Shell Fossil? (Near Fort Collins, CO)

[IonRocks]

So just wandering around my yard I discovered this. Not even sure what it would be, but even if it's not a fossil someone may have some ideas about it.

The age of this rock is unknown to me, only useless thing I know is that it was found at about 5280 ft in elevation, northeast of Fort Collins near Cobb Lake.

(positive and negative)

Edited June 10, 2016 by IonRocks

[CraigHyatt]

My opinion: nonbiological limestone concretion. See what the experts say.

Edit: what are the dimensions?

Edited June 10, 2016 by CraigHyatt

[Fossildude19]

Looks like it may have been a shell, but not sure you will get an ID with it in such bad shape.

The matrix looks like a sandstone or a marl of some kind.

Is it really hard or soft?

Regards,

[IonRocks]

Matrix is very soft and crumbles (as well as sand comes off of it when you crush over it)

(About an inch long)

Edited June 10, 2016 by IonRocks

[Jeffrey P]

I recall that fossils of bivalve shells are found on the south side of Fort Collins in sandstone, but what you have

there is too poorly defined to ID properly.

[CraigHyatt]

Now that the experts have spoken, I took a closer look. Do you see evidence of longitudinal (or radial) ridges along the margins? I think I see some at the bottom of the first photo and at the top in brown in the second photo. That, along with the general shape and size might possibly narrow it down.

[IonRocks]

(I will get more details on it later)

To me it looks like if it would have come off with the white she'll part together, and with the current positive being the negative, then it would probably be more identifiable haha!

[CraigHyatt]

I've played with that kind of soft, crumbly sandstone. If you soak it in water, you can carve it away with a pocket knife. This would be what we call a "destructive test". :-)

In my case, I was chopping up a sandstone cast, so I didn't care about preserving anything.

[Guguita2104]

I would say mollusc shell, probably bivalve.

However, as the other fellows said, it's not in a good shape for a proper Id.

Regards,

[IonRocks]

How would I go about figuring out the age of this sandstone? I am looking for more fossils, but so far I think j haven't found any yet, other than what might be some sort of tube/burrow (which I do not think is a fossil)

[CraigHyatt]

How would I go about figuring out the age of this sandstone? I am looking for more fossils, but so far I think j haven't found any yet, other than what might be some sort of tube/burrow (which I do not think is a fossil)

Other posters here gave me some ideas for dating a fossil bed:

1. Google around and find USGS maps and papers for the local area where you want to hunt. These are usually PDFs you can download along the lines of this (maybe not for your area): http://pubs.usgs.gov/pp/0503c/report.pdf. Here's some starter links:

http://geosurvey.state.co.us/Pages/CGSHome.aspx

http://www.blm.gov/co/st/en.html

http://www.westernpaleo.org/

2. Look for "index fossils" that universally date a fossil bed. In my case, confirming Late Cretaceous, I found Sphenodiscus ammonites.

3. Google around to see what other fossil hunters have found there. Check out local fossil shops. That gives you an idea what to look for. Here's one: http://www.thefossilforum.com/index.php?/topic/11892-northern-colorado-front-range-fossils/

4. As you find stuff, post it here and the experts will help identify it. That gives a clue as to the type environment and age. Go ahead and post what you find, even if you think it's nothing. About the first 5 or 6 things I posted weren't fossils but burrows, dendrites, concretions, chert, bubbles in limestone, Liesegang rings, differential weathering, etc. Might as well post them and get them out of the way. :-)

5. There's a lot of good advice in the pinned "Fossil Detective" post on this board.

6. (Edit) Look for places where a lot of layers are exposed, e.g. along highways, at construction sites, where water has eroded through layers. More layers increase the chance of a hit... especially if you see goodies on the surface. Check with your DOT website about fossil regs and politely ask permission on construction sites. I just stay far away from the hard hat areas. :-)