Fossil plants of the Mesozoic Era represent two distinct groups, gymnosperms and angiosperms. Gymnosperms have naked seeds, and most are cone-bearing. They include conifers, ginkgoes, and cycads. These gymnosperms evolved in the late Paleozoic Era and were dominant into the early Cretaceous Period. Angiosperms have covered seeds and are flowering plants. They became the dominant plant group during the Cretaceous Period and continue to be so today. 

Many organisms lived in the warm seas of the Mesozoic Era. Some of these were floating microorganisms, including marine algae and protozoans. These organisms produced the main ingredient of the sedimentary rock called chalk. The Cretaceous Period was named for creta, the Latin word for chalk.

Sand dollars, snails, clams, and ammonites (shelled squid) developed during the Mesozoic Era. In addition, numerous kinds of fish, amphibians, and reptiles flourished. Dinosaurs, a group of giant reptiles, ruled the land. Dinosaurs lived only during the Mesozoic Era. These reptiles died out completely at the end of the era. The first warm-blooded mammals appeared during the late Triassic Period, and the first birds appeared during the Jurassic Period.