Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals.

In other words: On land animals become (whether they are eaten or not) oil eventually which we make plastic out of and use to run gasoline and diesel engines and a special grade of  kerosene for jet engines.

So, it is ONLY in the oceans where you can find more easily fossils of ocean going animals still preserved for millions and millions of years for some reason.

Which is also interesting to think about because it isn't just dinosaurs and elephants and lions and tigers that we make gasoline and diesel out of, it is also all our ancestors back for thousands and thousands of years who died with these animals as well in unmarked graves. Because this is what oil actually is: human and animal remains condensed underground over millions of years into oil which is what we have been burning now in planes and cars and ships for more than 100 years now (maybe 150?)

Before that we only had steam engines that ran on wood or coal (coal is a hardened form of Oil I think). For example, you can make oil out of most kinds of coal through various processes. So, theoretically you can make gasoline out of coal even though it might be a lot of work to do. I think shale oil is a form of coal that is easier to process into oil and gasoline and diesel I believe. Here is more about this:

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Cannel shale has terrestrial origins, and is often classified as coal. It is made up from the remains of resin, spores, and corky materials from woody plants. It can contain the minerals inertinite and vitrinite. Cannel shale is rich in hydrogen, and burns easily.Mar 4, 2013

oil shale | National Geographic Society

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What is the difference between oil shale and coal?

Oil shale contains a lower percentage of organic matter than coal. ... At the same time, the organic matter in oil shale has an atomic ratio of hydrogen to carbon (H/C) approximately 1.2 to 1.8 times lower than for crude oil and about 1.5 to 3 times higher than for coals.