No. Brachiosaurus was a dinosaur that lived around 150 million years ago. By the time that Brachiosaurus became extinct, there were already early mammals called Eutheria living alongside the dinosaurs. The Eutheria gave rise to the placental mammals and then the Artiodactyla and, eventually, the modern giraffe. The most recent common ancestor of the Brachiosaurus and the giraffe would have been an amniote vertebrate – somewhere between a reptile and an amphibian – that lived about 340 million years ago. Confusion might arise from the name of one of these great sauropods: Brachiosaurus giraffatitan. This means ‘giant giraffe’, but the physical resemblance between the two animals is actually quite superficial. They are both large quadrupeds, but the long neck of the Brachiosaurus comprised dozens of separate vertebrae whereas the giraffe has just seven. This is the same number as you have in your neck; it’s just that the giraffe vertebrae are each a lot longer. It takes less time to evolve longer bones than it does to change the total number, and this is an indication that the giraffe is more closely related to humans than to dinosaurs. In fact, our ancestries diverged just 110 million or so years ago.
Answered by Luis Villazon