So imagine an Early Cretaceous shore next to a lagoon, with these deeply impressed trails running parallel to the average high-tide mark along that shore. With no vegetation along the way, the sauropods would have had a clear view, just in case they needed advance warning of the big predatory theropods waiting for them out there. Occasionally one or several of these theropods came down to the shoreline too, hoping to pick off a straggling sauropod for a big score, but for the most part they stayed away; this was sauropod country, and the uneven ground made ambush hunting and quick pursuits problematic. If seen from a pterosaur’s point of view, the coastal trails would have connected to more inland ones, criss-crossing the forested interiors and freshwater wetlands like a great spider web.
Such grand disturbances of pliable mud or sand, in which great numbers of overlapping footprints made by immense dinosaurs made trails or left churned messes in the geologic record, are sometimes called “dinoturbation.” I personally dislike this term, because it literally means “terrible [or awe-inspiring] mixing.” This handiwork, however, is not the exclusive domain of dinosaurs. After all, earthworms and ants also mix tons of sediment every day. This term also distracts from how a few modern vertebrates, such as elephants and hippopotamuses, are capable of doing their own awesome mixing of sediment, which we somehow manage to restrain ourselves from labeling “elephanturbation” or “hippoturbation.” Semantics aside, huge-sized dinosaurs, which sometimes traveled together in herds like the proverbial ships passing in the night, would have left sedimentary wakes with their passage, massively disturbing and altering terrestrial and freshwater ecosystems wherever their feet landed.
As the largest living land animals, elephants are the first analogs ichnologists reach for when trying to estimate the potentially far-reaching ecological effects of dinosaur trails. Elephants consist of three species: the African bush elephant (Loxodonta africana), African savannah elephant (L. cyclotis), and Indian elephant (Elephas maximus). Of these, the African bush elephant (L. africana) is the largest, with males weighing more than 7 tons, but Indian elephant males can also reach 5 tons. Elephants of all three species travel extensively and migrate annually. They normally walk in groups led by an adult female (matriarch), although adult males will go off on their own to make their own tracks. Thanks to fossil trackways recently discovered in the United Arab Emirates which show a series of parallel and overlapping tracks (group behavior) crossed by one trackway (a lone male), we know elephants and their relatives have likely held these same behaviors minimally for the past seven million years.
These behaviors also imply that local vegetation is normally worn down and sediments compacted by groups of elephants, not individuals, and that once a path has been cleared, it will be used repeatedly, perhaps by generations of elephants. Elephants also need plenty of water, so they try to stay near rivers or ponds, which they often enter and exit to drink or bathe. These habits mean they wear down banks, form wide divots on those banks, and muck up water-body bottoms, especially if they start wallowing. Elephant trails can also form depressions deep enough for water to flow along them, creating canals that connect previously isolated rivers or ponds. Trails on riverbanks similarly allow easier passage for floodwaters to cut through levees and pour out onto floodplains, depositing sediment in what are called crevasse splays.
Modern hippopotamuses (hippos), despite being smaller than elephants—with adults weighing in at 2.5 to 4 tons—have an even larger impact on their aquatic environments, which is where they spend most of their time. In a study by geologist Daniel DeoCampo published in 2002, he documented how hippos in Tanzania made a 30 m (100 ft) wide and 2 m (6.6 ft) deep muddy wallow pond, which connected to 1 to 5 m (3.3–16 ft) wide trails that imparted radiating and branching patterns onto the surrounding landscape. Hippos made these trails by frequently moving into and out of the wallow pond to feed on nearby vegetation; their activities, combined with their bulks, compressed and otherwise altered sediments. Most important, hippo trails actually changed the direction for water flow in the area, in which channels followed the trails, a type of channel abandonment called an avulsion. This channelization via hippo traces, in which their trails eventually turn into new river channels, is also well documented in the Okavango Delta of Botswana.
