Thalassosaurus

Thalassosaurus (meaning "ocean lizard") was a large carnivorous marine reptile from the late Iridaceous period. A distant relative of today's snakes and monitor lizards, Thalassosaurus was a formidable apex predator of the Iridaceous oceans. The family which it belongs to, the Thalassosauridae, is named after it.

Discovery
The first Thalassosaurus specimen was comprised of a jaw, partial skull, and associated vertebrae found in a limestone bed in the Razuni Sultanate by poachers. The poachers were illegally quarrying terracotta when they discovered a massive toothed jaw. The poachers attempted to sell it in exchange for an entire shulker box of Crimson Wine, and through a series of sales the fossil made its way to the University of Lhûmar, where it was described officially by Dr. Sanyanis Axeon in 841 ACA. The species name, T. razuniensis, honors the place where the original fossil was discovered.

The official description of this fossil shook many religious scholars in Twin Dragon philosophical schools, who previously held that most fossils were of large dragons or other giant versions extant wildlife, and venerated as holy objects. This was the first landmark discovery proving that organisms frequently went extinct, and that unique forms unlike anything alive existed in geologic history.

Description
Thalassosaurus had the same general body plan of any lizard, bearing four limbs and a long tail. However, instead of toed feet, Thalassosaurus had fins comprised of webbing between its digits. Its tail was flattened along the lateral axis, not unlike a fish. Whether or not the tail fin was a crescent-shaped fin like a shark's, or an elongate ovular shape like a tadpole or eel's tail, is not yet known and a subject of debate.

Thalassosaurus was a massive creature, with an estimated maximum size ranging from 10 to 17 meters long. Its head alone was nearly two meters long, large enough to swallow a human whole. It also bore a second set of teeth on the pterygoid bone around its throat, likely to secure thrashing prey in its mouth. It had large, conical teeth, but these teeth also bore serrated cutting edges, for both securing smaller prey and shredding flesh from bigger prey.

Ecology
Among the marine fauna of the late Iridaceous period, Thalassosaurus was one of the largest. Being an apex predator, it probably ate almost anything it could fit into its mouth, with large jaws able to crush through the shells of any turtle or shellfish it could find. It coexisted alongside various marine reptiles including sea turtles, plesiosaurs, and smaller thalassosaurids, as well as the various fishes and cephalopods that also inhabited the seas of the time. Fossils of the marosaurid plesiosaur Tlahatlsaurus have been found with bite marks on the bone that were likely made by Thalassosaurus teeth.

Smaller Thalassosaurus fossils are found in coastal sediments, while larger ones are known more from open ocean sediments. This implies that the youth of the species inhabited shallow-water environments, and larger adults lived in the open ocean.

It is unknown why the Thalassosaurus went extinct. It was the largest, and it and its contemporaries were the last of all Promesolithic marine reptiles apart from sea turtles. The Iridaceous-Cuiciloan Mass Extinction event roughly 69 million years ago is most likely what rendered the creature extinct, though the exact cause of the event is unknown.