The "that's a long time" for me occurred when I learned about the Mississippian Period (named after Mississippi in the United States) - part of the Carboniferou

The "that's a long time" for me occurred when I learned about the Mississippian Period (named after Mississippi in the United States) - part of the Carboniferous period, and only really useful within the US because the rock beds within that period are different than the adjacent Pennsylvanian period rocks. In this period, you've got crinoids - related to starfish (and still around - https://youtu.be/ror_fFswejM https://youtu.be/ror_fFswejM ). The classic ones had "stems"... https://www.wired.com/2008/03/friday-field-foto-46-crinoid-fossil/ https://www.wired.com/2008/03/friday-field-foto-46-crinoid-f... Look at all those circular things in that fossil that are scattered about - those are parts of stems that aren't intact. The Mississippian layer is up to 460 feet (140 meters) thick with these fossils. The crinoid fossils are so common you can buy them cheaply in bulk - by the pound. https://www.dinosaursrocksuperstore.com/collections/bulk-fossils-1/products/crinoid-stems-genuine-fossils-by-the-pound https://www.dinosaursrocksuperstore.com/collections/bulk-fos...