So did dinosaurs affect the courses of rivers with their trails, carving out new routes for flowing water through avulsions, or connect previously isolated water bodies? Given the known effects of much smaller modern large animals on rivers, the probable effects of individual dinosaurs that weighed 10 to 20 tons or more, herd sizes of these dinosaurs, and their geological longevity, I would be extremely surprised if they did not. If so, these effects may be detectable by geologists by more closely examining Mesozoic river deposits that also contain plenty of dinosaur bones and trace fossils—such as those of the Late Jurassic Morrison Formation in the western U.S.—for sauropod-width incisions in ancient river levees. They might also reexamine crevasse splays to see whether these connect to such divots, and whether these contain sauropod or other dinosaur tracks. In this respect, in 2006, two geologists—Lawrence Jones and Edmund Gustason—did indeed propose that avulsion features in the Morrison Formation of east-central Utah were likely caused by sauropod trails that created “channels” for the flow of floodwaters. Indeed, some former river-channel sandstones in the Late Jurassic Morrison Formation have dinosaur tracks on their bottoms, which may have been made by theropods or sauropods crossing rivers.
Amazingly, geologists have also determined that some modern rivers have been flowing more or less in the same valleys since before and well into the Mesozoic Era. These rivers include the Nile in eastern Africa, the Amazon of South America, the Macleay and Murrumbidgee of Australia, and the Colorado River of the western U.S., among others. All of these rivers are in places where dinosaurs—including big sauropods—used to roam, meaning at some point in their geological histories dinosaurs and previous versions of these rivers intersected. Considering the substantial human populations that settled along these rivers (some for as long as 10,000 years or more) and have since depended on the rivers and their floodplains for their lives, it is humbling to think that their present-day locations may have been at least partially determined by dinosaurs. Also think of how the ecological communities of these rivers evolved in those river valleys, beholden to these indirect effects of dinosaurs.
Now this is where a paleo-curmudgeon might preemptively scold me (augmented with much finger wagging) by saying, “Correlation is not causation!” Yet it is also unrealistic to accept the notion that dinosaurs, through their habitual movements, had no effect whatsoever on these or any other rivers during their times. Consequently, a valid question to ask is not whether dinosaurs affected the course of rivers or other water bodies in the past, but rather how much did dinosaurs affect these and other rivers? Although perhaps unanswerable, this inquiry is worth reflecting upon.
So while pondering this concept of how dinosaurs changed the very landscapes on which they walked, keep in mind how some may have been obliged to tread where generations had walked before. In that respect, here are the final two sentences from Thulborn’s 2012 paper about the sauropod tracks of Western Australia:
If sauropods were as wary as elephants in negotiating sloping terrain, they would naturally have tended to walk on the lower and safer ground—which, in practice, would be any area that was already trodden by earlier visitors. In doing so, they would automatically have followed, deepened, and widened the routes pioneered by their predecessors, thereby reshaping the topography of the landscape they inhabited.
In short, dinosaur trails influenced the behavior of dinosaurs, traces that affected their decisions in everyday life, perhaps extending back into the Jurassic but very likely by the Early Cretaceous. In turn, those dinosaur-made trails transformed the land and waterways for future generations, traces that extended well beyond the extinction of their species and descendants, affecting all life thereafter.
Dinosaur-Caused Avalanches
Did dinosaurs ever cause landslides? The short answer is “yes,” although it should be followed up by another question: “What do you mean by ‘landslide’?” In popular vernacular, this term refers to any movement of earth material, but most often conjures visions of a chaotic mass of rocks rapidly moving down a slope, perhaps toward a village, often preceded by a distant rumble and someone yelling dramatically, “Landslide!” or “Avalanche!” But geologists actually apply these two words to different mass movements of earth material: a slide moves along a defined plane, whereas an avalanche is a chaotic flow of rocks and air. Hence, the latter is the scientifically correct warning in this example, which is good to know as it prevents arguments with geologists who might object to your use of “landslide” as a descriptor while you are in a village being destroyed by an avalanche.
