Those look like sansapote (Licania platypus) to me - the size seems about right as well. They are only ripe when they fall to the ground, and they rot very quickly thereafter; if picked from the tree they tend to be very hard, although I couldn't imagine them taking a sledgehammer(!) hit without being pulverized. Still, I guess it would be possible in very immature fruit. I've never heard of anyone actually liking sansapote, by the way - at best people consider it tolerable. I've only seen them on trees, but from what I'm told it tastes either like a poorly flavored pumpkin or a poorly flavored canistel. Still, it is an unusual and interesting fruit to see (if it is, in fact, sansapote). Any other guesses?

You´re right Eldys. In México it hasn´t a very polite common name, though (caca de niño, only wrote it down for scientific matters). ;) Didn´t know it was edible actually.

It looks like the cannonball fruit to me, not too many sunsapotes around. Also, the fruit of the sunsapote tends to form low on the tree and nearly cauliferous in habit.

Sunsapote has one or a few seeds which are very big. You should have seen one if present.

Alan

or malformed sausage tree fruit.

Alan

Sunsapote?