Is the blue whale the largest animal that ever existed?

 Based on our current understanding, probably yes. The largest recorded blue whales weighed around 190,000 kilograms and were 29.9 meters long. Don't forget that. There are two animals that at least some scientists believe may have reached or even exceeded the weight of the blue whale.

The first of these animals is the Lilstock ichthyosaur , whose scientific name has not yet been published. Its length is estimated at 20 to 26 meters, but it was probably significantly thicker than a blue ichthyosaur, so a similar weight is possible.

As far as we know, it wasn't a plankton feeder, but probably not a fish or squid feeder either, since it was toothless. Instead, it likely sucked crustaceans from the seabed, like a gray whale, using benthic suction.

The second "candidate" is the Triassic octopus . The existence of this organism is claimed by one paleontologist, Mark McMenamin, and only by him. The hypothesis is based on rather weak evidence, but it is at least somewhat plausible.

The idea was based on the discovery of 205-million-year-old ichthyosaur bones arranged in a strange pattern and bearing suction cup-like imprints. McMenamin believes they were deliberately arranged that way by some kind of gigantic, intelligent cephalopod. Like I said, flimsy evidence.

Based on the size of the alleged sucker marks, this thing – if it existed – would have been 30 meters long and weighed 200 tons, making it the largest animal of all time. That is, assuming it had the proportions of a modern giant squid.

The Lilstock ichthyosaur is therefore – so far – too poorly understood for us to make an accurate mass estimate, and there is only scant evidence for the existence of the Triassic octopus. Until new evidence emerges, we can say with certainty that the blue whale is the largest animal ever known to science.

What was the largest river that has ever existed on Earth?

The largest river to have existed in the last half a billion years might have been the Mega Congo-Amazon.

In the contemporary world, the largest river by discharge is the Amazon, but the Nile, by some measures, might be the longest, and over its length, there are some disputes. However, both of these rivers pale in comparison to the mega Congo-Amazon, which existed when South America was still connected to Africa in the Gondwana and Pangea supercontinents and might have been around 10,000 km in length, almost twice the length of the Amazon alone. It was home to huge dinosaurs and other fascinating Mesozoic creatures.

The drainage area of the Amazon today is about 7 million sq km, the Congo about 4 million, and the Nile about 3.4 million. The Mega Congo Amazon had a drainage area of around 12 million square kilometers.

The flow rate of the Amazon is about 210,000 m3/sec, and that of the Congo is about 40,000 m3/sec. However, for millions of years, the climate was much wetter during the existence of the Mega Amazon-Congo, and its discharge was likely much higher than the combined flow rate of the Amazon and Congo rivers in Africa and South America.

When these two continents split, both the Amazon and the Congo continued to flow from east to west—Congo towards the Atlantic and Amazon towards the Pacific Ocean, but 23 to 50 million years ago Andes started to grow rapidly, and 10 to 15 million years ago, they became large and tall enough to block the flow of the Amazon towards the west.

For millions of years, the water accumulated into a vast inland sea and swamp, the Pebas, in what is now Colombia, Ecuador, parts of Peru, and western Brazil. It discharged into the Caribbean in the north of modern-day Colombia. It existed for so long that indigenous and fascinating animals and plants evolved to live in it, only for their habitat to be destroyed when the Amazon found a way, via sandstone, to flow east towards the Atlantic 7 to 10 million years ago.

Some of the most fascinating animals of the Pebas were Purussaurus, a 10-ton, 10 to 12 meters-long crocodile with the strongest bite force of any known animal, stronger than that of T. rex. There were also filter-feeding crocodilians with duck-like bills. Stupendemys was a 3-meter-long, up to 1-ton turtle. Freshwater snails of the Pebas had thick shells of up to 40cm in diameter.

The mega Congo-Amazon River might have been the largest in the last half-billion years. Earth experienced many supercontinental cycles over its 4.5-billion-year history, and if larger rivers existed on older supercontinents like Columbia/Nuna, 1.8–1.4 billion years ago, or Rodinia, 1.2 to 0.75 billion years ago, we have no evidence of their drainage basins due to erosion. Older supercontinents were still too small to harbor such large drainage basins. Therefore, the Mega Congo-Amazon is the largest we have evidence of ever having existed in the history of our planet.