Anyway, back to dinosaurs and their effects on mass movements. In an article by geologist David Loope published in 2006, he showed how Early Jurassic dinosaurs caused sand-dune surfaces to collapse as they walked across them. These ancient wind-blown dunes are preserved in the Navajo Sandstone, an Early Jurassic (190–180 mya) formation that is well exposed in many national parks of Utah and other spots in the western U.S. This formation is best known for showing off its spectacular cross bedding, which is the internal structure of sand dunes formed by Jurassic winds, its beautifully curved, parallel, and intersecting lines only much later becoming inert subjects of clichéd landscape photography. Fortunately for ichnologists, the Navajo Sandstone also holds tens of thousands of dinosaur tracks. In contrast, dinosaur bones are rare in the Navajo Sandstone, so these tracks show that dinosaurs were there, which ones were present, and what they were doing.
In Loope’s study, he looked at an area on the Arizona–Utah border that was particularly rich in dinosaur tracks, most of which were from variously sized theropods but also included prosauropods. The tracks could be observed both on bedding planes and in vertical sections, which helped to define just how much the dinosaurs were disturbing the sand dunes as they walked on them. Perhaps the most surprising conclusion of his investigation was that many of the dinosaur tracks had been made on dry sand, which was always regarded as a poor medium for preserving tracks. Even better, because the footprints were made on dry sand, each spot where a dinosaur stepped became a potential avalanche, which multiplied with every subsequent step. Other geologists had reckoned that the climate for that area during the Early Jurassic was monsoonal, so Loope figured that the tracks were made and preserved during the dry season, which was likely in the winter.
For anyone who has walked on a sand dune, especially one with dry sand, what happens to the sand depends on where you are walking on the dune. For instance, if you confine yourself to the dune crests by just walking along their tops, this motion causes minimal movement of sand, and when you look back at your path you’ll mostly see a softly defined track. But as soon as you start walking up or down the side of a dune, its sloped surface starts collapsing. As each foot pushes down into and against the dry sand and then withdraws, the sand grains become part of a miniature-scale avalanche, tumbling on top of one another as they move downslope. This sort of movement is called grainflow, in which sand grains flow as if they are in water, but in this instance are cushioned by air between the grains.
This phenomenon is best experienced while walking down a dune slope, in which one should note how each step causes sand to flow ahead of you, creating an apron of avalanched sand in front of where your next step will land. Once finished with a descent, take a look behind and you will see a wide, featureless area of disturbed sand, which probably will not show any clear outlines of your footprints. If you had not just done this personal experiment, you would have no idea if a person or other animal had caused the avalanches, or whether this change in the dune landscape was even caused by animals. Gravity happens, so dune slopes at high angles can also simply collapse under their own weight without the help of trackmakers.
So this is what the Navajo Sandstone dinosaurs did. As they walked down dry dune slopes, their feet initiated grainflows, which preceded each successive footfall. Step, grainflow, then step into a grainflow, which was repeated for however far the theropods or prosauropods walked down dune slopes. However, what the geologic record revealed, which was not readily apparent from contemporary examples of walking down dunes, is that outlines of the dinosaur feet were preserved below the surfaces of the original grainflows. In cross-sections of the sandstone, Loope found theropod and prosauropod tracks directly associated with grainflows, in which the dinosaurs caused downslope movements with each step, then stepped into the grainflow deposit they had just made. The dinosaurs had caused small avalanches, which then buried their tracks, leaving behind the most important clues needed to determine what caused these grainflows in a Jurassic dune field.
With this example in mind, we might wonder how else dinosaurs, through their walking on slopes, might have wrought other changes in their landscapes, such as triggering mass movements of materials in upland areas. For example, did dinosaurs that lived in mountainous regions ever start an avalanche by walking across a rocky slope? Did polar dinosaurs ever start snow avalanches? Both of such examples are extremely unlikely to have been preserved in the geologic record: for one, snow tends to melt, which was also true of Mesozoic snow. Nonetheless, sand dunes and other sedimentary deposits could reveal more secrets of how dinosaurs modified their environments, step by step.
