Here's why:

* Trace fossils are not the actual remains of an organism, but rather evidence of their activity. This includes footprints, burrows, nests, and even fossilized feces (coprolites).

* Body fossils, on the other hand, are the preserved remains of an organism's body, such as bones, teeth, shells, or even preserved soft tissues.

Since the footprint is a sign of an animal's activity, it fits the definition of a trace fossil.