Dinosaurs Without Bones

by Anthony J. Martin

  Thus it might behoove paleontologists who are interested in learning more about the origins of bird flight to pay attention to flying tracks associated with modern birds. After all, a continuing controversy in dinosaur paleontology—a real one, not a fake one like the old “Was Tyrannosaurus a predator or scavenger?” argument—was how self-powered flight evolved in non-avian theropods. Granted, nobody denies that self-powered flight provided some great advantages for those dinosaurs. For one, it took them to far more places than running, swimming, or gliding, but while using less energy than those means of transportation. Moreover, those places may have offered more choices in food, mates, nest sites, and habitats for raising offspring. This ability especially came in handy for migrating, in which these dinosaurs could more easily switch locations with seasonal changes or severe alterations of local climates. These are big questions that might be helped by looking at little tracks.

  Flying tracks are by far my favorite of all bird tracks to find, and many other people must share this feeling, as the more frequently forwarded photos I receive from ichnologically inclined fans are of these. One of these photos shows a snowy vista punctuated by repeating sets of four mouse tracks, which end abruptly as they coincide with the feathered outline of an owl. The tale is all there in the tracks: a small mouse galloping across the snow, knowing that it is risking its life by being out in the open; an owl spotting it from its roost, then taking off with a whispery flap; a glide down with talons extending and grasping the mouse just as it landed on the snow; a beat of the owl’s wings against the snow surface to continue its forward momentum and become airborne again, but this time carrying a little treat. Sadly for ichnologists, though, such detailed and evocative traces are more likely to be made in snow, which, once warmed, has a rude habit of melting and thus erasing all of the evidence.

  In my experience, the best places to look for flying tracks are in soft mud or sand along a seashore, lake margin, or river floodplain. While examining these tracks, watch for the paired ones, and for gaps in their trackway patterns. Nothing quite says “flight” like a right–left pair of bird tracks with no other tracks in front of or behind them. Do you see no tracks behind them, followed by a normal trackway? These are landing tracks. Do you see a normal trackway that ends with two tracks, and nary a track after that? These are take-off tracks. Other details to note in such tracks are linked to whether a bird was trying to control its descent or begin its ascent. For instance, in landing tracks, birds with rearward-pointing toes (digit I) on each foot direct these forward, leaving long claw-marks while “putting on the brakes.” As the other forward-pointing digits contact the ground, these will push against the mud or sand, forming mounds in front of those digits as the entire foot comes to a halt. For take-off tracks, these reveal whether a bird left the ground instantly—with a burst of wing-driven power, it is aloft—or needed a little more forward momentum, such as a running, skipping, or hopping start, aided by much flapping. In such trackways, distances between the sets of tracks become greater, reflecting how lift forces gradually took the bird farther upward into the wild blue (or mild gray) yonder.

  The controversy about the evolution of flight in dinosaurs is not whether or not it happened, but how it happened. At first only two hypotheses were offered, with pithy summary titles: “ground up” and “trees down.” The “ground up” hypothesis says that certain lineages of non-avian theropods at first were ground dwellers but later evolved flight through a combination of natural selection for longer arms, fast running, lighter body frames, and flight feathers. The “trees down” hypothesis states that those same originally ground-dwelling dinosaurs climbed trees and otherwise became more arboreal, and at first were gliders. But then natural selection favored those that could flap under their own power instead of just glide.

  Fortunately for scientists who do not like dichotomous arguments, a third hypothesis was offered just recently, and it synthesized elements of the other two with the special bonus of sounding more technical. Called the “wing-assisted incline running” (WAIR) hypothesis, the researchers who developed this model—mostly Kenneth Dial and his colleagues—noticed how baby birds, with only half-formed wings, flap these while running up inclines, including vertically oriented tree trunks. Thus, flapping combined with running helps the birds to get up those surfaces much more easily than if they did not flap at all. Half-formed wings also come in handy when baby birds fall out of nests, as flapping slows them down enough that they are more likely to survive once they strike the ground.

  This is where trace fossils could help to test and otherwise augment these hypotheses, especially in Middle through Late Jurassic rocks, which is when paleontologists suspect that birds began separating from dinosaurs, evolutionarily speaking. Track evidence supporting ground-up flight would show small theropod tracks with take-off patterns similar to those we see in modern birds, but probably more typical of those that need a little more of a running or hopping start before becoming airborne. Track evidence for trees-down or incline-assisted flight is a little more challenging from a trace fossil standpoint, as this would require paleontologists to recognize scratch marks on fossil tree trunks and branches, and to distinguish scratch marks made by climbing versus running while flapping. Although not completely impossible, such evidence would have a much lower preservation potential than those made on muddy or sandy surfaces. The best trace fossils related to this hypothesis, of course, would be those showing the landing marks of an awkward baby dino-bird—complete with feather impressions—that also show it walking away from its crash site, successfully surviving a fall from a nearby tree.

  Have non-avian dinosaur tracks indicating flight, however brief, been recognized from the fossil record? Not yet. However, some Early Cretaceous (about 120-million-year-old) small-shorebird tracks from Korea, reported by Amanda Falk and others in 2009, bear some of the same traits I just described for landing and take-off tracks. In 2013, Pat Vickers-Rich, Tom Rich, and I interpreted two of three closely associated anisodactyl tracks from Early Cretaceous rocks of Victoria, Australia as bird tracks, with the third coming from a non-avian theropod. These turned out to be the oldest bird tracks known in Australia, dating from about 105 million years ago. One of the tracks, made by a bird’s right foot, also had an elongated mark left by digit I, matching modern examples I’ve seen formed by herons in their landing tracks. Unfortunately, the slab of rock was broken along the front edge of this track, so we could not further test our provocative hypothesis by checking whether another track was paired with it, and bearing the same marks of flight. When we published our results, we encouraged other paleontologists to start thinking about evidence for flight whenever studying Cretaceous tracks, whether these were from birds or non-bird theropods.

