Wednesday, July 8, 2020

Partial placoderm plate from the Arkona formation

I have never found much in the way of fish remains at Hungry Hollow, Ontario, but once I did find a small fragment from a bony placoderm plate.  Placoderms were early, shark like fish that had external bony armor around their heads. There are a few species known from the middle Devonian aged rocks found at Hungry Hollow (Arkona formation and Widder formation) but this is just a fragment so I doubt I could narrow the species down.

I found this piece near the base of a slope of eroded Arkona formation and could not find any trace of more uphill. You can tell it is bone due to the pale blue color (evidence of the phosphate mineral Vivianite) and the spongy structure within the piece.

Top surface - this is what would have been the outer facing portion of the bone. Note the bumps, perhaps they helped streamline the fish for swimming purposes?

Two side views of the bone showing the spongy nature.

This was the interior side of the bone

I collected this specimen from the Arkona formation which is Devonian in age, Givetian stage.

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