The largest prehistoric organisms include both vertebrate and invertebrate species. Over 99 percent of all species that have ever existed are extinct. Some are celebrated, like the ferociously famous dinosaur Tyrannosaurus rex.
Many species mentioned might not actually be the largest representative of their clade due to the incompleteness of the fossil record and many of the sizes given are merely estimates since no complete specimen has been found. Their body mass, especially, is mostly conjecture because soft tissue was rarely fossilized. Generally, the size of extinct species was subject to energetic and biomechanical constraints.
Around 23 million years ago, the Andes had become high enough that they caused a huge increase in rainfall in Amazonia, because they blocked clouds from blowing further west. This, in turn, meant that the land was being eroded at a very high rate, and an enormous system of lakes and wetlands was created.
This is known as Lake Pebas, although it was more likely many lakes, or perhaps just a huge network of swamps. In any case, what is most fascinating about the Pebas ecosystem is the animals which inhabited it. They include:
One of the largest turtles in history. Its shell was over 3 meters long, and its carapace alone weighed as much as a small car. Additionally, males had a set of horns protruding from either side of their necks, which would have been used in combat.
Quite possibly the largest crocodilian ever to live. A type of caiman, it was probably between 10 and 13 meters in length and could have weighed over 8,000 kilograms. Additionally, it had an estimated bite force of 69,000 newtons, as much as twice that of Tyrannosaurus rex.
A very large gharial which was roughly 10 meters long. It was a member of an extinct group known as the Gryposuchus, which were generally seagoing species. However, Gryposuchus itself seems to have been more of an inland freshwater-dweller. Its name translates to "griffin crocodile".
Yet another giant crocodilian. It’s nicknamed the “pancake croc” for its bizarre snout, which was broad and flat, almost like a duck’s bill. It’s thought that it also had a large, grooved pouch in its throat as pelicans do. These were both adaptations for filter feeding - to me, the idea of a caiman filtering water like a whale is pretty amazing.
A genus containing several species. These were among the largest rodents ever to live, some may reach weights of up to 700 kilograms. In terms of ecology, they were probably similar to modern-day hippos - herbivores, spending most of the day in the water, perhaps grazing at night.