The Straight Scoop on Dinosaur Poop
Assume that every dinosaur pooped. If so, not all of these end products of dinosaur digestion were preserved in the fossil record. But you will have a load taken off your mind when you know that those found thus far have not gone to waste, nor remained the butt of jokes.
So let’s say you found what might be a dinosaur coprolite. After all, it looks like something your dog, your neighbor’s dog, or your neighbor left in the yard, except it’s a rock, and quite large. In your excitement, you dash to the nearest natural history museum or university, find a paleontologist, show it to her, and announce with much fanfare and dramatic flourish, “Behold, a dinosaur coprolite!” Before doing that, though, you really need to be a good little skeptic and go through a checklist that asks the following questions:
Was it from rocks of the same age as dinosaurs (Late Triassic through Late Cretaceous)?
Did it come from rocks formed in a continental environment, such as a former soil, river, or lake?
Were other dinosaur body and trace fossils in the same rocks?
Does it fall in the right size range for known dinosaur coprolites?
Does it contain any body fossils of what might have been digested, such as plant or bone fragments?
Only when you have answered this checklist with “yes” for every item should you take your rock to a professional scientist. Otherwise, she will tell you wearily that wrongly identified “coprolites” are the bane of her existence, rivaled only by wrongly identified “meteorites” and “gold.” In this respect, the most basic questions—dealing with age, environment, co-occurrence with other dinosaur fossils—are very important. For instance, if you found this rock in Cincinnati, Ohio, I would instantly tell you it is not a dinosaur coprolite. Cincinnati’s a great city with a lot going for it, but it has the wrong age rocks (Ordovician Period, 450 million years old) and wrong rocks (shallow marine limestones and shales), and hence no dinosaur fossils. As a result, its civic boosters should never add “dinosaur coprolites” to its list of local natural wonders, which would remain true even if you crossed the Ohio River and went into Kentucky.
Of these criteria, by far the most important one is that it contains body fossils of whatever the dinosaur ate. A suspected coprolite may look like fossil crap, feel like fossil crap, and taste like fossil crap, but does not qualify as fossil crap unless it holds fossil food. There had better be plant tissues, spores, seeds or pollen, bits of bone, insect or crustacean parts, or other bodily remains for a lumpy chunk of rock to qualify as a genuine, bona-fide coprolite.
The least important criterion applied to coprolites is size, and that is because of its variability. Dinosaur dung may have been as big as footballs (American or Australian rules), or it could have been as small as chocolate-covered raisins. Coprolite size would have depended on: the age and size of the defecating dinosaur; its health; time of the year; or what happened to the dung after it emerged. Even the mere act of dinosaur-cloacal pinching would have affected the size of each exiting nugget, meaning the total volume of feces might have been quite high but composed of many pieces snipped by a well-honed sphincter. Of course, falling and landing on the ground would have altered the size and shape of such deposits depending on mass, anal altitude, and relative solidity. Some feces would have gone “thud” and flattened slightly on impact. Others would have gone “splat” and spread out over a sizeable area. Those left by swimming dinosaurs might have been floaters, making no sound at all. Smell would have entered this equation, too, as dung-loving insects or other animals in the area would have picked up on any distinctive odors and hurried to indulge.
Nonetheless, by far the most common question about dinosaur coprolites I’ve heard is not “How do we know that this is dinosaur dung?” Instead, it is “How did it fossilize?” This inquiry is understandable, considering how the closest encounters most urban dwellers have with dung either involves a brief experience in the morning, picking up or stepping on dog doo, or cleaning a kitty-litter box. In contrast, people from rural communities, and especially those who live on farms, cannot avoid feces, as livestock and all other animals leave “land mines” often and everywhere.
