Shark Tooth Fossil

My company just had a haiku session by Zoom. We were challenged to describe something natural nearby. When I made my choice, I didn't realize we'd be encouraged to include a hint at the time of year -- not easy with something that hasn't changed much in millennia. The organizer did not require us to follow the 5-7-5 rule, but I felt like it. The group still rather liked my output, so here it is:

Sea bane's long dread knife,
Now a long-dried-up knickknack,
Trapped in dusty stone.