Dinosaurs Without Bones (51 page)

Another intriguing idea about dinosaur nesting inspired by modern birds is that perhaps some made massive nest mounds for incubating their eggs. Remember the mallee fowls (
Leipoa ocellata
) of Australia and the 4-m-high hills they make with their nest mounds? How about the recently extinct mound-nesting birds of New Caledonia,
Sylviornis neocaledoniae
and
Megapodius molistructor
, which made mound nests so big that archaeologists at first mistook these for human burial mounds? Of these two birds,
Sylviornis
was the bigger one, weighing about 25 kg (55 lbs), but still much
smaller than most Jurassic and Cretaceous theropods. In at least one recent documentary (
Dinosaur Revolution
, 2011),
Tyrannosaurus rex
was depicted as a mound nester with an accordingly huge mound for its eggs. Sadly, though, such nest mounds may have been made mostly of vegetation, not sediment, so these would have had a much lower chance of being preserved as trace fossils. Alternatively, if composed more of sediment, remnants of massive nest mounds like these might have been left behind, although if unaccompanied by dinosaur eggshells or embryos these might be tough to discern.

Still, there is much we need to learn about dinosaur nesting. For example, no one has found nests of stegosaurs, ankylosaurs, and many other dinosaurs, including those commonly preserved as adults, such as
Allosaurus
and
Triceratops
. Perhaps some of these dinosaurs—especially those for which we have evidence of gregariousness, like ceratopsians—had immense nesting grounds that played a major part in shaping their local environments. However, if such nesting grounds lacked eggshells and bones, geologists and paleontologists may not have recognized them for their true nature and merely regarded them as strangely bumpy ground of uncertain origin. So it is with some hope that an ichnological outlook and a little bit of luck may aid in the discovery of large-scale dinosaur nesting grounds, and the effects these grounds had on their local environments.

Dinosaur Flatulence and Global Warming

Farts do not fossilize. This is an extremely fortunate situation for geologists and paleontologists worldwide who otherwise would have to wear gas masks every time they cracked open a Mesozoic rock with their hammers. Let’s face it, the Mesozoic was probably a very stinky time in earth history, given the considerable numbers and sizes of dinosaurs actively eating, belching, vomiting, pooping, peeing, and, yes, breaking wind in a way that likely generated its own measureable wind speeds.

Probably the only way for ichnologists to detect whether a specific dinosaur let one rip is to carefully examine the sediment
surrounding the few theropod resting traces described from Early Jurassic rocks and see whether any unexplained structures might be in the former vicinity of the theropod trackmaker’s cloaca. However, I am not hopeful about our finding such trace fossils, nor do I expect anyone to receive government-funded research grants to reproduce modern examples under laboratory conditions.

So although individual trace fossils of dinosaurs’ gaseous emissions may not be recognizable, their collective effects likely were. Paleontologists, geologists, and
paleoclimatologists
(people who study ancient climates and how these changed through time) are considering the intriguing possibility that dinosaur farts may have contributed to global climate change.

The sudden release of gases from intestines, politely termed
flatulence
, can cause localized changes in atmospheric chemistry that may or may not be detectable through smell, but normally is preceded by sonic qualities. The mixture of gases, called a
flatus
, varies considerably within or between species of flatus emitters. For example, in humans, a flatus produced by swallowed air that made its way through the digestive tract approximates normal proportions of atmospheric elements and compounds, which is 78% nitrogen (N
₂
), 21% oxygen (O
₂
), and less than a percent of carbon dioxide (CO
₂
), methane (CH
₄
), and other gases. However, if originating as a by-product of digestion, the proportions may include as much as 30% carbon dioxide and/or 50% hydrogen and methane. Most of the unpleasant smells we associate with flatulence come from sulfurous gases, such as hydrogen sulfide (H
₂
S). In contrast, carbon dioxide is odorless, and methane, although detectable by smell, is not as offensive as its sulfurous companions. Most of these gases, methane included, are among the waste products of anaerobic bacteria and bacteria-like organisms (
archeans
) living in our guts.

