Xiphactinus

Xiphactinus (name meaning "Swift Shark"), often nicknamed the "Bulldog Fish", is a genus of large predatory bony fish that originated during the Late Cretaceous period in what is now North America, Europe, and Australia. Measuring 6 meters long and famous for its ugly appearance, it would have been in a middling role in the marine food chain; terrifying in its own right and fed on Hesperornis as well as the occasional, unlucky low-flying Pterosaur, it was also prey animals for mosasaurs, like Tylosaurus.

In the Season 2 episode, "Monster of the Deep", a couple of Xiphactinus were lived their lifes from Late Cretaceous 65 million years ago.

Era & Discovery
Xiphactinus lived in the seas of North America, Europe, and Australia during the Late Cretaceous period from 85 – 65 million years ago. One of the top predators in the seas, it died out with the dinosaurs at the end of the Cretaceous. The first Xiphactinus fossil was discovered during the 1850s in the Niobrara Chalk.

Physical Attributes
Measuring around 13 – 20 feet (4 – 6 m) long and weighing 1 – 2 tons (2,000 – 4,000 lbs.), Xiphactinus is described as one of many types of fish the Devil himself would have as pets.

One of the fastest and most agile creatures in the Cretaceous seas, they were a grotesque species of predatory fish that looked more like bulldogs than fish. Its lower jaw was specially hinged so that its mouth could be opened particularly wide. These fish could also sallow a Hesperornis down whole in one go.

Its ugly appearance has earned it the nickname "bulldog fish". With all that aside, Xiphactinus was a swift and powerful carnivore. It had a streamlined body and a broad and muscular tail, allowing it to reach speeds of 60kph. At this speed, it could outrun nearly all the fish around it.

Behavior & Traits
Xiphactinus spent most of its time cruising the surface waters of the shallow seas hunting fish. Xiphactinus would pursue its prey and swallow it whole. Its diet wasn't exclusively fish, it also hunted seabirds, like Hesperornis, and even possibly pterosaurs. Xiphactinus had the ability to jump clear of the water like modern dolphins. This was probably done to dislodge any loose scales or attached parasites.

However, Xiphactinus was vulnerable to attack. Larger predators, like Tylosaurus, could easily kill it. Xiphactinus was a carnivore of the Cretaceous seas, but not a top one: it was prey to giant mosasaurs that lived alongside it at that time. However, it is likely that if Xiphactinus lived and hunted in small shoals, it could possibly predate injured Giant Mosasaurs. Alone, Xiphactinus hunted anything that was smaller than itself.

Trivia

 * A teleost, bony fish, Xiphactinus was not a close relative of the sharks, though its name, translated from Latin, means "Swift shark".