Fish Fin?

[Kosmo]

I live in Fountain, Colorado (south end of Colorado Springs), at the base of the Rockies.  I found a perfectly preserved sea shell a couple years ago had me think it might be ancient; a fossil from when the region was part of the Western Interior Seaway.  And that shell (not shown in these photos) has me "browse" the ground on occasion.  Recent earth movers' excavation of the same grayish colored earth I found the shell had me looking for fossils the other day.  And in relatively short order, I found this.

My first thought was...fish fin, but the small (1-3 mm), whitish fragments of what might be shell fragments seem odd, combined with the fact that whatever it is, is more than just a surface imprint.  See 2nd photo showing how it's got some thickness associated with it, unique to the rest of the rock.  The 3rd photo shows the lines curling down along that side of the rock.

Though I've watched a fair amount of shows on archaeology presented on channels like NatGeo, Discovery, etc., showing the process of removing rock from fossils as very tedious, I'm as amateur and unexperienced as it gets with such processes.  So I'm reluctant to try to remove any of the attached rock, which looks like might reveal more of the curved (and underside) of whatever it is.   It's heavy for its size and the rock is too hard and dense to easily break or scrape away.

[Coco]

Hi,

 

I think it is a part of an ammonite.

 

Coco

[ted coulianos]

Possibly part of an ammonite as Coco said; but I'm getting more of a bivalve vibe, something like the impression left by a scallop or pecten.  Much too fragmentary to make a positive id.

[Kosmo]

Very interesting.  What's odd to me is how the ridges go straight through what appears to be small fragments of...(whitish colored) shell?  As if most of it is just an impression in the rock but is seen (uninterrupted) through the white spots.

I'm tempted to try to gently pick away at (could use a good dentist's tool or two) the rock around the more curved area... 

[Mark Kmiecik]

17 hours ago, Kosmo said:

I'm tempted to try to gently pick away at (could use a good dentist's tool or two) the rock around the more curved area... 

 

I don't think you'll find enough to make any difference. I would venture that about 95% of what is there is already exposed.