Can you please identify this weird bone or shell

I found this in a small stream on my property in Conifer, Colorado. We are 15 miles west of Denver in the foothills, at 8,150ft elevation. I have done some internet searching, I cannot determine what it is.

First, is this a shell, bones of a fish, or a skull? Are those horns, antlers, or fins? Is this from a baby animal or full grown?

It measures about 3" long, and 2.5" wide with the "horns".
I have changed the settings on the web album to make sure its public. View photos of the specimen here:
https://picasaweb.google.com/leehoffman/SkullSkeletonShellPhoto?authuser=0&feat=directlink

List of colorado wildlife:
http://wildlife.state.co.us/WildlifeSpecies/Profiles/Pages/WildlifeProfiles.aspx

You'd probably have to consult a professional zoologist who's well versed in the local wildlife to get a definitive identification. Even then, it may be difficult. The photographs you have are photographs of a skull fragment. More specifically, they're the cranium, the upper part of the orbits of the eyes, and a bit of the sinus cavity. The lower part of the bones encompassing the eyes, frontal part of the sinus cavity, lower jaw, and dentition are all missing from your specimen.

That said, given its size and overall shape (wide exterior orbits, no prominent tooth ridges on the outside of the skull, but small curved indentations in the middle of the bottom which are most likely where teeth were attached, etc), it looks like part of some type of rodent skull.

I was thinking more of a large rodent such as a marmot. I know people often aren't used to thinking of them that way, but porcupines and beavers are also rodents, and the largest rodent (the capybara) can grow to more than four feet long and weigh well over a hundred pounds...

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Not that there are likely to be any capybaras eight thousand feet up in Colorado, or anywhere else in the North America, for that matter, but there are definitely rodents in Colorado large enough to have a three inch skull.

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It is definately a well-preserved fossil of an ancient marine creature. Please consult a professional paleontologist! I thought it might be a trilobite, but it is not. It might be related though.

Here is a site on trilobites and one on marine paleontology:

Can't say for sure, but the "horns" are incomplete broken off cheek bones.
The photos are a bit dark, makes it hard to really see the anatomy.
I'm thinking badger.