From the Super Mario Wiki
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The Cretaceous Period is a period of time that spans seventy-nine million years, from the Jurassic Period to the Paleogene Period. During this time, flora and fauna alike grew in size and diversified. In Mario's Time Machine, Bowser travels back in time to about eighty million years ago and steals a Dinosaur Egg, and Mario later travels back in time after taking the Dinosaur Egg back. After Bowser entered the area, it became infested with Koopa Troopas and Bodyslam Koopas that try to stop Mario (although there are also regular dinosaurs in the background). Eventually, Mario makes it to a cave (at the far-right of the area), drops off the Dinosaur Egg, and time travels forward to the present.
The good endings for the SNES and PC versions of Mario's Time Machine also feature the Cretaceous Period. If everything is done correctly, Bowser attempts to use the Timulator to escape Mario, but it malfunctions and sends him back to the Cretaceous Period. Bowser is soon squished by a large dinosaur. In the PC version, his crushed remains are then thrown away like a flying disc. Exclusively in the PC version, Mario also ends up here if he enters an incorrect date in the Timulator. He can only escape by catching some falling eggs from a Pterodactyl while Bullet Bills attack him. After catching enough eggs, Yoshi appears, and he hops onto him and travels back to the present.