First of all, preservation was really helped if these feces already had minerals in them. This means carnivorous dinosaur scat had a better chance of preserving than that coming from insectivores or herbivores because meat eaters were more likely to ingest bone, and bone is composed of apatite. Second, anaerobic bacteria in the feces could have assisted in preserving it, in which their metabolic processes caused chemical reactions that made more minerals precipitate, and do so rapidly (geologically speaking). In a few instances, this bacterially mediated precipitation replaced undigested muscles and other soft tissues, leaving ghostly mimics of these body parts in a dinosaur coprolite. And third, rapid burial, such as from a nearby river flood, would have prevented fresh droppings from getting eaten, poked, prodded, sniffed, trampled, washed away, or otherwise damaged. Under the right geochemical conditions, mineralization also could have accelerated once the feces were buried, as more anaerobic bacteria would have joined the mineralization party.
However, a dinosaur coprolite turned to stone is not necessarily the end of its journey, in which it waits patiently for a well-trained paleontologist to recognize it for its true fecal nature. Coprolites are among the few dinosaur trace fossils—such as toothmarks in bones, and gastroliths—that could have been transported before and after burial, or reburied. Before burial, a dinosaur turd could have fallen down a hill slope, had part of it rolled by a dung beetle, or carried some distance by flowing water. After burial, it could have been exhumed by wind, streams, tides, or waves, and moved to a new place before getting buried again, or not—in which case it might have weathered, eroded, and vanished from the fossil record millions of years before human consciousness. This also means a dinosaur coprolite, like dinosaur bones, feasibly could be reburied in geologically younger sediments. In short, where you find a dinosaur coprolite in the field should never be assumed as representing the same place or time where that dinosaur took a dump.
Given all of these special conditions for preserving dinosaur coprolites, they understandably are among the most precious of dinosaur trace fossils. They are also among the most difficult to attribute to a specific dinosaur. Once a coprolite is identified, paleontologists often limit themselves to saying it belongs to a carnivorous or herbivorous dinosaur based on its contents (bone or plant fragments, respectively). This is where bigger is better, in that large coprolites are easier to connect to dinosaurs big enough to have made them, whereas smaller ones could have been made by a wide range of small dinosaurs. Dinosaur body and trace fossils also help, in which paleontologists can play the much-cherished game of “match the defecator.”
Of dinosaur coprolites identified thus far, the best understood ones are attributed to the Late Cretaceous hadrosaur Maiasaura of Montana. Some of these coprolites are quite large; although most are broken into smaller pieces, some suggest original volumes of about 7 liters (1.8 gallons). (For perspective, a regulation U.S. basketball is about 8.5 liters.) Other large coprolites are credited to Late Cretaceous tyrannosaurids, such as Tyrannosaurus rex; one of these is more than twice the length of a 12-inch sub sandwich. Moreover, coprolites in Late Cretaceous rocks of India have been connected to sauropods.
Nonetheless, the vast majority of probable dinosaur coprolites fall into the aforementioned nebulous categories of “carnivore” or “herbivore.” For example, Early Cretaceous coprolites from Belgium have bone fragments in them. Hence, these are allied with carnivorous theropods, but nothing more can be said about them. This vagueness is especially apparent once dinosaur ichnologists admit, with much embarrassment, that we have no idea how to distinguish whether therizinosaurs, ornithomimids, ankylosaurs, nodosaurs, stegosaurs, ceratopsians, pachycephalosaurs, or lots of other dinosaurs made some dinosaur coprolites. We also have not yet discovered a dinosaur coprolite showing
any evidence of insect eating, nor of clear omnivory in which, say, a dinosaur had a salad with its steak.
Of these, coprolites showing that some dinosaurs ate insects as a regular part of their diet would qualify as fantastic finds. This seemingly un-dinosaur-like behavior, which is extremely common in modern birds, was proposed for the bizarre theropod Mononykus from the Late Cretaceous of Mongolia and a few other theropods. So if insect-bearing coprolites of the right size were found in strata of the right age, environment, and place as Mononykus bones, this would be one way to confirm an idea that is now mostly speculative.
Dinosaur Dung, Conifers, Insects, Bacteria, and Snails: A Love Story
Every day, I give thanks to dung beetles. My thanks is offered for what these insects do to keep our planet clean, because otherwise we would be up to our waists in waste. The tight relationship between large herbivores and their diligent insect cleanup crews is easy to witness today: wherever elephants, cows, horses, or other plant-eating mammals loosen their bowels, dung beetles are not far behind. Why are these beetles and other insects, such as dung flies, attracted to feces? Because it is irresistible as baby food. Some beetles roll balls of this nutritious stuff as take-out, which they push into burrows, lay eggs on them, and seal off the burrow. Other beetles burrow below these patties, or into a patty itself, and lay eggs there so that the beetle larvae are surrounded by food when they hatch, which they can then devour. (It’s dinner and a nursery!)
