[GOPJ]

What would cause that?

[NativeSon]

The animal walked upon fresh sediment or some such, compressed the material underfoot.

Uncompressed sediments washed away over time.

[going hot]

What you are looking at is the negative of the actual footprint.

The rock was the sedimentation that filled in the print.

It is now (recently?) flipped over, showing the actual cast of the footprint, rather than the actual foot print, which would be a depression, by definition.