(145 mya to 66 mya) was similar to the Jurassic. The climate was still warm, with numerous shallow inland seas. Dinosaurs continued to dominate, and mammals and birds continued to diversify. Angiosperms, or flowering plants, appeared for the first time and quickly became the dominant group of plants on Earth. North America was split lengthwise by the Western Interior Seaway, which was dominated by marine reptiles called mosasaurs. Some of the most well-known dinosaurs that walked the land included Tyrannosaurus, Velociraptor, and Triceratops. This Period, and the Mesozoic Era, ended with the namesake-Paleogene Mass Extinction. A worldwide layer of iridium dated to 66 mya sparked the Álvarez Hypothesis, which posited that the extinction was caused by a huge impact event. This idea was strongly supported by the discovery of the massive Chicxulub crater on the Yucatán peninsula. 75% of all species went extinct, including all non-avian dinosaurs and large terrestrial animals.