Charig and Milner also mentioned gastroliths as being present and associated with the skeleton; sadly, they gave no other description of these trace fossils. Oddly enough, they mention “gastroliths” in their 1986 paper, but only “an apparent gastrolith” in the 1997 one. Did the others get misplaced during that eleven-year gap? However, if Baryonyx only had one or a few gastroliths, such rocks could have been accidentally grasped and swallowed as it scooped up a slippery fish. Lastly, a few broken bones in Baryonyx were explained as the trace fossils of other dinosaurs that irreverently stepped on its body after it had already died and been buried.
Nearly all of the trace fossils mentioned thus far were enterolites, food items that only made it down as far as the proventriculus, gizzard, or whatever else functioned as a stomach in a dinosaur. What about cololites, in which this food was more digested, passing through the intestines for further nutritive absorption, and almost ready to explode from a dinosaur’s other end but never quite made it?
Only a few such trace fossils are known in dinosaurs, but one of the best is also in the most complete dinosaur from an entire continent, Minmi paravertebrata. This Early Cretaceous dinosaur lived about 115 mya in Queensland, Australia, and was an ankylosaur; recall that ankylosaurs were armored dinosaurs that lived throughout much of the Jurassic and all of the Cretaceous periods. I have seen a cast of this specimen displayed at a small museum in Richmond, Queensland, and it is incredible. Although flattened so that it looks like Cretaceous road kill and about the size of an adult sheep, nearly every hard part of its body is intact and articulated.
Fortunately, this exceptional preservation included its gut contents, which consisted of a compacted mixture of leaves, seeds, small fruits, and other parts from a variety of plants inside the area of its hip. All of these plant remains were the size of coarse coffee grounds, measuring only a few millimeters across. “Then it must have had gastroliths!” you shout excitedly. In response, I smile, nod, and thank you for outwardly expressing such enthusiasm for those trace fossils, but then gently inform you that this ankylosaur had no gastroliths. This means that it more likely chewed its food thoroughly in its mouth before swallowing, rather than relying on its stomach to reduce food to a more digestible texture.
Okay, time to get skeptical. Dinosaurs, like other vertebrates, probably spilled their guts after dying, whether from decomposition, scavengers, or a combination of the two. Once opened, a dead dinosaur’s body cavity would have invited the outside world to intrude on its inner spaces. This means river currents could have carried in the finely ground and varied plant material and deposited it in the open body cavity of this Minmi after it died. Therefore, these plants might not represent its final meal after all.
Good thinking, but in my preceding description of Minmi and its cololite, I omitted a key piece of information, one that crushes that hypothesis like ankylosaur jaws would a Cretaceous plant. It turns out this specimen of Minmi was deposited in a Cretaceous sea, and no river was anywhere near its body when it was laid to rest in a shallow marine grave. Central Queensland abounds in fossils from this Cretaceous seaway that divided Australia then. In the same museum where I stared at the cast of Minmi, hundreds of fossils of ichthyosaurs, plesiosaurs, ammonites, squid, clams, and other sea life surrounded it, all from the same rocks.
No serious paleontologist suggests that the heavily armored ankylosaurs paddled out to sea, just as medieval knights were not likely keen about going for a swim in full armor. So this Minmi must have been a “bloat-and-float” dinosaur. It died on land with a belly full of terrestrial plants and was filled with enough gas from decomposition to buoy it into the ocean. Once out there, it somehow escaped scavenging by marine carnivores; no toothmarks or other signs of nibbling were on its body. Once it sank, it also must have made quite an impression on any sea life living on the bottom before burial. Most amazing of all, this is an example of a dinosaur that carried its trace fossil with it after death and on a long journey to sea.
