In the deposit, amid the mastodon’s bones, Gustafson found a rib that was different from the others.

Most of the bones, however fragmentary or complete, showed the kinds of modifications that time and sediment pressure produce: mineral replacement, cracking along natural stress lines, distortion from decades or centuries of soil movement above the remains.

Normal taphonomy — the study of what happens to organisms after death and during fossilization — could account for most of what was visible.

This rib had something else.

Embedded in its outer surface, penetrating through the cortical bone and into the interior of the rib, were fragments of dense bone material that had not come from the mastodon itself.