Dinosaurs Without Bones (36 page)

Streaming on Demand: Dinosaur Urolites

We know that dinosaurs defecated and probably threw up, but what about urination? This is a tougher question to answer than one might think. Considering how dinosaurs were evolutionarily “in
between” crocodilians and birds, we could examine the liquid-waste excretion of these two groups and see which best fits dinosaurs. But the first place to start thinking about dinosaur urination is with modern birds, which then can be compared to their non-avian theropod ancestors more directly than with crocodiles. Given an appreciation for the super-soaker potential of penguin excretion, we can also think about the sorts of structures that might have been produced by dinosaurs’ liquid wastes, especially if delivered from cloacas on high.

Fossilized structures formed by urination, called
urolites
, are either rare or rarely recognized, with only a few thus far attributed to dinosaurs. For anyone who has seen or made their own modern examples, these structures are best defined and most recognizable when a forceful (high-velocity, low-diameter) stream of urine hits and erodes soft sand. An idealized urination structure made under such conditions should have a central impact crater, closely associated splash marks, and, if a slope is present, linear rill marks caused by excess fluid running down that slope. Although such traces have low fossilization potential, they feasibly could be preserved if made in sand dunes with the right conditions for fossilization: for instance, if dry wind-blown sand stuck to the wetted sand and filled these structures, making natural casts of them.

So far, only two dinosaur urolite discoveries have been reported. The first of these was in the Late Jurassic Morrison Formation near La Junta, Colorado. This urolite was mentioned in a poster presentation at a paleontology meeting in 2002 and understandably garnered much media attention, especially for how the paleontologists reporting these—Katherine McCarville and Gale Bishop—tried to replicate this structure on a Georgia beach by using a combination of water buckets, funnels, hosing, and ladders. The suspected Morrison urolite, which is preserved in a former lakeshore limestone, is a shallow, oblong, trench-like depression, which McCarville and Bishop described as “bathtub-shaped,” a descriptor that does not sit well when imagining it occupied by liquid waste. Because this depression cuts across
bedding, scouring of some sort formed it, but in a specific spot on an otherwise nearly flat surface.

Had paleontologists encountered this feature just by itself, most might have shrugged, written “weird bathtub-shaped depression” in their field notebooks, and moved on. However, additional trace fossil evidence led McCarville and Bishop to consider sauropod pee as a possibility. The same limestone bed with the suspected urolite also holds more than eighty dinosaur trackways, divided almost evenly between sauropods and theropods. Known as the Purgatoire site (after the Purgatoire River, which runs through the area), it is one of the most spectacular dinosaur tracksites in the western U.S., including some of the best-known examples of herding-sauropod trackways. This circumstantial evidence meant that large potential peers were at the site, and the size of the possible urination structure pointed toward a sauropod as the culprit.

Based on its present dimensions, it would have held about a cubic meter (265 gallons) of liquid, which would easily overflow a small wading pool. Even the largest of Late Jurassic theropods living in this area, such as
Allosaurus
, would not have had enough juice to produce such a huge structure, no matter how much they drank and how long they held it. Also, an adult sauropod cloaca—especially those of
Apatosaurus
or
Diplodocus
—would have been much higher off the ground (3–6 m, or 10–20 ft) than those of contemporary theropods (1–2 m, or 3.3–6.6 ft). Hence, their liquid wastes would have generated more force and imparted greater erosion.

How likely is it that this enigmatic feature is a urolite? I rate it a definite “maybe.” Fortunately, I have seen it in person and did not have to rely just on these paleontologists’ descriptions of it or their critics’ guffaws. In 2002, my colleague Steve Henderson, a bunch of our eager undergraduate students, and I visited the Purgatoire tracksite. Steve and I were co-teaching a dinosaur field course and swung by La Junta to see the incredible dinosaur trackways south of town. Our guide—U.S. Forest Service paleontologist Bruce Schumacher—happily showed us the hundreds of dinosaur tracks there, wowing students and instructors alike. He then
detoured our group to the suspected urolite to talk about it as an example of “science in progress.”

