Ancient Skeleton

This skeleton belonging to a presumably extinct Leviathan Class Organism is found outside the Lost River Laboratory Cache surrounded by scanners and cables. The skeleton is the only one known to exist and is approximately one million years old. It appears to be some sort of huge, carnivorous life form. The skeleton is composed of many bone plates. Running parallel on down either side of the skeleton are twelve large holes, the purpose of which is unknown. It has six eye sockets and many sharp teeth, each tooth being larger than the Player. This skeleton is large enough that the player can swim through the gaps left by its missing teeth.

The Precursor Race had discovered these remains, and set up a Laboratory near it for the purpose of studying the skeleton. This is evidenced by multiple Precursor lamp-like objects surrounding it.

In Life
It is near impossible to imagine what this creature looked like in life, as it is unknown how much of the skeleton is visible to us, and even how the creature moved.

Lateral Openings
There are many openings running down the sides of this creature's skeleton, the purpose of which is unknown. Assuming they did not simply serve to lighten the overall weight of the skeleton, they could potentially have served as openings for flippers, limbs or limb-like structures to protrude from, though the openings do not appear to be ball-and-socket joints. It is unclear what method of locomotion the creature used in life, though the extremely thick skeleton does not imply it was a capable swimmer.

Armor
Despite being armored this creature appears to have descended from a different group of animals than the living armored creatures, these being the Research Specimen Theta, Boneshark, Sand Shark and Rockgrub which appear to have evolved armor independently of the ancient skeleton. The skeletal features of the Ancient Skeleton support a distinct lineage, as unlike Research Specimen Theta and and its relatives which have dermal armor that is visibly separate from its skeleton, the Ancient Skeleton's armour appears to be derived from solid bone extensions of the spine and ribs themselves, the outline of these bones being visible from within the skeleton.

This makes the Ancient Skeleton's shell more similar in design to that of a tortoise or turtle, where the shell is formed by the spine and ribs. As opposed to the other armored creatures which possess a more armadillo-like set of armor, being derived from hard plates on the skin, not joined with the skeleton. It is possible that when alive, the creature's shell was covered by protective scutes in order to protect the bone, like the shell of a tortoise or turtle.

Teeth
The teeth of the creature are very large, the largest being over two metres in height. There are ten teeth in the lower jaw and eight in the upper jaw. The upper jaw lacks teeth in the middle, instead having a thin flat surface, possibly an adaptation to slicing for a diet comprised mostly by soft-bodied organisms.

Eyes
Unusually, this creature appears to possess six eye sockets. This feature is not shared by any living creatures on the planet.

Trivia

 * The Ancient Skeleton was once named Ancient Skull. It was named this up until a livestream in which it was pointed out to narrative designer Tom Jubert that the incorrectly designated "skull" had a spine and ribs within it, so was clearly not just a skull, but also the body.