The battle for the world’s largest dinosaur title is a complicated one. One of the troubles is that archaeologists rarely discover an entire skeleton. It’s more likely that they will get fragmented bits of bone, and try to estimate a profile of height and weight.
That said, the largest dinosaur based on the most evidence is the Argentinosaurus, discovered in — you guessed it — Argentina in 1993. With a length of up to 130 feet long and an estimated weight of 110 tons, it dwarfs other known fossils of dinos. Argentinosaurus, known as a sauropod, lived during the middle cretaceous period, 100-90 million years ago.
Though it has fewer fragments available, A. fragillimus — a dino in the Amphicoelias family — might be the biggest dinosaur on the block. Based on surviving descriptions of a single fossil bone, researchers estimate that fragillimus may have been 130 to 200 feet long, and may have had a mass of up to 135 tons. Sadly, evidence of the giant dino’s existence was lost after being studied in the 1870s, so modern inquiries are impossible. All that remains is drawings and field notes. [How Big Were Baby Dinosaurs?]
So how did dinosaurs grow so big? Some researchers hypothesize that dinosaurs were able to grow quickly and fuel large bodies because they lived at a time when temperatures were warm, oxygen levels were high, and giant land masses provided abundant living space. Others say that dinos had to grow large to outcompete the myriad of small-and medium-sized creatures on the planet.
Being big came with costs, though: when the asteroid impact at the end of the Cretaceous period 65 million years ago, there were only a few small dinosaur species, and the group was almost obliterated – only birds survived. The many small mammals living at the time, however, were uniquely suited to dominate a world that was primed for the little guys.