[wbarmy]

could be an early ancestor of either reptiles or mammals

or it could be an early bird, or snake ancestor, or even an early Democrat. What the heck do we know from a rock. Even the 300,000 year age is a total WAG from some other fossils, seashells, in the area. They have a big book of all kinds of seashells and date the fossil from the seashells found in the area.

The seashells have an assumed age from the idea of how many mutations must have occurred to make them look different than other sea shells.

That assumption goes all the way back to the first ideas about mutations and has never changed. Rock radioactive dating is selectively done to reinforce that original dating.

[JBW1949]

If it voted last year, it’s definitely a democrat....

[BroJoeK]

"Rock radioactive dating is selectively done to reinforce that original dating."

There are dozens of different methods for dating ancient materials, including radiometric and fossil comparisons.
For some sites several methods can be used to verify each other.

When multiple methods produce similar answers then confidence levels are pretty high.
When fossils of the same kind are found in different strata many miles apart, then it's reasonable to suggest those strata are roughly the same age.
Especially when confirmed by other dating methods.

It's how science is supposed to work.