For such a famous and influential dinosaur--it's been featured in countless movies, most notably the first installment of Jurassic Park--Brachiosaurus is known from surprisingly limited fossil remains. This isn't an unusual situation for sauropods, the skeletons of which are often disarticulated (read: picked apart by scavengers and scattered to the winds by bad weather) after their deaths, and more often than not are found to be missing their skulls.
It's with a skull, however, that the story of Brachiosaurus begins. In 1883, the famous paleontologist Othniel C. Marsh received a sauropod skull that had been discovered in Colorado. Since so little was known about sauropods at the time, Marsh wound up mounting the skull on a reconstruction of Apatosaurus (the dinosaur formerly known as Brontosaurus), which he had recently named. It took nearly a century for paleontologists to realize that this skull actually belonged to Brachiosaurus, and for a brief time before that, it was assigned to yet another sauropod genus, Camarasaurus.
The "Type Fossil" of Brachiosaurus
The honor of naming Brachiosaurus went to the paleontologist Elmer Riggs, who discovered this dinosaur's "type fossil" in Colorado in 1900 (Riggs and his team were sponsored by Chicago's Field Columbian Museum, later to be known as the Field Museum of Natural History). Missing its skull, ironically enough--and no, there's no reason to believe that the skull examined by Marsh two decades before belonged to this particular Brachiosaurus specimen--the fossil was otherwise reasonably complete, evincing this dinosaur's long neck and unusually long front legs.
At the time, Riggs was under the impression that he had discovered the largest known dinosaur--bigger even than Apatosaurus and Diplodocus, which had been unearthed a generation before. Still, he had the humility to name his find not after its size, but its towering trunk and long front limbs: Brachiosaurus altithorax, the "high-thoraxed arm lizard." Foreboding later developments (see below), Riggs noted the resemblance of Brachiosaurus to a giraffe, especially given its long neck, truncated hind legs, and shorter-than-usual tail.
Giraffatitan - The Brachiosaurus that Wasn't
In 1914, a little over a dozen years after Brachiosaurus was named, the German paleontologist Werner Janensch discovered the scattered fossils of a giant sauropod in what is now modern Tanzania (on the eastern coast of Africa). He assigned these remains to a new species of Brachiosaurus, Brachiosaurus brancai, even though we now know, from the theory of continental drift, that there was very little communication between Africa and North America during the late Jurassic period.
As with Marsh's "Apatosaurus" skull, it wasn't until the late 20th century that this mistake was rectified. Upon re-examining the "type fossils" of Brachiosaurus brancai, paleontologists discovered that they were substantial different from those of Brachiosaurus altithorax, and a new genus was erected: Giraffatitan, the "giant giraffe." Ironically, Giraffatitan is represented by much more complete fossils than Brachiosaurus--meaning that most of what we supposedly know about Brachiosaurus is actually about its more obscure African cousin!