Coping with coprolites
Carl Mehling (USA)
Generally, we have no use for it, or at least we convince ourselves we don’t, conveniently ignoring the fact that faeces of one kind or another (even our own) have fertilised our food for millennia. Organic waste products are an integral part of the living system and don’t tend to sit around for long. And it’s a good thing too, because, without the recycling of waste in Nature, we’d certainly be swamped by the stuff.
Alas, this is a problem for students of coprolites – those droppings from Deep Time – as it reduces the probability of good coprolite fossils. However, everything in Nature has a story to tell, and there’s usually someone eager to listen, whomever or whatever that storyteller might be. I am one of those palaeontologists drawn to make order out of ordure and, thanks to the whims of the fossil record, enough of these now-inoffensive offerings have fortunately survived to the present.
My first coprolite emerged from the Late Cretaceous marine sediments of Big Brook, New Jersey. It was a coprolite of spiral morphology – surprisingly common, once one’s search image is tuned – which my mentors credited to a shark. All ‘experts’, both amateur and academic, reflexively parroted this identification. Later, I learned that other ‘primitive’ fish groups, many present in Big Brook’s palaeofauna, also have a spiral gut capable of producing a similar product – sharks were only one group of spiral coprolite suspects.
As a young scientist, I took this science lesson to heart and, over the years, noticed that coprolite studies are a great place to learn about. As a young scientist, I took this science lesson to heart and over the years noticed that coprolite studies are a great place in which to step gingerly. As seasoned scientists, we have learned, sometimes grudgingly, that first, indeed all, identifications are provisional. And with so little to go on, interpretation of coprolites encourage extra caution.
When an animal defecates, it tends to move on. And what is true now was almost certainly true then. Therefore, for coprolites, identifying the guilty party is usually a distinct challenge. However, clues abound. Size, palaeoenvironment, coprolitic content, geologic age, shape or local palaeofauna can help in the ‘process of elimination’. Diminutive coprolites exhibiting a hexagonal cross-section are sometimes found packed in galleries within petrified wood. These are very likely made by termites – today’s termites leave the same calling card. A huge, terrestrial, bone fragment-filled, Late Cretaceous coprolite from western North America is almost certainly from a Tyrannosaurus rex. There are even rare cases of palaeoexcreter and excretion found in indubitable association: tree resin has encased more than one terrified insect, mid-poop, suspended for all time in an amber embrace.
However, just because something looks like a coprolite doesn’t make it one. The fossil market is loaded with alleged coprolites from the Jurassic of Utah. People who actually study coprolites find little to convince them of the feculent origin of these masses, except possibly from the perspective of the feculence of the seller’s motives. What’s a motivated salesperson to do with amorphous masses of minerals that are hard to move? Slap a lurid and lucrative title on them – “Dinosaur poop!” – and the market is now a dumping ground for the previously hard-to-sell.
Even scientists can have a hard time identifying coprolites. Miocene deposits from Washington State, Upper Cretaceous sites in Saskatchewan and Madagascar, as well as Permian exposures from China abound in objects that certainly look the part. However, debate has raged in the literature about whether or not they are bona fide bowel movements. Does an overwhelming external similarity to faeces qualify them, irrespective of the strata’s absolute deficiency of any other vertebrate remains? Could they be siderite coprolites, or maybe even cololites, because they appear stooly even though their composition and lack of inclusions would seem to suggest otherwise? And if not coprolites, then what? The non-copro camp suggests that they are mud that was squeezed through knotholes in buried trees and later petrified.
Each explanation, seriously lacking in evidence, is basically speculative. But the market cares not, for such items, phony or authentic, are understandably easy to sell. Those from Washington are the most commonly passed as genuine, whether they be confidently sold as “turtle,” “mammal,” or even “dinosaur” dung (never mind that only non-avian dinosaurs were still around in the Miocene). For now, the best description of them remains “excrement-shaped ferruginous extrusions”.
If you can’t dig up your own fossil faeces, and trust me, they are out there, the most trustworthy samples on the market are small, white spoors from the White River Formation of western North America. Undoubtedly, they are shaped like scat, but they are also phosphatic, often contain bony inclusions and are found in rocks with abundant fossil vertebrate body fossils. They smoothly qualify as relict poo.
Among the first coprolites correctly recognised as such were small, spiral, marine examples from England’s fossiliferous Jurassic Coast. The Rev William Buckland, who coined the term “coprolite” in 1829, saw them for what they were and spread the word. Certainly, he must have been aware that these very same productions, originally identified as fossil fir cones, often adorned ladies’ earrings. That market quickly relieved itself of the now-offensive trinkets. However, I would wager that these very same coprolites, now accurately identified, would today be worn with pride and levity.