Brachiosaurus: Beast of the Week

 It's time to check out one of the most famous giant dinosaurs.  Make way for Brachiosaurus altithorax!

Brachiosaurus was a huge plant-eating sauropod dinosaur that lived in what is now Western United States during the late Jurassic period, about 153 million years ago.  It's genus name, Brachiosaurus, translates to "Arm Lizard/Reptile" because its arms were longer than its legs, and its species name, altithorax, translates to "Deep Chest" because of its particularly tall ribcage.  From snout to tail an adult Brachiosaurus is estimated to have been between 60 and 70 feet long and its head was held about 30 feet off the ground when standing.  

Brachiosaurus altithorax watercolor life reconstruction by Christopher DiPiazza.

Brachiosaurus was originally only known from a few bones, including the pelvis, some vertebra, and limb bones.  Over the years, however, more fossils from this dinosaur have been unearthed from multiple individuals that altogether give us a relatively complete skeleton. Sauropod skeletons are notorious for being found incomplete because they're so large and therefore their bones were more prone to getting eroded, carried away by predators, or scattered after death.  Fossilization most commonly happens when the animal's body is buried rapidly and completely after death.  Therefore it's takes a lot more forces of nature for this to happen to a large animal than a small one.

Brachiosaurus, as stated above, is famous for having longer arms than legs, which makes its family, brachiosauridae, unique among dinosaurs.  In addition, its arms were placed farther below its spine than its legs, causing it to be even taller in the front.  This unusual posture allowed Brachiosaurus to reach especially high with its already long neck to take advantage of leaves on the tops of trees that even other sauropods couldn't easily reach.  Unlike other sauropods, brachiosaurids had proportionally shorter back legs, and tail which were held relatively low because of this sloping build.  Having shorter back limbs also would have highly limited Brachiosaurus' ability to rear up on its hind legs, which other kinds sauropods, like the diplodocids, likely were able to do more easily.  Despite what that iconic scene in Jurassic Park* may show you, real brachiosaurs likely very rarely stood on just two limbs, and if/when they did, only for extremely short periods of time, like to mate or possibly intraspecies combat.  Since Brachiosaurus' center of gravity was in its torso, not its hips, that kind of posture would put extreme pressure on its back end which could seriously injure the dinosaur.  All things considered, if an animal could reach thirty feet in the air from a standing position...rearing up probably wasn't necessary often.

Brachiosaurus' neck bones were relatively rigid closer to the skull, but the part of the neck closest to the shoulders would have had more flexibility.  This means that despite its very front-loaded posture, it could still bend its neck so its head was close to the ground if it need be.  This would have been necessary when Brachiosaurus needed to drink, or maybe even eat certain kinds of low-growing plants at times.  It wouldn't have needed to bend or spread its front limbs in order to achieve this either.  The neck bones, along with many of its other bones, were hollow and would have been filled with air sacs in life.  Some of the spaces around the bones (which can be inferred by the negative space around the edges of the vertebra) was also filled with air sacs in life.  These structures were connected to the animal's respiratory system and allowed the body to be more efficient in distributing oxygenated air to where it needed to go.  This also made the animal much lighter for its size, so moving that neck around wouldn't have drained as much energy as it would have appeared.  

Brachiosaurus could have lowered its head down to ground-level relatively easily when it wanted to.

The skull of Brachiosaurus was found and published in 1998.  It had a relatively long snout with chisel-shaped teeth concentrated to the front of the mouth, an adaptation for stripping leaves off branches.  It had no teeth in the back of the jaws, implying it simply swallowed all its food and relied on its insides to do all the work of breaking everything down.  It is possible dinosaurs like Brachiosaurus swallowed rocks, which would stay inside the stomach for extended periods of time, which would have helped mechanically break down its food as they mixed around in the stomach, called gastroliths.  Brachiosaurus' eyes were set high, towards the rear of the skull and the naris (holes in the skull where nostrils were) are elevated in a sort of bony crest between the eyes.  For a long time it was thought that Brachiosaurus had its nostrils high on the head exactly where these openings are on the skull, but it is more likely that these openings were connected to soft tissue that placed the nostrils closer to the front of the snout.  The evolutionary reasoning why Brachiosaurus, and many other sauropods had these openings on the top of the skulls, if not the true placement of the nostrils, is still somewhat of a mystery.  Some have suggested that this area was a space where air could flow through and help keep the blood in the animal's head and brain from overheating.

Photograph of the mostly complete (partially reconstructed) Brachiosaurus skull from Carpenter's 1998 paper.

As an adult, Brachiosaurus likely had few to no predators to worry about.  It was just too big.  When imagining it in its environment I sometimes stop thinking of it as an animal and more as a roaming force of nature more than anything else.  That being said, young Brachiosaurus were probably easier prey for all sorts of predators from the moment they hatched for years until they hit a certain size.  These younger animals probably relied on camouflage or possibly living in groups to minimize their chances of becoming another animal's meal.  

Special thanks to Dr. Heinrich Mallison for lending his sauropod expertise during the process of me writing this post!  See you next time!

*The dinosaur in Jurassic Park that's referred to as Brachiosaurus is actually based on a close relative called Giraffititan.  Giraffititan used to be considered a species of Brachiosaurus back when the film was made, so it being referred to as such in 1993 wasn't wrong.  

References

Carpenter, Kenneth, and Virginia Tidwell. 1998. Preliminary description of a Brachiosaurus skull from Felch Quarry 1, Garden Park, Colorado. Modern Geology 23:69-84.

D'Emic, M. D.; Carrano, M. T. (2019). "Redescription of brachiosaurid sauropod dinosaur material from the Upper Jurassic Morrison Formation, Colorado, USA". The Anatomical Record. 303 (4): 732–758. 

Riggs, E.S. (1903). "Brachiosaurus altithorax, the largest known dinosaur". American Journal of Science. 4. 15 (88): 299–306.Taylor, M.P. (2009). "A re-evaluation of Brachiosaurus altithorax 

Russell, D. A. (1989). An Odyssey in Time: Dinosaurs of North America. Minocqua, Wisconsin: NorthWord Press. p. 78. ISBN 978-1-55971-038-1.Witmer, L. M. (2001). "Nostril position in dinosaurs and other vertebrates and its significance for nasal function". Science. 293 (5531): 850–853.