Rygalisaurus - "Rygali's Lizard", named by Cassandra Rygali, who unearthed the first specimen. Supposedly, this was in the name of her husband xp - was a mosasaur, native to the Australian seas during the Late Cretaceous era.
A massive predator, rygalisaurus averaged thirty to forty feet in length. Like all mosasaurs, it breathed air and bore live young. However, this species was very distinct in its large pectoral flippers, blocky skull, flexible yet muscular neck and of course, the obvious spinal armor.

The spinal scutes were vertibral extensions, composed principly of bone. While this did limit its flexibility, it also provided considerable protection from similar predators. Bite marks in the skulls of a specimen indicate that the animals fought regularly.

The rygalisaurus' dorsal spines were edged, creating a line of fins extending down the animal's back. This allowed it to move with relative ease, despite its great weight.
The rygalisaurus' body was also more muscular than other mosasaurs, granting it the extra strength it needed to keep up with its prey.

Rygalisaurus' jaws were thicker than other mosasaurs and its teeth were MUCH larger. These adaptations ensured that its bite killed prey with one blow, allowing it to maximise food intake.
Like other mosasaurs, rygalisaurus was able to unhinge its jaws in a snakelike fashion, allowing it to gulp down massive chunks of meat or completely swallow smaller prey. Specilized muscles allowed the jaws to extend to ridiculous proportion, then suddenly slam them together full force. In fact, the very first specimen of a rygalisaurus was a female, which came complete with the skull of a similarly sized male wedged in her throat!

Rygalisaurus was a terror of its domain in the Cretaceous, but eventually succumbed to extinction with the rest of its mosasaur kin.

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To those who haven't figure it out, yes, this is indeed Rygama's non-mutated form.
And yes, that female was the same one which was later assimilated and rebuilt as a giant monster.