Umoonasaurus, an extinct plesiosaur from the family Leptocleididae, once inhabited the Early Cretaceous period, approximately 115 million years ago, during the Aptian-Albian stages. Its domain covered parts of what is now Australia. This was a relatively diminutive creature, measuring around 2.5 meters (8.2 feet) in length. One of Umoonasaurus's distinguishing features was the presence of three crest-ridges on its skull.

In terms of anatomy, Umoonasaurus was a small plesiosaur, a marine reptile equipped with four flippers. The holotype, typically the standard individual for this genus, was believed to have reached a length of 2–2.5 meters (6.6–8.2 feet) and weighed about 70 kilograms (150 pounds). Umoonasaurus had a small, triangular skull with crests that were most likely covered by keratin during its lifetime, thus making them substantially taller. These crest structures, although seemingly too delicate for functions such as defense, intraspecific combat, or skull reinforcement, were probably employed for communication.

It's plausible that these crests served dual purposes in species recognition and courtship displays and could have exhibited vibrant colors when the creature was alive. Examining the remains of the type specimen, scientists found 60 gastroliths (stomach stones) and 17 isolated vertebrae from an unidentified teleost fish, indicating that Umoonasaurus had a varied diet. This particular fish likely ranged between 182 and 296 millimeters in length. This suggests that Umoonasaurus primarily fed on pelagic prey from the middle trophic level but also opportunistically targeted benthic prey. This dietary insight reinforces the notion that Umoonasaurus preferred smaller prey and lacked specific adaptations for hypercarnivory.