A Baby Brachiosaur?

Brachiosaurus used to hold the title of biggest dinosaur ever. I remember when, as a young dinosaur fanatic, books and documentaries told me that this long-necked dinosaur was the ultimate prehistoric titan. Then Supersaurus, Argentinosaurus and other super-sized dinosaurs came along and ruined all the fun. Even worse, paleontologists recently realized that we actually know very little about what Brachiosaurus really looked like.

In 1903, paleontologist Elmer Riggs described Brachiosaurus altithorax from fossils discovered in the 150-million-year-old Late Jurassic strata of western Colorado. The dinosaur, which Riggs believed to be the largest known, was represented by a huge humerus and assorted elements of the shoulder girdle, hips, hindlimbs, vertebrae, ribs and a few other miscellaneous parts. Despite the relative smattering of material, though, the proportions of the bones led Riggs to conclude that he had found a previously unknown dinosaur that was significantly larger than Apatosaurus, Diplodocus and other giants which lived at the same time.

Fossils discovered by German expeditions to Tanzania seemed to fill out the form of Brachiosaurus. At the beginning of the 20th century, the Late Jurassic deposits of Africa were believed to be roughly equivalent to those of western North America, and so dinosaurs discovered in Tanzania’s Tendaguru Formation were often assigned to genera known from the Morrison Formation of Colorado, Utah and Wyoming. As a result, paleontologist Werner Janesch described partial skeletons and skulls of a large sauropod dinosaur from Tanzania under the name Brachiosaurus brancai. The fact that the material from Africa was more complete allowed paleontologists to get a better idea of just how big the dinosaur was—Brachiosaurus brancai reached over 80 feet long and may have weighed more than 25 tons.

But there’s a major problem with this approach. Paleontologists recently determined that the brachiosaurs from Africa and North America don’t actually belong to the same genus after all. Artist Gregory S. Paul noted differences between the two brachiosaurs in 1988, and in 2009 sauropod expert Mike Taylor confirmed that the two dinosaurs were different enough to warrant placement in separate genera. Furthermore, a skull fragment tentatively assigned to Brachiosaurus hints that the traditional picture of the dinosaur may have been skewed by reliance on fossils from Tanzania. While the North American form has retained its name, Brachiosaurus altithorax, the dinosaur from Tanzania is now called Giraffatitan brancai. Thanks to a name change, we know significantly less about Brachiosaurus than we thought we did.

Then again, a reevaluation of another Jurassic dinosaur skeleton may provide a rough idea of what Brachiosaurus looked like as a baby. In 2007, Daniela Schwarz-Wings and colleagues described a juvenile sauropod skeleton found in Wyoming’s Howe Stephens Quarry. This Late Jurassic specimen was designated SMA 0009, and was initially thought to be a young diplodocid dinosaur. But in a new paper published in Palaeontology, Schwarz-Wings, José Carballido and colleagues have amended their diagnosis. Additional preparation of the partial skeleton revealed that the dinosaur was not a close relative of Diplodocus and Apatosaurus at all, but was more closely related to Brachiosaurus.

Schwarz-Wings and co-authors refrained from assigning SMA 0009 to a particular dinosaur species. The fact that the animal is a juvenile confounds precise identification attempts—dinosaurs changed significantly as they grew up, and the traits seen in adult dinosaurs may not have been present in juveniles. Likewise, the revised idea that SMA 0009 is a brachiosaur makes comparisons difficult since paleontologists have yet to assemble a complete picture of an adult Brachiosaurus. Still, since the young dinosaur is grouped closely with Brachiosaurus, and Brachiosaurus was the only dinosaur of its kind present in the Morrison Formation, there is a good possibility that SMA 0009 is a young Brachiosaurus. Until someone finds more complete remains of this rare and enigmatic dinosaur, however, Brachiosaurus will remain a dinosaurian enigma.

References:

CARBALLIDO, J., MARPMANN, J., SCHWARZ-WINGS, D., & PABST, B. (2012). New information on a juvenile sauropod specimen from the Morrison Formation and the reassessment of its systematic position Palaeontology DOI: 10.1111/j.1475-4983.2012.01139.x

RIGGS, E.S. (1903). “Brachiosaurus altithorax, the largest known dinosaur.” American Journal of Science (series 4) 15(88): 299-306.

TAYLOR, M.P. (2009). “A Re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropod) and its generic separation from Giraffatitan brancai (Janensh 1914).” Journal of Vertebrate Paleontology, 29(3): 787-806

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.