Found this in rocks at Lake Ontario

[MrG]

I found this rock and smashed it open while taking a walk at Lake Ontario.  What I found inside seems rather interest.  It is sort of egg shaped but what got my attention is 3 near parallel lines in the possible pattern of a bird or reptile foot, maybe too.  Possibly branching lines like blood vessels or roots?  It's not very big, maybe a couple inches in length at most and a good part of it is still embedded in the rock.  I don't know if there may be more embedded in the rock.  I couldn't identify the other fragments that came off the rock after I broke it open unfortunately.  I'd love to hear some opinions.

 

[Meganeura]

Very much a newbie when it comes to ID'ing Ontario fossils beyond Crinoids (Which is funny cause I grew up there) but that looks to me like a Trilobite, possibly? Or at least the head?

[Fossildude19]

Looks more like an internal mold of a gastropod, to me.

[Mark Kmiecik]

Gastropod mold was my first impression as well. It is a fossil, just not the one you expected.