What is the X in X-mas?
in reference to "geological period between the Triassic and the Cretaceous," 1847, from French Jurassique, literally "of the Jura Mountains," between France and Switzerland, whose limestones were laid down during this geological period. Used in English in a literal sense "pertaining to the Jura Mountains" by 1831. The name is said to be from Gaulish *iuris "wooded mountain."
The second and middle period of the Mesozoic Era, from about 208 to 144 million years ago. During this time the supercontinent Pangaea continued to split up and numerous shallow seas inundated the new continents. Dinosaurs were the dominant form of terrestrial animal life, and the earliest birds appeared. Marine life was dominated by ammonites and belemnites, and sponges, corals, bryozoa, and gastropods all flourished. Gymnosperms and cycads were the dominant land plants. See Chart at geologic time.