Possible Fish Fin North York coast?

[Chris Kimo]

Good morning guys! I’ve recently been exploring the coast of North York and found ammonites in abundence I also split open a rock to reveal what appears to be a fish fin of some sorts can anyone clarify this for me!? It would be greatly appreciated! I’ll also post some pics of the anmonites found and if anyone knows what species they are could you inform me please! Thanks for your time! Also sorry about the poor lighting of the ammonite picture Hope they can still be made out.

[Rockwood]

I think the characteristics are that of shell rather than fin. 

[Randyw]

+1 for shell

[TqB]

I think it's a piece of the ammonite Phylloceras heterophyllum - it can grow very large and then the fine ribs don't show a lot of curvature. I think you also have two of the larger undulating ribs that  show on the outer whorls of big specimens, as in this photo:

 

From this thread http://www.thefossilforum.com/index.php?/topic/81338-big-ammonite-found-in-whitby/&page=2

 

 

 

[RJB]

 Im with all these other guys.  My first impression was fine ribbing of an ammonite.

 

RB