All the pretty fishes

The Green River Formation of Southwest Wyoming is has to be one of the favourite fossil localities of every fossil dealer on the planet. The many layers of micrite - muddy limestone - that compose the formation contain fossil fish of exquisite detail and in numbers that would make your eyes bulge from your head, pop and dribble down the front of your shirt.

And not just fish - bats, birds, reptiles and even a small horse have been found. These are rare, though, and fetch tidy sums. The beauty of the site is that it offers so much high quality material that there is sufficient to supply the academics with as much as they could possibly need whilst also providing a fantastic commercial line for the fossil trade.

The site(s) have been very well studied over the years and a great deal is known about the geological history. The rocks were laid down as a series of lakes, shifiting over time from about 58 million years ago for perhaps 15 million years. By far the most common fish found is the Knightia, below left, a relative of the modern herring, followed by Diplomystus, above right. Most of the fossil-bearing layers date to around the 50 million mark. There are 10 or 11 private quarries, many offering digging expeditions, and also a National Park, where some of the rarer material is carefully protected.

The amount and quality of the fossil material in the Green River Formation allows great insight into the environment of the area at the time. Leaves, seeds and even flowers from a number of plants can be found, and a good idea of the fauna is preserved within the rock - a snapshot of the area's wildlife.

Wyoming weather permits only a few months fossil hunting each year and the professional diggers make the most of it. They have to strike a delicate balance between keeping their potential finds intact whilst also shifting tons of rock about with heavy machinery. The more valuable finds are often from deposits from the edges of the lakes - mammals, birds, reptiles - and this material is also the hardest to access.

I've been invited to go digging a number of times. I'd love to go, but it would be difficult to justify financially, as I see the dealers at the Tucson show and buy from them there. One day, though.