Why would polar bears not make good index fossils?

Polar bears would not make good index fossils because they have a relatively short geological lifespan, as they only evolved around 500,000 years ago. Additionally, their limited habitat in the Arctic makes them less widespread compared to organisms that thrived in diverse environments. For a fossil to be considered a good index fossil, it should represent a species that existed over a broad time frame and across various locations, which polar bears do not.