Amazingly, dung beetles have been performing this essential ecological service virtually unchanged since at least the Late Cretaceous Period, and we know this because of dinosaur coprolites. Thanks to the careful and insightful collaboration of paleontologist Karen Chin and entomologist Bruce Gill, they convincingly showed how the Late Cretaceous hadrosaur Maiasaura, dung beetles, and conifer trees interacted with one another as part of a food web about 75 mya.
As is often the case in ichnology, body parts had little to do with this discovery, which Chin and Gill documented in 1996. Plenty of dinosaur bones and eggs were in the same area, as this was the same place near Choteau, Montana where Maiasaura and Troodon nested, preserved in the Two Medicine Formation. Yet as of this writing, not one body fossil of a dung beetle—legs, wings, abdomens, antennae, or anything else—has been recovered from the rocks of that area. The only body fossils involved in this research came from the conifers, which were represented as blackened bits and pieces in calcite-cemented coprolites.
As mentioned before, some of these coprolites were big, spanning about 34 cm (13 in) wide, although others were mere cobbles. They were also plentiful, showing up in sixteen spots within a square kilometer. Such a coprolitic concentration might imply that the area was a Cretaceous latrine. However, a more likely scenario is that dinosaur feces were nearly everywhere on dry land then, but a few places—like river floodplains—were better at burying them rapidly, which aided in their fossilization.
To have such fine fossil feces in the same field area as dinosaur eggs, babies, adults, and nests was a very lucky find and thrilling enough in itself. Yet when Chin and others also realized these coprolites had burrows in them, their paleontological importance skyrocketed: a dreamy ichnological two-for-one deal. Some of the burrows were open, but in others the insects had actively filled them, having packed a mixture of sediment and dung behind them and leaving distinctly visual “plugs.” The burrows also varied considerably in size, from about a millimeter to 3 cm (1.2 in) wide, all of which were insect-sized and with circular outlines. This variation implied that more than one species of insect made them.
Which insects made these burrows? First of all, they had to be ones that loved tunneling into dinosaur manure. This narrowed down likely candidates to two major groups, dung flies and dung beetles. Dung flies are relatively small and normally just lay their eggs on feces; their larvae then hatch on this food supply and start chowing down, which they continue doing until they pupate. In their life cycles, dung flies do not dig wide and lengthy burrows into the dung, let alone backfill them. But dung beetles do. Accordingly, Chin and Gill focused on these insects as the most likely suspects for these trace fossils.
Direct observations of many modern dung-beetle species and their traces served as guides for figuring out how these Cretaceous beetles’ lives depended on dinosaur waste. Dung beetles today employ three different strategies in handling feces: tunneling, dwelling, or rolling. Tunnelers burrow into and below a patty, making and storing a brooding chamber with dung and eggs. Dwellers make themselves at home in the patty itself, digging out brooding chambers so that the larvae emerge in the dung-beetle equivalent of a candy store. Rollers scrape dung off the surface of a pile, shape it into a big ball, and leave the neighborhood with their prizes, evoking Sisyphus as they roll dung balls larger than themselves. They later stuff these dung balls in burrows dug elsewhere. Given the three choices, Chin and Gill figured these Cretaceous burrows were from tunnelers, which had burrowed into the dinosaur feces while it was still gooey, gathered some of this organic goodness, and placed it into nearby burrows.
The most exciting conclusion drawn from this discovery was how Maiasaura interacted with and affected plants and insects in its surroundings, which in turn provided a sketch of how a Late Cretaceous ecosystem might have functioned, and with a dinosaur as a possible keystone species. As a large herbivore, Maiasaura may have had an impact comparable to elephants in savannah ecosystems today, in which dung beetles played an important role in the flux and flow of elements consumed by such herbivores.