Other than initiating temporary social discomfort, adverse olfactory sensations, embarrassment, or lowbrow humor, every flatus by all humans or other animals contributes gases to the global atmospheric budget. Although each emission is small, the joint effect of these gases from animal- and non-animal sources is what
changes the earth’s atmosphere and ultimately affects climate. As everyone now knows (with the exception of a few billionaires), the overproduction of certain gases—such as carbon dioxide and methane—has insulating qualities in our atmosphere, trapping infrared radiation (heat) and thus contributing to warmer global temperatures. This is not just a matter of quantity, but also the type of gas. Although carbon dioxide is often vilified for its role in climate change, methane in smaller concentrations has a greater impact, as it insulates much better than carbon dioxide—like the difference between using nylon or wool for a blanket.

In a 2012 paper, three scientists—David Wilkinson, Euan Nisbet, and Graeme Ruxton—asked themselves this: Did dinosaur-produced methane contribute to global warming? They then tried to answer it by applying a few biological principles, ecological concepts, atmospheric chemistry, and a whole bunch of math. In their model, they concluded that, yes indeed, sauropods alone could have produced almost as much methane in a year as is made by all sources (human and non-human) today: about 570 million tons versus 600 million tons, respectively. Among the factors they considered in their model were:

• Sauropod metabolism, such as whether they were endothermic or ectothermic;
  
• Mode of digestion, in which they assumed sauropods used gut bacteria or archeans to break down low-quality plant matter;
  
• An average sauropod size of 20 tons, which was typical for the Late Jurassic sauropod
  Apatosaurus;
  
• Estimated sauropod herd size;
  
• Land area needed to ecologically support this dinosaur biomass;
  
• High ecological productivity of land ecosystems during the Mesozoic, which was partly a function of how average global temperatures were much warmer than today.
  

So did Mesozoic global warming begin before dinosaurs added their gaseous contributions to the atmosphere, or afterwards? This is actually a tough question to answer, because unlike other times in earth’s history, the Mesozoic lacked long periods of global cooling. Instead, it was warm throughout almost its entire 185 million years, from the start of the Triassic Period to the end of the Cretaceous Period. Dinosaurs were a part of this equation for the last 165 million of those years, but the grandest of herbivorous gas-bag dinosaurs—sauropods—did not show up until about the middle of the Jurassic, which is when they were joined by stegosaurs, ankylosaurs, and nodosaurs. Later, in the Cretaceous, sauropods were accompanied by great numbers of big ornithopods and ceratopsians. This means that a warm climate was started before the Late Triassic—which was when the dinosaurs arrived on the scene—but dinosaurs may have done their part to keep the home fires burning once they evolved, spread, and farted throughout the Jurassic and Cretaceous.

Nevertheless, as intriguing as this model of dinosaur-caused warming might sound, it’s based on many assumptions and with lots of leeway granted for some of the factors. For instance, the jury is still out on whether or not some herbivorous dinosaurs—such as sauropods, ornithopods, stegosaurs, ankylosaurs, and ceratopsians—used fermentation in their hindguts to digest their food. As mentioned previously, gut-produced methane comes from anaerobic bacteria and archaeans, which together are called
methanogens
. Many modern animals, from termites to cattle, have this symbiotic relationship in which these methanogens live in their intestines and through fermentation break down otherwise low-quality plants (think “woody”). Although paleontologists know from coprolites that some dinosaurs had gut bacteria, the evidence is otherwise scanty for methanogens having shared time with dinosaur innards.

As a result, paleontologists sometimes have to point to an absence of trace fossils in order to say hindgut fermentation (and hence methane production) was common in large herbivorous dinosaurs. In this instance, the rarity of gastroliths in most herbivorous dinosaurs is
now regarded as a good reason to think that sauropods and other hefty herbivores used hindgut fermentation. Gastroliths were always considered as one way for herbivorous dinosaurs with pencil- or leaf-like teeth to grind up their food, thus easing their digestion. But with an awareness that gastroliths are not as common in sauropods as once supposed—or used more as mixing stones, rather than grinding stones—and that they are almost unknown in stegosaurs, ankylosaurs, ceratopsians, and large ornithopods, other digestive aids have to be considered. Granted, some of these dinosaurs, such as ceratopsians and hadrosaurs, had terrific teeth for shearing and grinding plants. But no amount of chewing can break down cellulose, which requires biochemical intervention. So without gastroliths, these dinosaurs were likely getting help from their little anaerobic friends, which in turn released great volumes of methane.

