According to our current understanding, the modern mammalian jaw and teeth appeared around 195 mya. If a fossil like that were found that was 300 million years old, it would either suggest that this had evolved more than once convergently (less likely) or that the fossil record had been incomplete and that true mammals had appeared much earlier.
Work Step by Step
Please see Fig. 25.7 for the information on our current understanding. Logically, if an older fossil is found with true mammalian features, it either has to be convergence (review in earlier chapters-- this is a much less likely explanation) or that the fossil record as known was missing important evidence seen in the newly discovered fossil.