Maelstrom and fellow sea reptile Cretaceous were once creatures of the era of dinosaurs that had worked in tandem to catch prey. After having been frozen in the waters they swam in, both reptiles remained in stasis until the ice ages began, thawing them out into a world ruled by mammals instead of reptiles.
The Ice Age
Frozen long ago with Cretaceous during the era of the dinosaurs, Maelstrom was imprisoned in a great block of ice that was jarred loose when the wall of ice that it had been part of began crumbling, due to global warming loosening it.
The ice melted, releasing both Cretaceous and Maelstrom into the lake which was all but abandoned by most animals. When they made it to the ice lake, the first animal that they had encountered and eaten was Stu the glyptodon, leaving behind only his shell. The two sea reptiles then swam further down the series of rivers where they eventually reached Manfred's herd as well as Crash, Eddie, and Ellie. Maelstrom intended to eat Sid but could not catch him, as Sid was already escaping with Diego, and had only just escaped Maelstrom as he broke off a massive chunk of ice where the sloth and Saber-tooth Tiger had stood only moments before.
Later when the dam broke, the two reptiles went swimming on the wall of water that was slowly approaching the escaping masses and pulled down Manny when he was swimming to save Ellie. Manny devised a plan to rid himself of the two sea reptiles, by having them try to chase him, he tricked them into hitting a log that was wedged under a large rock. The plan worked, freeing Ellie, and knocking the two reptiles down with the boulder.
Maelstrom, like his smaller companion Cretaceous, was incapable of speech and so communicated in loud growls, bellows, or roars. Maelstrom was a large pliosaur, thickly built with a large mouth filled with sharp teeth ideal for breaking bone, faintly glowing yellow eyes and armour-like teal scales that were topped off with spikes and bumps, propelled through the icy waters with large, flat fins. Within Maelstrom's digestive tract were several parasites: a number of stunted giant single-celled organisms and a large parasite with multiple eyes and a beak.
Behind the Scenes
Maelstrom is revealed to be a pliosaur in the Essential Guide, though his appearance is altered for artistic purposes, as actual pliosaurs had longer necks, no armor plating, a more streamlined body, proportional fins and a crocodilian-like snout. It's also possible that he and Cretaceous came from the Dinosaur World (despite its existence has been revealed only in the following movie), and during the millions years in which the pliosaurs and the ichthyosaurs lived there, they changed their appearances in the ones of the two specimens seen in the second movie. This would also justify what is written in the Essential Guide, namely that they have been frozen a million years before the ice age, while actually both of their species went extinct about 90 million years ago (even though it's possible some still survived).
The sounds used for Cretaceous and Maelstrom's vocalizations were those of whales, coupled with tigers growling and human voices to give an eerie quality.
Initially, according to director Carlos Saldanha, there were to be three water-based predators, and concept art showed a small, amphibian creature similar to an axolotl. Moreover, Saldanha intended the two sea reptiles to be truly dangerous foes, a feat accomplished by having them kill off one character so as to prove their threat: Stu.
The fate of both Cretaceous and Maelstrom remains unclear: while it is overt that they were vanquished by Manny, different sources present different endings, with the storybook of the film showing them being eyed by vultures while the Essential Guide stated that they were eaten off by mini-sloths. Furthermore, in the book Ice Age: The Great Escape, it was stated that both reptiles were sucked out with the waters that were pulled through the giant crack in the ice wall that Scrat had created: from there, the mini-sloths found their bodies and carried them away, referring to them as "good sushi". In the video game of the second movie however, it is possible to find the corpses of both reptiles in a level which takes place after the climax, where they are both in an advanced stage of decomposition, with most of their meat reduced to the bone. Rather than decomposing very fast, it is most likely the two reptiles were eaten offscreen by the mini-sloths, as many sources state.
Maelstrom's species, in the online game Ice Age Village, are erroneously referred to as 'Maelstrom'.
Maelstrom resembles more like a type of mosasaur called Globidens.