Is this a coincidentally symmetrical bone fragment, or something more significant?

[Chippewa]

This was found on a dive in Venice FL, where there are lots of shark teeth, whale, dugong fossils, as well as land mammal fossils (mostly horse, mammoth).  

(As most of you know, this area transitioned from sea bed to dry land numerous times as ocean levels fluctuated, so there is a mix of land and marine animal deposits on top of one another)

I almost discarded this one, but decided to query anyone interested for some help.  It seems to be more than a fragment, but I'm not sure.  It's obviously quite symmetrical.  The "bottom" is slightly concave.  There is a distinct seam detail at the apex of the "top", but photo doesn't capture it very well.  Any thoughts on this guy are appreciated.

Thank you,

 

[M Harvey]

I don't know what it is but the shape reminds me of a human patella.   

[Rockwood]

Or a worn epiphysis.

[hemipristis]

ray denticle?

[Rockwood]

Or even a turtle bone ?