So what if I told you there was never such thing as Brontosaurus?
Well, not technically anyway. The animal who's fossils we found and subsequently named "Brontosaurus" certainly existed. But here comes my dark raincloud to hover over the parade dedicated to our love for this dinosaur. To understand why Brontosaurus is no longer used by scientists today, we first have to look at some rules.
There's a strict set of them guiding how exactly we're allowed to name species. Not their common names mind you. Those aren't always reliable and are subject to variation, as some animals are known by different common names in different areas (for example- the cougar, as it's called in Canada, is also known as the mountain lion in the USA and the puma in South America). It's the species scientific names that have rules attached to them. These names are binomial and are the same everywhere, no matter what. Going back to the cougar, although its common name varies between people, to science it's always known as Puma concolor. That way, there's no question about what species you're talking about. As a side note, as far as dinosaurs go, we don't really have common names for them, so all dinosaurs are known by the genus part of their scientific names. That's the first word in their binomial scientific name. For example, in Tyrannosaurus rex (one of the few dinosaurs often referred to by its full name), the "Tyrannosaurus" part is the animal's genus, while the "rex" part is its species.
So what does all that have to do with Brontosaurus? Well, back in the late 19th century, there was a very prominent palaeontologist names Othneil Charles Marsh, who discovered a huge Sauropod dinosaur, and named it "Apatosaurus". For those who have never heard of it, Apatosaurus looked a lot like what you think of Brontosaurus looking like. Exactly like it, in fact. Shortly after this discovery, Marsh dug up more, similar fossils from a site close by and, in his zealous attempt to name more dinosaurs than his arch-rival Edward Drinker Cope, he named the animal that these new bones belonged to "Brontosaurus". Everybody immediately thought Brontosaurus was pretty awesome, with artists such as the legendary Charles R. Knight doing several famous paintings of the creature. From a very early point, Brontosaurus became engrained into popular culture along side such celebrities and Triceratops and T. rex.
Then, years later, a scientist named Elmer Riggs made an startling discovery: he was closely examining and comparing the fossils of both Apatosaurus and Brontosaurs. Riggs noticed that the two animals were so similar that they actually should belong in the same genus together (this type of event all too commonly creeps up in palaeontology). Now that it had been decided that the two animals were similar enough to be grouped under the same genus, the question arises- do we call them both Apatosaurus or Brontosaurus? Well, the codes of zoological nomenclature have an answer to this: if two specimens of an animal, once thought to represent two distinct types, are found to be the same thing, the name that was first given to describing the the animal is the name that is kept. Marsh coined the name Apatosaurus in 1877, and Brontosaurus in 1879. So, since Apatosaurus was the first name used to describe the animal, everything that we used to call Brontosaurus now should be called Apatosaurus. That's why there was, in a sense, no such thing as Brontosaurus.
Obviously the name Brontosaurus is dying a very slow death. While scientific institutions consistently refer to the animal in question as Apatosaurus, it seems like many people are having a lot of trouble breaking the Brontosaurus habit. So should the people who love Brontosaurus so dearly just grit their collective teeth, accept Apatosaurus and throw Brontosaurus reluctantly away? Well, yes and no. It still is technically correct to us Apatosaurus and only Apatosaurus when you're dealing, scientifically, with this dinosaur. The naming system is designed to avoid confusion (although this isn't always the result), so switching between the two names isn't a logically sound solution. We all should learn to love Apatosaurus. However, this doesn't mean the Brontosaurus has to go away completely. I'm acquainted with a few people in the field of palaeontology who make a habit of colloquially referring to the Sauropods in general as the "Brontosaurs" (Brontosaurus does mean "thunder lizard", so it's a pretty fitting name for the group). At the end of the day, when all the politics of naming organisms has got you down and confused, it's worth taking a moment to reflect on the fact that these creatures certainly never knew, nor cared, what names we would later give them.