Dr Neale Monks (UK)
Our look at those fossils that commonly bought rather than collected has so far looked at the fossil remains of animals, whether shells, teeth or whatever. But this time we’re looking at a trace fossil; that is, a fossil produced by a living organism but not actually part of that organism. Trace fossils include such things as burrows and footprints, and are of huge importance in telling us about the behaviour of animals in ways their skeletal remains might not. The trace fossils we’re looking at today are fossilised faeces, also known as coprolites.
It might be a surprise to know that faeces fossilise at all, but essentially the same principles apply to them as to any other type of organic material. Provided they are quickly buried in some type of sediment lacking in oxygen, there’s a chance they’ll become fossilised. In some cases the faeces eventually decompose away to nothing, but not before leaving a mould of some sort in the surrounding sediment, so what the geologist uncovers will be a cast of the original droppings, composed of some type of mineral, such as marcasite (iron pyrites).
But often some elements of the original faeces remain, usually the harder and more durable parts that persisted long enough to become mineralised, such as fish scales, fragments of bone, even certain types of plant material. These coprolites are of very great value because if the palaeontologists can determine what types of animals produced those coprolites, they can then say something about what sort of foods those animals were eating.
Ichthyosaur coprolites are good examples of fossils that have helped palaeontologists in this way. In this case the usual contents are fish scales and bones, but some ichthyosaur coprolites also contain fragments of bones from smaller ichthyosaurs. This suggest that big ichthyosaurs sometimes consumed smaller ichthyosaurs, though it’s difficult to say whether we’re talking about adults taking juveniles or bigger species viewing smaller ones as prey.
Cannibalism within species is not uncommon among reptiles, but where this is a major risk the juveniles often adopt behaviours that help to keep them away from the adults. Juvenile Komodo dragons for example spend most of their time up trees, where the adults cannot get them. Did young ichthyosaurs do something similar, favouring shallow water or rocky reefs that afforded them some degree of protection? Or perhaps cannibalism within species wasn’t all that common, and it was the bigger ichthyosaur species that the smaller ones had to worry about?
Spiral coprolites are among the most peculiar examples of coprolites known, so-called because they have a distinctive spiral twist that appears to be related to a structure called the spiral valve seen in sharks, rays, and certain primitive bony fish. There’s some debate as to whether spiral coprolites had actually been evacuated from the animal in question, or whether they’re actually the contents of the spiral valve in situ, with the rest of the animal’s remains now decomposed away. Certainly, sharks don’t normally leave any trace in the fossil record beyond their teeth, and some scientists have noted that both teeth and spiral coprolites can be found associated with each other at some horizons. But many spiral coprolites are fragmentary, which has been taken to suggest that these really were shark or ray droppings, deposited somewhere in the sea, and then broken apart by water currents, scavengers or whatever.
As with the ichthyosaurs, spiral coprolites can be used to determine the diet of prehistoric sharks and rays, but they’ve also revealed a paradox in the quality of the fossil record. In chalk sediments for example, spiral coprolites are far more numerous and more diverse in morphology than any other type of coprolite, and these spiral coprolites seem to have come from sharks and rays. But bony fish, which lack spiral valves, should have been much more common organisms that either sharks or rays, and a lot more diverse as well, so where are all their coprolites? Is there something special about shark droppings that means they preserve better than those of other marine vertebrates? That does seem to have been the case, but until this observation can be properly explained, the overwhelming abundance of spiral coprolites from sharks and rays does seem to be paradoxical.
Dinosaurs were big animals that produced a lot of waste, and it shouldn’t be too surprising that dinosaur coprolites are both abundant and well studied. Tyrannosaurus coprolites contain worm trails indicative of endoparasites similar to modern roundworms and tapeworms, and the presence of whole as well as crushed bones would suggest these dinosaurs wolfed down big pieces of meat without chewing it first.
As well as dinosaurs, the droppings from giant mammals can be studied as well, including such beasts as mammoths and giant ground sloths. One curious example came from a mastodon, an elephant-like animal native to North America. In this case the coprolite was found in Florida and contained a well preserved hazelnut, but hazelnuts haven’t been native to Florida in historical times, being naturally limited to more northern parts of North America. So this coprolite is a piece of evidence that helps to understand how the flora of North America changed over time.
For the collector, the main thing with coprolites is to be able to tell actual coprolites apart from irregularly shaped lumps of rock. The ones sold from reputable traders should be the real thing, coprolites being quite common and requiring relatively little cleaning to be turned into sellable objects, thus there isn’t the same incentive to create forgeries as there is for, say, trilobites.
That said, historically the term ‘coprolite’ has been bandied about a bit loosely, which can be confusing if you’re out looking to find your own specimens. Phosphatic nodules for example have been confused with coprolites many times. This is perhaps most famously the case with the ‘coprolite beds’ of the Cambridge Greensand (Late Albian). While some of these nodules might have been coprolites, most were formed when organic detritus became jumbled together and cemented together with phosphatic material. During the late nineteenth century, these beds supported quite a large industry that extracted the nodules to make fertiliser.