I've seen this question asked before and the answer was we don't know because of the poor fossil record.

I'm unsatisfied with that as it seems that other means like sedimentation might be a starting point to get some idea. Furthermore, we have some idea of the productivity of Earth's land surface now and may have a reasonable idea of how much of the Earth's surface was productive in the Eocene for example.

I've learned that 30% of the current Earth's land is "productive" which means as I remember 4kg of biomass per square meter. I'm speculating that perhaps 90% of the Earth's surface was "productive" during the Eocene. And making a presumptive guess that Earth's biodiversity was an order of magnitude higher during that time compared to today.

The only reference I have found is the book by Donald R. Prothero titled The Eocene-Oligocene Transition: Paradise Lost.



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