Of course, elephant bones were present and recognized as such by those who were familiar with them.
I recently was thinking about "elephantos, India quorum milibus e multis vallo munitur eburno" "Elephants, many thousands of which fortify India with an ivory wall " (DRN 2.537), I always thought this "wall" was just poetic. I still do, but there is this:
Dio Chrysostrom (Discourse 79.4) “Again, if ivory is a marvelous possession and worth fighting for, the Indians are of all men most blest and pre-eminent by far, for in their land the bones of the elephants are tossed aside and no one troubles to go near them, just as in our land the bones of cattle and of asses are treated; they even say that in many places the skulls of the elephants, tusks and all, are built into their house walls”