Dinosaur Puke
So how about food that a dinosaur’s gastrointestinal tract rejected instead of passing through, or more quiescently as gastric pellets like those emitted by predatory birds today? Tragically, examples of dinosaur regurgitalites are either extremely rare or unrecognized from the fossil record. One regurgitalite inferred from Late Triassic rocks contains pterosaur remains, but is thought to have come from a large fish rather than a land-dwelling animal. I also mentioned one example interpreted in 2009, which was apparently fossilized beside the mouth of a Coelophysis. As much as I want to believe this is Triassic puke, the coincidence of vomiter and vomit seems a little too fortuitous. For instance, how did this former meal stay put while river currents buried its supposed producer?
Nonetheless, I am hopeful that dinosaur regurgitalites are more common than previously supposed, and paleontologists just need a combination of training and imagination to find them. So here are some criteria for finding dino-barf. For one, because the contents of regurgitalites only spent a short amount of time in an animal’s gut, they will be noticeably less digested than anything coming out its other end. Still, any solid items, such as bones, may have some acid etching, while also being more broken, poorly sorted, and chaotically arranged than if they had not been chewed, swallowed, and partially digested by a dinosaur.
Unless preserved as a pellet, like a tightly packed modern-day owl pellet, vomit should have contained enough liquids to cause it to spread, especially if delivered from a great height. For a moment, just imagine the Late Jurassic sauropod Brachiosaurus puking its guts out, with its mouth as much as 14 m (46 ft) above the ground. This likely would have caused immediate panic amongst any animals in the vicinity, which would have tried to escape such a harrowing aerial bombardment, or at least been spooked by the unusual noises accompanying a purging sauropod. Trace fossils would thus show the following:
Large sauropod footprints at a standing position, and behind a crater with splatter marks radiating several meters around its center;
A concentration of bigger, more solid items in the middle;
Run-off marks from included fluids, especially if influenced by slopes on the former ground surface;
Trackways of other animals with ratios between footprint lengths and stride lengths showing they were moving at high speeds and fleeing in directions away from the crater;
The remains of one or two small unlucky theropods knocked out and drowned by the initial shower of debris;
Thousands of trackways of the “cleanup” crew, such as insects specialized at eating dinosaur throw-up. Just as there are insects that specialize in eating dinosaur poop, there may have been some that enjoyed a pre-digested (but totally vegan) meal.
Only one instance of fossilized dinosaur vomit, containing a mix of dinosaur and turtle bones in Early Cretaceous rocks of Mongolia, is inferred to have come from an actual dinosaur. Another possible dinosaurian up-chuck is embodied in a mass of four birds from the Early Cretaceous of Spain. However, the paleontologists reporting it thought it also may have been from a pterosaur. Otherwise, most Mesozoic regurgitalites are credited to marine-reptile contemporaries of dinosaurs: ichthyosaurs. These deposits, found in Late Jurassic rocks of the U.K., consist of concentrated collections of belemnite shells. Belemnites, which were squid-like animals that shared the seas with ichthyosaurs, also show up as stomach contents. However, these masses of cigar-shaped shells were found by themselves and were acid-etched. This suggested they spent time bathed in low-pH stomach acids, but because they were not inside an ichthyosaur skeleton, they were either regurgitalites or coprolites.
Coprolites were quickly eliminated as an explanation, though, because belemnite shells are pointy on one end. Now imagine being an ichthyosaur that swallowed several dozen pointy-ended shells and passed those out its other end: no, thanks. The risk of internal injury also would have been too high to pass such shells all the way through intestines too sensitive to handle such a payload. Hence these were the m
arine version of gastric pellets, in which ichthyosaurs snapped up lots of belemnites, kept them down long enough to digest their useful nutrients from the soft parts, and then coughed up their shells.
Other than gastric pellets or illness, what would have been other reasons for a dinosaur to barf? One would have been from the desire to feed their children warm meals. And if it were regurgitated as meals for hungry youngsters and puked right into their waiting little mouths, a trace of this in the fossil record would be very rare indeed. As mentioned earlier, though, bird parents commonly seek out food, ingest it, partially digest it, and then egest it into their chicks’ mouths. Keep this in mind for later, and how other trace fossils opened our eyes to this particular nurturing behavior as a possibility in dinosaurs.
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 ex
plain 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.
Dinosaurs Without Bones Page 27