It's relatively unclear and practically impossible to judge exactly which species of dinosaur was the largest, due to the fact that many of the fossil remains discovered to date aren't totally complete. Partial fossils have been discovered relatively recently that allude to the fact that there may have been dinosaurs that were much larger than what scientists had perceived to have been the largest, but trying to judge based on the incomplete evidence that exists is a difficult task.
Despite the information and evidence that has already been unearthed, and what is discovered in the near and the distant future, there will probably always be questions when it comes to determining which dinosaur was the largest or which was the fastest, which could fly or which were adept in the water. Regardless of what exactly is discovered, scientists will probably never be able to prove beyond a shadow of a doubt what exactly went on sixty five million years ago, or that we will have enough information to make any truly conclusive decisions.
The best that we can do is judge based on the most complete evidence that we currently have available to us, which may only be an infantile percentage of what the age of the dinosaurs really has to offer.
Judgment based on complete evidence
With a relatively complete skeleton to go by and enough information to make a reasonable judgment as to overall size, the largest dinosaur discovered seems to be the Brachiosaurus.
The Brachiosaurus most likely lived at the end of the Jurassic period or at the beginning of the Cretaceous period.
How big did it get?
It should come as no surprise to anyone who has some familiarity with dinosaurs that the biggest of all the dinosaurs was beyond what could be considered massive. The Brachiosaurus could have gotten to in excess of eighty feet in length, and with a body type similar to that of a giraffe, it could very well have been able to raise its head nearly forty feet in the air.
Though it can be pretty difficult for scientists to accurately estimate the weight of any dinosaur, the estimates on this giant herbivore range in the neighborhood of thirty tons on the low end, all the way up to forty tons at the other end of the spectrum.
Safety in Numbers (numbers of pounds, that is. )
Brachiosaurus was an herbivore, dining exclusively on plant material. Many herbivores were particularly susceptible to predators, most likely because they posed less of a threat to their attackers. Quite a few known dinosaur herbivores had extra protection like armor plating or extra tough, spiny skin, but not the Brachiosaurus. It's speculated that the brachiosaurus wasn't too threatened by even some of the larger predators in the dinosaur world, simply because they were so large in stature.
Because of the unique distinction of being the biggest beast to ever roam the earth, the brachiosaurus has gained some notoriety in the popular media. We've seen the brachiosaurus in some major motion pictures as well as on television; in fact, the brachiosaurus has become popular almost to the point that it rivals the tyrannosaurus as the most popular of all known species of dinosaur .
It's probably pretty likely that, in the not too distant future, another nearly complete fossilized skeleton will be unearthed and put back together like an enormous jigsaw puzzle, to show the world a dinosaur that is even more immense than the brachiosaurus. If in fact this does happen, it certainly won't change the fact that the brachiosaurus was one of the most impressive animals to ever walk the earth and it probably won't change the fact that the brachiosaurus is also one of the most popular species of dinosaur to have ever existed.
Looking for information on Dinosaurs? Be sure to check out Walking with Dinosaurs on Discovery Channel's Dinosaur blog for the latest information and resources on dinosaurs and their prehistoric existence.