Today's world is remarkable for a lack of dinosaurs. Big ones, anyway. Those critters would have been among the first things someone would notice here for upwards of 200,000,000 years.
Then, about 66,000,000 years back, something awful happened. The only dinosaurs left are those little tweeting, chirping, and cawing critters we call birds.
Ammonites had been around for even longer, but whatever finished the 'thunder lizards' wiped them out, too. We showed up much more recently, and are learning that there's a very great deal of our past, and Earth's, that we don't know. Not yet....
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