addition to the new look, you may notice a fringe of wires along its neck and back. Those don't represent quills or protofeathers—fossil evidence does not support sauropods bearing feathers. Instead, those are there to prevent birds from adding their own, ahem, touch-ups to the new paint job.
Riggs, E. S. (1903). ART. XXX.--Brachiosaurus altithorax, the largest known Dinosaur. American
Journal of Science (1880-1910), 15(88), 299.