Nesting Grounds: Turning Flat into Bumpy
As we learned from the Late Cretaceous titanosaur nests in Argentina, sauropod nesting grounds probably changed the appearances of their landscapes, especially because they kept coming back to the same area over generations. This meant that sauropod nests, which were probably made by scratch digging, were placed above the graves of former nests, eggs, and embryos. In Montana, the Late Cretaceous hadrosaur Maiasaura and theropod Troodon also seemed to nest in the same places over time, implying that these dinosaurs also may have habitually used nesting grounds. High concentrations of dinosaur eggs elsewhere in the world, such as in South Africa, Spain, and India, likewise point toward how dinosaur parents kept choosing the same places over generations for their nesting. All of this nesting over time meant many ground nests, some of which may have been reused; but once buried by a river flood, new nesting grounds would have been stacked on top of previous ones.
Although we do not have sauropods or ornithopods today that we can study for the effects of their nesting on local landscapes, we do have birds, or theropods, making vast nesting grounds. As mentioned in a previous chapter, many shorebirds make simple scrape nests, but they may also concentrate these nests in vast breeding colonies, congregating by the thousands along oceanic and lake shorelines or on small islands. In some instances, birds in breeding colonies, such as Australasian gannets (Morus serrator), make more prominent nests as raised platforms with a central bowl-like depression for holding eggs. They then place these nest structures close together—just slightly more than the body length of the gannet next door—which creates regularly bumpy and dimpled surfaces with hundreds or thousands of regularly spaced nests throughout their nesting grounds. While on a trip to New Zealand, I remember seeing and marveling at the gannet nest sites there, which were on a few isolated marine platforms just offshore. But while gazing at these busy colonies and their nesting traces, I also wondered whether Mesozoic theropods might have had similar structures, and over extensive areas.
Now, one would think researchers looking for examples of ground-nesting birds that modify their environments via nesting, and as analogs to Mesozoic dinosaurs, might be tempted to look first at ratites. But this would be a mistake, as ostriches, emus, cassowaries, and rheas do not form breeding colonies but instead act more like rugged individualists, albeit family-oriented ones. Rather, one of the best examples of breeding grounds in which nesting birds remade their environments, even affecting the locations of deltas and river channel
s, comes from those long-legged pink birds we revere as lawn-ornament idols, flamingos.
In an article published in 2012, Jenni Scott, Robin Renaut, and Bernhart Owen described the terrain-altering effects of flamingo nesting grounds along lakeshores in the Kenya Rift Valley of eastern Africa. Flamingos there consist of two species, the lesser flamingo (Phoeniconaias minor) and the greater flamingo (Phoenicopterus roseus). These birds gather and breed by the millions next to high-salinity lakes in this area. Feeding, flirting, and fornicating flamingos significantly trample the nearshore and shallow-water muds of these lakes, which these birds also use for bathing. They further contribute to this “flamingoturbation” by constructing thousands of nests out of gooey shoreline mud. Their nests look like miniature volcanoes, wider at their bases (30 cm, or 12 in) than at their tops (20 cm, or 8 in), and about 15 to 20 cm (6–8 in) tall, with a shallow bowl at the top to secure the egg clutch.
Flamingos mine the lakeshore mud, which they do by dragging it with their beaks or scooping up mud in their bills and spitting it onto the nest. These actions result in shallow rings or semi-circles (“moats”) around each nest. Mixed in with the mud shaping the nests were feathers, vegetation, and (somewhat morbidly) the bones of dead flamingos. Insects may add their own traces to the nests, burrowing into and forming brooding cells in their sides, and plant roots sometimes invade abandoned nests. Based on their detailed descriptions of these modern nests, the researchers were confident that they could easily identify ancient examples. Not surprisingly, then, they promptly did this in the same study area, discovering flamingo-nest trace fossils that were more than 10,000 years old, and sometimes directly next to or beneath modern nests.
Dinosaurs Without Bones Page 38
