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About half of the half of the original amount (1/2 * 1/2 = 1/4) of U-235 has decayed into other materials - meaning that only half of its half life has passed - therefore: ~300 mya.
Other forms of dating are: The most common geological methods of dating are the decay of Uranium into Lead, a natural process that occurs in Uranium ore, and the Potassium-Argon method, useful with volcanic deposits.
These elements have much longer half-lives than Carbon, and in some cases can be cross-referenced if more than one of these elements is present in a volcanic tuft.
There are other methods for dating fossils, such as thermoluminescence.
The radioactive isotope Carbon 14 has a half-life of 5,730 years.
This has made it useful for measuring prehistory and events occurring within the past 35 to 50 thousand years.
Of course C-14 would never be of any use for dating dinosaur bearing deposits, unless you want everything to date to around 40,000 years!
Radio-Carbon dating can be used for dates up to ~80,000 years ago.
Usually the radioactive 'clocks' for these elements are started when the elements are deposited by a volcanic eruption (usually in the form of ash).
For example, Uranium (U-235 or U-238) runs into the Thorium series then breakdowns into Radium and Radon, and finally, into Lead (the stable isotope).
Volcanic tuft containing U-235 also contains (stable) Lead associated directly with it.
(The item being tested must be organic based, and must be dead - tests on live mollusks showed an age of 2000 years).
If a fossil is completely replaced (permineralized), then it would be useless in a similar test - because it no longer is organic.