Essential collectibles #6: Green River fishes

So far our essential collectibles have been invertebrate fossils, remains of animals without backbones such as ammonites and trilobites. Vertebrate fossils are much rarer, and consequently more expensive, the pocket money-priced ones tending to be things like sharks’ teeth or small fragments of reptile bone. Certainly, even a small fully articulated dinosaur skeleton is the sort of thing that’ll set you back many thousands of pounds! But exceptions to this rule can be found among the Green River fishes, well preserved fossils with all their bones in place thanks to the rapid burial and fossilisation of the original animals.

Of course there are rare species like stingrays that command high prices, but individual Knightia and Diplomystus can be had for a few tens of pounds, while the less common Priscacara goes for a little bit more. Either way, these are fish most collectors end up with sooner or later, whether bought for themselves or received as gifts.

Let’s start by explaining why the Green River Formation is so productive. While the name refers to a nearby modern-day river, the fossils themselves were formed in a series of three large freshwater lakes that existed during Eocene times and spanning the states of Colorado, Utah, and Wyoming. The lakes were part of an extensive mountainous environment across this part of North America, and at various times a range of different sediments were laid down in the lakes, including sandstones, limestones and shales.

Of particular interest to us though are the two muddy limestones where conditions were ‘just right’ for fossil preservation. The key to this was the lack of oxygen at the bottom of these very deep lakes, so when animals died and sank to the bottom bacterial decay was effectively halted. Lack of oxygen also meant an absence of scavengers that might have disturbed and disarticulated the fish skeletons, and the very fine sediment particles meant that the quality of preservation was extremely high. In short, the fossiliferous beds of the Green River Formation are classic examples of what palaeontologists call lagerstätten.

The two comment fish fossils are Knightia and Diplomystus, examples of what fish biologists refer to as clupeomorphs, a group that includes the herrings, anchovies and sardines. Today the clupeomorphs are mostly marine fish, but there are some species, including shad and alewives, that spend all or part of the lives in freshwater. Knightia and Diplomystus would seem to be strictly freshwater clupeomorphs, and like their modern counterparts Knightia in particular appear to have been small, schooling fish that feed on planktonic algae (Diplomystus, oddly enough, was big enough to take small fish, including Knightia).

It’s a basic law of ecology that the higher up the food chain you are, the smaller the population a given habitat can support. That’s why top predators like killer whales and tigers are comparatively rare animals. Herrings are among the few fish with gill rakers fine enough to trap planktonic algae, which are the producers at the very bottom of aquatic food chains. That means herrings have the potential to exist in huge numbers, and that was certainly true for Knightia, far and away the commonest fish fossils from the Green River Formation.

In turn the large populations of Knightia were able to support a considerable variety of predators, including the perciform fish Priscacara. At least two species are recognised within this genus, and while certainly perches in the general sense, they have usually been placed in their own family, the Priscacaridae, though recent research has placed Priscacara firmly within the family Percidae as true, if primitive, perches.

Priscacara were by no means the only predatory fish in the lake, or even the largest, though they were certainly among the commonest. Of course Priscacara were eaten by a wide range of larger fish species including gars (Lepisosteus) and bowfins (Amia) all but identical to species still to be found in parts of North America. There were also a host of other fish-eating animals as well, including alligators and turtles.

Because so many of the animals known from the Green River Formation are similar to ones still around today, it’s tempting to equate the Green River Formation environment with that of the modern Great Lakes spanning the Canada-USA border. But there were significant differences. For a start, the presence of alligators and freshwater stingrays implies a relatively warm climate as those animals cannot tolerate frost, let alone extended periods of cold. Fossil leaves indicate the surrounding forests included not only things like sycamores and ferns but also warm climate plants such as palms. Evaporites, sediments rich in salt left behind with the water evaporated away, indicate periods when the lakes dried up, and along with other lines of evidence, a picture emerges of lakes that changed considerably over time, not just in terms of size and depth, but also salinity, to the degree some of the lakes became saline and lifeless bodies of water more like the Dead Sea than one of the Great Lakes.

So while fossil Knightia aren’t valued as rarities, they are extremely informative. The fine details preserved tell us about the anoxic conditions at the bottom of the lake and the fine sediments that quickly smothered the corpse. As herbivores of a sort, these fish occupied a position low down on the food chain meant they could exist in huge numbers, and by dint of that success they were able to support a whole range of predators that either fed directly on them, as with Diplomystus, or further up the food chain. Finally, the fact Knightia can only be found at certain horizons tells us something about the dynamic nature of giant lakes. They might seem unchanging on the human timescale, but on the geological timescale they do change, from being full of life at one point, then hypersaline and dead the next, perhaps even evaporating away completely some time later.

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