Brontosaurus was merely a mistaken naming duplicate of Apatosaurus, invalidated in 1903 and never a legitimate genus.
Brontosaurus was named by O.C. Marsh in 1879 but was officially synonymized with Apatosaurus in 1903 due to confused fossil assignments. However, a 2015 study by Emanuel Tschopp, Alexander Benton, and colleagues published in PeerJ re-evaluated the remains and demonstrated that Brontosaurus and Apatosaurus were indeed separate sauropod genera based on distinct anatomical characteristics. Brontosaurus was officially resurrected as a valid genus, stunning the paleontological community and vindicating a name that had been dismissed for over a century.