Unfortunately, given this revelation about the probable role of dinosaurs in methane production and their contribution to Mesozoic atmospheres, a few climate-change deniers have latched on to it and made pithy statements such as, “Dinosaurs caused global warming, too. So what’s the big deal now?” The big deal is in the numbers provided by Wilkinson and his colleagues: in a contest for methane production in which dinosaurs take on modern humans, humans would still win. This means that humans are unwittingly conducting an experiment to find out whether we can reproduce Mesozoic temperatures without dinosaurs. If we succeed by confirming the hypothesis—elevated levels of methane in our atmosphere cause significantly greater global temperatures—the consequences are dire for many species, including ours. Hence, this possible dinosaur trace, one that may have maintained globally high temperatures and therefore high sea levels—thus affecting ocean ecosystems—is a lesson in what trials of earth history do not need repeating during our time here.

Look at a Flower, See a Dinosaur

As hinted at in previous chapters, paleobotanists are often the Rodney Dangerfields of paleontology, garnering only slightly more
respect than ichnologists. Fortunately, their relevance among vertebrate paleontologists—and especially dinosaur paleontologists—has been bolstered in the past thirty years or so through their pursuing the answer to a provocative question: Did herbivorous dinosaurs have something to do with the evolution of flowering plants? In short, are modern flowering plants an evolutionary trace of Mesozoic dinosaur behavior?

What is brilliant about this question is that, unlike many rhetorical inquiries with “yes” or “no” answers that are uttered with confident finality, the answers are probably “Yes, in part” or “No, not exactly.” These uncertain responses appropriately lead to broader discussions of whether herbivorous dinosaurs were the primary movers and shakers in the evolution of land plants during the Mesozoic, or whether plants were in the driver’s seat, or whether a combination of the two took place. In other words, more good science came out of following and testing this possible dinosaur–flower connection.

Robert Bakker was the first dinosaur paleontologist to suggest that dinosaur feeding might have “invented” flowering plants (
angiosperms
) from non-flowering seed plants (
gymnosperms
), such as cycads, ginkgoes, and pines. His reasoning was that during the Early Cretaceous (about 135
mya
), grazing and low-level browsing ornithopods mowed down gymnosperms so much that angiosperms, with their low-lying flowers and fruits, were favored over these plants. Consequently, flowering plants proliferated, diversified, and took over terrestrial ecosystems, which they have ruled ever since. This was an imaginative idea, but Bakker was not a paleobotanist, so his supposed connection between herbivorous dinosaurs and flowering plants seemed tenuous to most other paleontologists.

Among the first paleobotanists to start examining whether flowering plant and land-dwelling vertebrate evolution went hand in hand (or rather, leaf in mouth) were Scott Wing and Bruce Tiffney. In several articles published in the late 1980s, they proposed that herbivorous dinosaurs and the earliest flowering plants
had what they termed a “reciprocal relationship,” which resulted in their coevolution. This relationship meant dinosaurs that ate these plants helped with dispersing their seeds (more on that topic later). Moreover, as angiosperm seeds were placed in new ecosystems, this affected their natural selection and ultimately their evolution throughout the rest of the Cretaceous Period. In turn, as new species of angiosperms arose, some produced more specialized flowers and fruits, or defenses against herbivory (such as toxins and spines), which contributed to the natural selection of herbivorous dinosaurs. In other words, the plants were partially responsible for driving the evolution of new species of herbivorous dinosaurs that were more specialized in their feeding. In contrast, the first herbivorous dinosaurs to eat angiosperms may have been more generalists, eating every item from their Early Cretaceous all-you-can-eat salad bars.

However, other paleontologists, including some paleobotanists, have critically examined and questioned a direct cause-and-effect of herbivorous dinosaurs on flowering plants. In a 2001 study done by dinosaur paleontologist Paul Barrett and paleoecologist Katherine Willis, wherein they examined how herbivorous dinosaurs and fossil angiosperms overlapped in time and space during the Cretaceous, they could not conclusively say “Dinosaurs invented flowers.” Instead, they concluded that other animal–plant interactions—such as between insects (especially bees and wasps), mammals, and plants—probably had a greater impact in angiosperm evolution, as well as extended periods of global warming. Yet they also admitted that dinosaurs and flowering plants likely affected each group’s evolutionary histories, even though the evidence supporting this was still fairly meager.