Although I’m still skeptical of its proposed identity, it nonetheless provided an enjoyable experience for our students, in which we asked basic scientific questions such as “What evidence supports the hypothesis?” “How would you test this hypothesis?” and more specifically “How much pee would be needed and from what height would it have been delivered to make something like this?” Urolite or not, this enigmatic structure presented us with a fine educational opportunity, and a memorable one.

Given the controversy over the Morrison urolite, I was much relieved to later find out that paleontologists Marcelo Fernandes, Luciana Fernandes, and Paulo Souto had also interpreted dinosaur urolites in the Late Jurassic–Early Cretaceous Botucatu Formation of southern Brazil. Although they found only two such trace fossils, both were beautiful textbook examples of what urolites should look like, bearing all of the marks of steady but focused streams of liquid hitting dry sand. Preserved in sandstones, these trace fossils are teardrop-shaped craters connected directly to streamlines. The structures were likely made in sand dunes, with the craters having formed by erosion of upper sand layers—caused by urinary impact—and the streamlines from liquids trickling downslope on a dune surface.

The craters and streamlines of the two specimens were nearly identical in size and form, measuring about 2 cm (<1 in) deep, 16 to 19 cm (6.3–7.4 in) long, and 11 to 13 cm (4.3–5.1 in) wide, about the size of a gravy boat. The craters would have held about 300 to 400 cc of liquid (a cup and a half), but some of this would have soaked into the underlying sand with first wetting, so the original volume was probably more like 500 to 1,000 cc. Because these trace fossils were in the same strata as ornithopod and theropod tracks, and those dinosaurs were the only animals large enough to have produced such vigorous bursts of fluid, the paleontologists concluded they were the most likely to have left such marks.

Nevertheless, Fernandes and his colleagues, just like McCarville and Bishop, further tested their results by trying to make their
own structures in sand. Fortunately, this did not require any self-experimentation, such as chugging liquids and running to a nearby sand dune. Instead, they took two liters of water and poured it from 80 cm (2.6 ft) above and onto a loose sand surface with a slope having the same angle (about 30°) as the fossil ones. This procedure successfully created a structure strikingly similar to the interpreted urolites, with an elongated central crater and streamlines caused by water dripping downslope. However, their triumph did not satisfy completely, so they then “cheated” by watching a modern dinosaur urinate: an ostrich, that is. Like many countries, Brazil has ostrich farms, so these researchers simply went to one and watched these big birds take a leak. Sure enough, what they observed matched their interpretations.

Because ostriches and other ratites eliminate liquid wastes first, then solid wastes, this also would explain how ornithopods and theropods could have made both urolites and coprolites. That is if they had plumbing comparable to ostriches, versus most other birds that only produce a mixture of the two. Also keep in mind, though, that if dinosaurs peed more like birds and less like mammals, their liquid waste would have been evacuated behind them, not in front; this is how ostriches tinkle. Oh, and one more thing: Remember that modern male birds do
not
use their tools to urinate. So even if a male dinosaur had a penis, you still would not be able to tell whether a male or female dinosaur made a urolite, which is definitely not the case with any mammal urination structures I have seen. Regardless, if paleontologists are lucky enough to find urolites directly associated with dinosaur tracks, they must be careful in defining “pre-pee” and “post-pee” footprints in that trackway.

Given that this is all we know so far about dinosaur urolites, it doesn’t take a whiz to figure out that more studies are needed to better recognize these trace fossils in the geologic record, which should be abundant. After all, these dinosaurs had to go sometime, so the traces must be out there. So I will suggest a fun follow-up to this research, which would be to try duplicating what was done with penguin-poo physics. In other words, figure out minimum cloacal heights and diameters of these peeing dinosaurs, velocity
of flow, liquid viscosity, and other factors, and then model the resulting structures. These could then serve as search images for similar trace fossils. For example, minimum cloacal heights for a urinating dinosaur would have been about the same as their hip heights; as we learned earlier, these can be calculated from dinosaur tracks. Paleontologists who do such research could be assured of making a big splash with it, while also going against the flow of others’ prejudices. Afterwards, they will be flushed with success, and their colleagues pissed off.

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.