As a result of continued warming, the polar ice caps melted and much of Gondwana became a desert. In Eastern Antarctica, seed ferns or pteridosperms became abundant and large amounts of sandstone and shale were laid down at this time. Synapsids, commonly known as "mammal-like reptiles", were common in Antarctica during the Early Triassic and included forms such as Lystrosaurus. The Antarctic Peninsula began to form during the Jurassic period (206–146 Ma), and islands gradually rose out of the ocean. Ginkgo trees, conifers, bennettites, horsetails, ferns and cycads were plentiful during this period. In West Antarctica, coniferous forests dominated through the entire Cretaceous period (146–66 Ma), though southern beech became more prominent towards the end of this period. Ammonites were common in the seas around Antarctica, and dinosaurs were also present, though only three Antarctic dinosaur genera (Cryolophosaurus and Glacialisaurus, from the Hanson Formation, and Antarctopelta) have been described to date. It was during this era that Gondwana began to break up.