The 3 million year old gargantuan fossil found in the Lost River belonged to a giant eel-like super predator that would have been at least a mile in length. The PDA also reveals that the remains are only 1/3 of the complete specimen. This is my reconstruction of what I will dub the Lost Leviathan (due to its remains being found in the Lost River).
Life on planet 4546B wasn’t just restricted to the volcanic crater, and so it radiated and diversified to adapt to living in the open ocean (although the only niches available would be filter feeder, predator, and creatures that feed on marine snow/carcasses). The Lost Leviathan may have returned to the crater to lay its eggs, and died due to unknown causes. The Lost Leviathan is a Mesopelagic predator that is able to go long periods in both the epipelagic and bathypelagic zones, and spend some time in the abyssopelagic zone as well. It will eat anything that fits in its mouth, which didn’t actually open that wide, but didn’t really need to considering its size. The Lost Leviathan lacks any nostrils or nasal cavities, having evolved to rely on its vision. It has 6 extremely large light sensitive eyes. The frontmost pair was somewhat binocular and faced upward. The second pair looked off to the sides of the creature and ever so slightly up as well. The final hindmost pair faced slightly backwards. It has red bioluminescent patches that act like a search light, and only it can see this light since it’s eyes are adapted for it and red is the first color to turn invisible in the dark. It’s skin is also a brown-red to make it barely visible as well. The Lost Leviathan has three dorsal fins as well as 4 rows of fins that run along its body that are formed from extensions of the ribs. It also has 2 horns on its head, which are presumably display structures. The Lost Leviathan would have been a massive predator that could eat anything in its path using a variety of hunting methods. It’s first and second eye pairs could have helped it hunt epipelagic prey items, which it would have struck from below. While hunting for prey in deeper waters it’s thick heavyset skull would have helped it dive while it used its red bioluminescent patches on its face to scan for food while remaining perfectly invisible. It could have also used the blue lure on its tail (which is visible to other species) and may have adopted a feeding posture not unlike a gulper eel from Earth.
And that’s it for the Subnautica fossil reconstructions. Admittedly the colors could be better and the body is a little off sized.