Mr Wood 's Fossils - mwfossils@blueyonder.co.uk

About 50 million years ago, when Europe and America were closer, and part of the American continent was still under sea, 3 enormous lakes covered the present states of Wyoming, Colorado and Utah. During their 10 million year history enormous quantities of fresh water sediments were deposited in these lakes and their fine consistency created extraordinary favourable preconditions for the preservation of plant and animal fossils, particularly fish.

Today, fossils are being excavated in these quarries. In recent years many spectacular finds have been made - not only of fish, but also of birds, turtles, small alligators, crabs, countless species of insects, butterflies, palm leaves, small seeds, fruits, blooms and wood. It is one of the great fossil-finding areas in the world, comparable in significance with the Solnhofen quarries and the Monte Bolca near Verona. With the exception of land mammals there is an almost complete picture of the Eocene group. This is the starting point for the development of life in the tertiary period and is therefore particularly interesting for palaeontology.

The Green River fossils are named after the small town of Green River in Wyoming. In 1870 the geologist Hayden described for the first time the extensive, white fresh water layers found there. The complete Green River fossil region extends over a desolate and desert-like mountainous area the size of Switzerland, large parts of which still remain undiscovered.

The main interest of the quarry owners is focussed on fish fossils. About 12 men are exclusively concerned with their excavation, plus each year another 2-3 dozen people will spend some weeks or months digging. The fish of Wyoming are easily comparable with the finest fossils from other parts of the world. Their natural dark-brown colour sets them off distinctly from the light-coloured stone. The fine quality of the depositing sediments means that they are preserved down to their smallest detail; not only bones and fins but even scales are perfectly visible after roughly 50 million years.

Many of these fish can be prepared without any damage to the bones. The rock usually separates on the level in which the fossil is lying, and the fossil is split in to the two parts termed "positive" and "negative". In some quarries the rock splits just above the fossil and it can only be recognised by means of a slightly raised area on the rock surface. The result of such intensive and time consuming preparation is very rewarding, producing some of the finest examples of fossil fish.

The fish are not evenly distributed throughout the layers of stone, so fossil hunting, although exciting, is hard work for a very modest yield. A typical day's yield would consist of several small fish and one or two of a medium size. In all, 17 different fish species have been located in the Green River area. There are 5 common species - Knightia, Diplomistus, Priscacara, Mioplosus and Phareodus. Together they constitute 90% of the excavated fish. The rest are rare to very rare.

KNIGHTIA

Knightia is the most common fish found. The average size of the fish varies from dig to dig and gives rise to a number of names. It still has not been established whether these are different species or a different sub-species. As the Green River rock formations were formed over a period of 10 million years, one species could have easily changed its appearance during this time.

The size of Knightia varies between 3 - 5 cm. The fins on all parts of the body are roughly of the same size and the scales are often well-preserved. On the back between head and back fin, and on the belly, there is a row of bone plates which probably would have acted as a kind of armour plating.

Knightia belongs to the herring family (Clupeidae). Like the present representatives of this family (herring and sardine), they would have lived in enormous shoals. Evidence of this is found within the rock, where large numbers of fish are found grouped together. Knightia fed on planktonic organisms and in some well-preserved fossils the stomach contents can actually be recognised. Countless large predators in the Green River Lakes would have preyed on Knightia; their abundance would have made them a very attractive food source.

DIPLOMISTUS

Diplomistus is the second most frequent fish of the Green River Formation. Its size varies from very young fish a few centimetres in length, to enormous specimens measuring 60cm. Characteristic features include rows of bone plates with serrated edges on the back and belly; relatively small fins, and an extended dorsal fin which reaches as far as the root of the tail fin. It also had a protruding mouth and a distinct "lump" is seen on the back of fully grown specimens. The small scales of the fish are mostly indistinctly preserved, whereas the skeleton of bones is clearly visible.

Diplomistus, like Knightia belongs to the herring family. Layers of rock showing mass deaths, indicates that they also lived in shoals. The young fish were eaters of plankton, the older ones hunted small fish in the open water. Fossils of Diplomistus have also been found in the chalk rock of Lebanon and in the tertiary rock of Brazil and Australia. Close relatives of these fish live today in the seas off Peru and off the East of coast of Australia.

PRISCACARA

This is probably the most popular Green River fish and can instantly be recognised by the strong spines on its' dorsal fins. It is known as the "sun-perch" by the locals because of its oval, sometimes almost round, body.

The length of this type of fish is normally about 15cm, but there are also quite small specimens about 3cm in length. In exceptional cases they attain 30cm or more. The conspicuous dorsal fin usually consists of 10 fin spines which in cross-section have a V-shaped indentation towards the back. Such indentations are found in fish with poisonous spines and it is therefore quite possible that Priscacara was a poisonous fish. Adjacent to the dorsal fin spines are the soft fin rays. These are used to distinguish between the two distinctively different species of Priscacara. These fish have also been found on another excavation near Horsefly in British Columbia, Canada.