The upgraded model seen in Half-Life 2

The ichthyosaur (Xenotherus ichthycanthus, roughly meaning "foreign-beast spiny-fish" in Greek) is a fictitious aquatic animal in the first-person shooter computer game Half-Life. It is one of many alien species encountered in Half-Life that have been teleported to Earth from the borderworld Xen.

Although it is named after the extinct reptile group, the ichthyosaurs, it resembles them only in its carnivorous ecology and its general size. When an ichthyosaur is first encountered in Half-Life, a scientist remarks that while he has been told it originated in the Challenger Deep (in the Pacific Ocean), he is of the opinion that its first taste of a terrestrial ocean was its arrival at the Black Mesa Research Facility.

In Half-Life, ichthyosaurs are encountered in several water bodies, including a flooded laboratory and in a reservoir behind a hydroelectric dam. They are often placid at first, but quickly become hostile, especially should the player take action against them. The speed and viciousness of the icthyosaur make the underwater areas of Half-Life some of the most frightening parts of the game.


In Half-Life: Opposing Force, a wall poster depicts the creature's skeleton with the title "Xenotherus ichthycanthus."

Half-Life 2[]

The ichthyosaur also makes a single appearance in Half-Life 2. During a teleport malfunction ("harmonic reflux"), Gordon Freeman is momentarily transported underwater, and is almost attacked by an ichthyosaur before being transported elsewhere. Although this encounter is scripted, when an ichthyosaur is spawned using console commands or a mod, such as Garry's Mod, it also attacks the player, suggesting that the developers planned on Gordon Freeman encountering them more than once.

Some implementations find that ichthyosaurs are able to survive on land for limited periods before dying but also show that they will actually drown if underwater for too long. This latter behavior may indicate that the creature was not fully implemented in the final game, although equally this may be a design feature, as it is what might be expected from an air-breathing organism similar to dolphins, whales, and even real ichthyosaurs. The latter interpretation is supported by the frequent surfacing of icthyosaurs during the game (although this may simply be a ploy by the game designers to attract the player).

Most of the sound files associated with the ichthyosaur contain the prefix "watery death" (the same prefix is used with files and sounds associated with the Carnivorous Leeches), suggesting that the designers may have originally intended the ichthyosaur to serve the purpose of the Leech: to appear out of the foggy waters in the ocean, attack the player, and disappear in a similar manner, thus keeping the player out of deep water. Since the "watery death" and the Leeches themselves are included in a large trigger entity in the Valve Hammer Editor, using the ichthyosaur in this fashion would have made an easy way to keep the player out of water and may also explain the creature's lack of artificial intelligence. Though it doesn't aggressively attack the player, the ichthyosaur may suddenly turn or nudge towards the player. It is actually easier to attack the ichthyosaur than to receive attacks from it. In order to be attacked by an ichthyosaur, the player must be positioned directly in front of the ichthyosaur's eyes. The ichthyosaur attacks when the player is close enough so that he is not able to swim away without receiving damage. In the leaked beta of Half-Life 2, upon entering deep water, no Leeches will appear; instead an ichthyosaur will come and kill the player, but the ichthyosaur is not visible. This ichthyosaur will emit growls and such and sometimes will let out a high-pitched noise that is similar to a that of a whale.

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