However, another mystery about the Two Medicine coprolites was how the pieces of conifer wood had become so blackened. The answer came from within, as in fossil bacteria that originally lived in Maiasaura guts. In a paper published in 2001, geochemist Thomas Hollocher, Karen Chin, and two other colleagues detected both abundant body fossils and chemical signatures of anaerobic bacteria in the coprolites. These bacteria invaded vascular tissues in the wood and left distinctive black organic residue called kerogen, the same mix of organic compounds in oil shales. The simplest explanation for how these bacteria got into the plant tissues is that they were in the dinosaurs’ intestinal tracts. This made sense, as any modern herbivores likewise have gut microflora that aid in breaking down cellulose and other compounds in consumed plants.
This discovery of bacteria that lived inside a dinosaur was important enough. But the bacteria also did paleontologists a 75-million-year-old favor by helping to fossilize the coprolites. Once these researchers examined thin sections of the coprolites under microscopes, they realized that the calcite in the coprolites was probably precipitated in two stages: inside the vascular tissues of the fragments, then in the areas between the fragments, including the dung-beetle burrows. They proposed that bacteria could have initiated this precipitation, starting with live bacterial colonies in the original feces hardening these droppings. Once these proto-coprolites were buried, calcification would have continued, turning what was originally dark, mushy, and smelly into just dark and rocky.
Yet the story of these coprolites does not end with these two studies. As often happens in paleontology and other sciences, this research raised more questions. For instance, as Chin looked more closely at thin sections of the fossilized wood, she realized that something was rotten in the Cretaceous. The hadrosaurs had not been masochistically masticating hard, fresh, living conifers. Instead, they went for long-dead and already-decayed wood. The fossilized wood lacked lignin, a connective tissue that holds wood fibers together. Without this “glue,” wood falls apart. In modern forests, fungi aid in this disintegration, which rots the wood throughout. Then wood-boring insects—such as termites, beetles, and ants—break it down further. Although the fossil wood was too ground-up to tell whether insects had bored into it, Chin found some evidence of fungal damage in the wood fragments.
So Chin asked the best question of all: Why? As in, why would a dinosaur eat decayed wood? The nutritional value of the wood fiber itself would have been neglig
ible, hardly worth the effort involved in chewing and digesting it. So there had to be something more behind this behavior than just exercising jaw muscles. The fungi on and in the wood must have provided some sustenance, but probably not enough to keep a hadrosaur going. Also recall that Maiasaura is the “good mother” dinosaur, with a scientifically earned reputation for its child-raising skills. Think about the disappointment (not to mention hunger) baby dinosaurs would have felt if their parents simply brought them degraded wood to eat.
This is when Chin thought about both woodpeckers and vomiting. The first part of her reasoning—woodpeckers—was prompted by how these birds feed. As most people know, woodpeckers drill into dead wood not to eat it but to gain access to yummy and protein-rich larvae of wood-boring insects living just under wood surfaces. Woodpecker parents also do this for their chicks, flying back to nest cavities with insect treats for their offspring before drumming up their own meals. So perhaps these hadrosaurs were also breaking up wood to get insects, and carrying these back to their nests.
However, this same strategy would not have worked very well for a 6-to-7-ton Maiasaura trying to feed more than a dozen ravenous hatchlings, especially as their appetites grew with them. The image of a hadrosaur mother or father wearily carrying one beetle grub at a time in its mouth to a nest, dropping it into one of many competing maws, then going back for another, is too absurd to consider. It also did not explain why their coprolites contained wood, meaning the hadrosaurs didn’t just break wood, but swallowed it. So one solution that neatly explained such wholesale consumption of woody tissues is that these dinosaurs were eating large volumes of this decomposing wood for the insects (and perhaps some fungi). These caring dinosaur parents then transported these MREs (meals-ready-to-eat) in their stomachs back to their nests, where they obligingly regurgitated them into the waiting mouths of their hatchlings. Admittedly, this is a difficult hypothesis to test more directly, unless paleontologists some day find juvenile Maiasaura skeletons with enterolites matching the content of the adult’s coprolites.
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