Palaeobiology

101 uses for fossilized faeces

Fossils are familiar objects by which we can trace the evolution of life on Earth and document past climatic events. Less familiar are the so-called trace fossils — traces that once-living animals have left behind them as a result of something they did during life. On page 680of this issue, Chin et al.1 describe a piece of fossilized dung from Canada that, from its large size and context, can be reliably assigned to a very large carnivorous species of dinosaur, possibly Tyrannosaurus rex.

Trace fossils are examples of fossilized behaviour, and, if they can be assigned to a fossil species, this tells us something about the behaviour of that species. For terrestrial vertebrates, the most common trace fossils are footprints, and many examples of dinosaur2, mammal3 and even human4 footprints have been found. Not as well known — and for obvious reasons much less glamorous — are fossilized faeces or pellets, usually referred to as coprolites. There are many examples of coprolites from mammalian or avian carnivores5, but few have ever been found that could be assigned with any certainty to carnivorous dinosaurs.

The contents of coprolites are the main issue here. These provide direct evidence about how and where the coprolite-producer fed. Ideas on dinosaur behaviour have changed dramatically, and the new discovery not only changes our thinking on how carnivorous dinosaurs ate their food, but it gives us clues about the association between predator and prey. Predators are very effective in sampling their environment, but they are selective, only taking prey of particular size or type. They may transport prey from one place to another, so the more mobile the predator the further a bone may have been removed from where the prey originally lived. The same goes for pollen grains that may be inside the coprolite. These could be derived from the diet of the herbivorous prey, or they may have been trapped by the sticky surface of the coprolite when it was still fresh6. The species content of coprolites therefore depends on the hunting preferences of the predator, assuming that it is the product of hunting as opposed to scavenging. On the other hand, many cases are known where animals turn up in predator scats in places where their presence had never even been suspected.

The theropod coprolite described by Chin et al. is extremely large, and it contains the remains of a sub-adult ornithischian dinosaur. Finding the remains of food in faeces or pellets, or sometimes in the stomach of a well-preserved specimen7, gives a direct insight into what a predator had been eating. Although this is interesting in itself, it becomes still more fascinating when the bones of the prey are examined. All predators modify their prey, either by the way in which they ingest it or by the way that they digest it. By analogy with living reptiles, dinosaurs would have been expected to ingest their prey in relatively large pieces8, so breakage of bone should not be extensive. But this was apparently not the case. The authors found that the theropod coprolite contained many bone fragments, ranging from a few millimetres to less than 100 micrometres in length. This bone was evidently crushed — one could say pulverized — during ingestion, which implies a considerable degree of food preparation before it was swallowed. This may have been similar to the repeated biting of small prey animals by crocodiles, the largest living reptiles, before they swallow them.

But just the reverse happens after ingestion. By analogy with crocodiles9, we can predict that ingested bone would have been extremely heavily digested in carnivorous theropods (Fig. 1). Although some of the small (less than 1 mm in diameter) bone fragments from the coprolite seem to be completely unstructured (Fig. 3 on page 681), the photomicrograph of one of the larger fragments (Fig. 2 on page 680) shows the internal histology to be intact and unmodified. In fact, this bone is in such apparently pristine condition that it might be worth looking for DNA in it. The edges of all the bone fragments are rounded to some degree, but this is little more than would be expected in most mammalian carnivores5, and nothing like the degree of modification seen in crocodile scats.

Figure 1: Photomicrograph of a rodent femur digested by a crocodile.
figure1

Predictions can be made about bones found in fossilized faeces (coprolites), such as those discovered by Chin et al.1, by comparison with bones in the faeces of modern-day crocodiles. Crocodiles have low enzymatic activity during digestion, and their gastric juices are almost pure hydrochloric acid. This results in very aggressive destruction of the inorganic fabric of bone tissues, so only organic collagen fibres remain.

The contrast between the extent of breakage and the low level of digestion is the most surprising finding from the newly discovered coprolite. Not only does it throw new light on the ability of large carnivorous dinosaurs to break down the bones of their prey in their mouths, but it also tells us something about the physiology of digestion once the prey animal has been swallowed. All of that from a piece of dung!

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Andrews, P., Fernandez-Jalvo, Y. 101 uses for fossilized faeces. Nature 393, 629–630 (1998). https://doi.org/10.1038/31356

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