Size and Weight:
Kronosaurus was a classic example of a pliosaur, marine reptiles characterized by their thick heads, short necks, stocky trunks and outsized flippers. The pliosaur body plan stood in stark contrast to that of their close relatives of the Mesozoic Era, the plesiosaurs (typified by Elasmosaurus and Cryptoclidus), which had relatively slender, graceful trunks, long necks and small heads, though often attaining the same impressive lengths.
The middle Cretaceous Kronosaurus appears to have made its living much like a modern Great White Shark, simply eating anything--fish, squids, and other marine reptiles--that swam across its path (the "Kronos" in this giant's name derives from the ancient Greek god, who ate his own children in a zealous attempt to preserve his power). As big as it was, though, Kronosaurus didn't approach the bulk of the most massive pliosaur of all time, Liopleurodon, which may have weighed as much as 35 tons (compared to about 10 tons for the largest Kronosaurus individuals). And both of these reptiles were outclassed by the giant shark Megalodon, which lived tens of millions of years later and attained weights in the 50 ton range.
As is the case with many other marine reptiles (most notably Mosasaurus), the fragmented remains of Kronosaurus were discovered fairly early in paleontological history, in 1899 in Queensland, Australia (a much more complete specimen was unearthed three decades later, during a fossil-hunting expedition to Australia sponsored by Harvard University). A few decades after that, a second Kronosaurus species was discovered in Colombia, South America, proof that this fearsome reptile attained an especially wide distribution during the Cretaceous period.