👉 A hydromastodon is a type of extinct dinosaur that lived during the Late Cretaceous period, around 68-79 million years ago. It was a small theropod, with features such as a long neck, large teeth, and short legs. The term "hydromastodon" likely refers to its resemblance to the large, muscular hydromasts that are found in the ocean, which might have been used for swimming or diving.