Spectember 2025 Day 30: Eobasilosaurus giganteus
What would mosasaurs look like if they have evolved to become baleen whale-like filter-feeders?Already towards the last 20 million years of the Cretaceous, from the Turonian to Maastrichtian stages, these aquatic cousins of monitor lizards were reaching the peak of their evolutionary diversity following the extinctions of the icthyosaurs and giant pliosaurs. Some of them such as Globidens and Platecarpus were specialized ammonite or fish eaters, some such as Mosasaurus hoffmani and Tylosaurus proriger were giant apex predators, and others such as Megapterygius and Plotosaurus developed large flippers and even small dorsal fins and were highly specialized, fast-moving pelagic or open-ocean hunters. But had the Cretaceous-Palaeogene extinction event never occurred, the shapes and sizes achieved by the mosasaurs might have been even greater, and some of them could become the largest marine animals the world has ever known.
Eobasilosaurus giganteus is a species of gigantic tylosaurine mosasaur which inhabits the shallow tropical ocean waters of an alternate earth in which the asteroid impact never occurred and the non-avian dinosaurs never went extinct, as well as the seas which surround Sekaia’s island continent of Aetherosia. Having evolved from a Tylosaurus-like ancestor around 66-56 million years ago, Eobasilosaurus represents a peak in mosasaur evolution in that it is a filter feeding hunter of small to-medium-sized fish and marine reptiles which occupies a similar niche to the late Triassic icthyosaur Icthyotitan, and its long, unhinged jaws and small but sharp teeth support a wide system of baleen rakes that can be opened or shut to trap in hundreds of fish in a single gulp. Like all mosasaurs, Eobasilosaurus is oviparious, giving birth to about 4 to 6 offspring, and the young stay very close to their mother until they reach the age of 10 and wander off to forage on their own. The species is also highly territorial, with adult males patrolling vast swathes of open ocean hunting or mating grounds and fighting off intruders with their toothy jaws and slaps of their large flippers and tails. With a head and flipper length ranging from 4.5-5.5 meters and a total body length of 34-35 meters from the tip of the snout to the end of the tail, Eobasilosaurus giganteus is not only the largest marine reptile of the Cretaceous, but also the largest Mesozoic reptile ever to have existed.