Though there have been a few instances of scientists purporting to have found larger specimens, the longest known dinosaur that is widely accepted by paleontologists seems to be the Diplodocus, a type of sauropod dinosaur with a long neck and long tail that is said to have measured as much as 175 feet in length. There seems to be some controversy and debate around the question of which dinosaur is truly the longest, with scientists introducing competitors such as the Seismosaurus, which was once thought to be a distinct species but was eventually found to be a very large Diplodocus specimen. The Seismosaurus' discovery was announced in 1991, but its existence as a unique species was debunked just over a decade later, a fact that was announced at an academic event in 2004.
The revelation of this massive new Diplodocus specimen caused scientists to actually amend the dinosaur's scientific name, changing it to Diplodocus hallorum. Because dinosaur discoveries happen at such a rapid rate, there may continue to be specimens proposed to be bigger than the Diplodocus, including the Amphicoelias, another proposed member of the sauropod family that is said to be the longest dinosaur ever. However, this assertion is based on very limited fossil evidence consisting of a single bone that was lost in the late 1800s.