However, it's a safe bet it came from the Green River Basin, which is known world-wide as a tremendous source of excellently preserved fish and other fossils. The thing that's kind of special about this particular piece is that there is a single nice fossil on each side. In the above picture, there is a third head above the head of the complete fossil.
I just went looking for some links to stick in here, and a photo of a Green River fish is the first one listed at the wikipedia page for Lagerstätte. If you want more, a Google search for "Green River Fossils" produces nearly 400,000 hits; some of the commercial dealers on the first page have some very nice material, for which they are asking some very nice prices. There are also some archives if you just want to look at pictures.
Above is a crop from the second picture, showing the incredible detail preserved in these fossils.
The other thing about the Green River Formation that you should know is that it holds an enormous amount of fossil fuel in the form of oil shale. Quoting the oil shale article at wikipedia, A 2005 estimate set the total world resources of oil shale at 411 gigatons — enough to yield 2.8 to 3.3 trillion barrels (520 km3) of shale oil. This exceeds the world's proven conventional oil reserves, estimated at 1.317 trillion barrels (209.4×10^9 m3), as of 1 January 2007. The largest deposits in the world occur in the United States in the Green River basin, which covers portions of Colorado, Utah, and Wyoming; about 70% of this resource lies on federally-owned or -managed land. Deposits in the United States constitute 62% of world resources (...)In other words, the amount of oil in oil shales is at least double that known as petroleum reserves, and of that amount, the US has a bit less than 2/3 of the total. When "peak oil" deniers start talking about how there's enough oil to last for a century or more, this is what they're referring to- though they don't seem to actually know that, based on conversations I've had. I'm not necessarily against extraction of oil shale, but it's not as simple as all that. It is much more energy intensive to extract than crude, and creates much more negative enviromental impact. So even if you don't account for the enviromental costs, simply extracting it and converting it to a form useable by current technology means it will be much more expensive than traditional crude. It was looking like a hot area last summer when gasoline was at $4.oo a gallon; now... not so much.
Though I haven't actually read anything on this topic, I suspect that the low-oxygen, high-organic-carbon environment that led to the development of the kerogen-rich "oil" shales (technically, it isn't oil, and technically, the rock is more silty than shaly) is the same set of factors that allowed for such marvelous, highly-detailed preservation of fish and other organisms. And it is quite possible that in the future, a distillate from rocks like this one will be making your car run.
The Green River basin also has a hypothsized connection to my home turf, which I describe in the third from last paragraph in this post. Sorry I don't have a reference for you geology types out there; it was the cover article from a GSA Bulletin in the late 80's/early 90's.
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