  One last point I would like to make about bird tracks, and a surprising one to many people, is how these traces can actually alter environments. For instance, if you ever see mudcracks in an area frequented by birds, take a closer look at the geometry of those cracks and you may see the familiar three- or four-toed patterns of bird tracks in them. I first noticed this phenomenon during a hot summer in 2004 while walking along the edge of a pond on San Salvador Island, Bahamas. Yellow-crowned night herons (Nyctanassa violacea) and black-necked stilts (Himantopus mexicanus), both long-legged birds with thin toes, had been hunting for crabs in the muddy areas around the pond, and both species made hundreds of tracks. Wherever these birds’ feet had punctured the muddy surfaces, sunlight dried the mud exposed along the edges of the footprints, which started cracks that grew and radiated from these edges. The mudcracks grew enough that they sometimes joined, especially where the birds’ tracks were closer together (a result of slowing down while stalking crabs). From then on, I’ve looked for avian-caused mudcracks elsewhere and was gratified to find them in Arctic environments. In the summer of 2007, while doing field work at a dinosaur dig site on the North Slope of Alaska, I noticed bird tracks—this time from seagulls, geese, swans, and plovers�
��in the muddy patches of a river floodplain. Sure enough, wherever the muddy surfaces had dried, mudcracks developed, and nearly all connected perfectly to the bird footprints. The extended sunlight of a polar summer—with the sun setting for only a few hours during each 24-hour cycle—accelerated the drying after birds had pierced the muddy surfaces with their feet.

  Did non-avian theropods ever cause mudcracks from their tracks? Yes, indeed. Among the extensive collections of dinosaur tracks at the Amherst Museum of Natural History in Massachusetts are slabs of Early Jurassic sandstones with gorgeously defined natural casts of mudcracks. Some of these mudcracks have theropod tracks in the middle of them, with cracks joining theropod clawmarks. The same apparent causal relationship between theropod tracks and mudcracks shows up in Early Jurassic rocks of southwestern Utah, in the same strata holding thousands of dinosaur swim-tracks, and is one of the better surviving examples of such intersections between dinosaur trace fossils and their correspondingly altered landscapes.

  Nests, Empty and Otherwise

  Bird nests are extremely variable as traces, ranging from simple scrapes in the ground to some of the most elaborate structures made by any living vertebrates. Examples of scrape nests are those of penguins or some shorebirds, such as oystercatchers (species of Haematopus) or plovers (mostly species of Pluvialis and Charadrius), which use their feet to scratch out slight depressions into which they deposit a few eggs. Among the latter are the massive individual nests of bald eagles (Haliaeetus leucocephalus), as well as the communal nests of the sparrow-sized sociable weaver birds (Philetairus socius). The largest bald-eagle nest yet measured was 3 m (10 ft) wide, 6 m (20 ft) deep, and weighed more than two tons. These nests are usually constructed in trees sturdy enough to hold such nests or on the tops of rocky cliffs. Sociable weaver nests of South Africa are among the most spectacular of all avian-made traces. Unlike the individual typical cup- or saucer-shaped nests people may normally see in urban settings, these nests are made collectively, shared, and reused by hundreds of weaver birds. Placed in stout trees or on telephone poles, weaver nests are composed of multiple chambers and entrances, are as much as 3 to 7 m (10–23 ft) in outline, and are used for both nesting and shelter by non-nesting adults. They are so massive, other bird species nest in or on them.

  Not to be outdone, the mallee fowl (Leipoa ocellata) of Australia constructs gigantic incubation mounds for its eggs. These mounds can be as much as 4 m (13 ft) tall and 10 m (33 ft) wide, representing the movement of more than 200 m3 (7,000 ft3) of soil. For incubation, the parents let anaerobic bacteria do all of the work for them, breaking down organic material in the mounds and generating enough heat to keep the eggs toasty. In short, birds can make modest nests, incredible nests, and everything in between.

  Although bird nests are almost as diverse as birds themselves, they can be placed into eight basic kinds, based on where they are located or their overall shape: scrape, platform, crevice, cup (or saucer), spherical, pendant, cavity, burrow, and mound. Of these, probably the one most people see are cup nests in trees, in which various songbirds arranged sticks or other vegetative debris—perhaps cemented by mud, bird spit, or feces—into a semi-circular bowl that keeps eggs from falling out. Indeed, of the nest categories, most of these are in trees, although a few are on or in the ground, such as scrape, platform, burrow, and mound nests.

  Based on what we know about the evolution of birds from dinosaurs, bird nests probably started on the ground, but with underground as another possibility. The earliest known dinosaur nests, made by the Early Jurassic sauropodomorph Massospondylus of South Africa, were definitely on ground surfaces. At the other end of the Mesozoic Era, Late Cretaceous Maiasaura, Troodon, and titanosaur nests were bowl-like depressions surrounded by raised rims. A burrow entombing the small Cretaceous ornithopod Oryctodromeus and two of its offspring may or may not have been a nest site, although it was likely used as a den for raising its young. Of these dinosaurs, Troodon was the most closely related to birds, with at least one species of feathered troodontid known. Moreover, based on dinosaur egg porosities, nearly all were likely partially buried, whether in sediment or vegetation. So for now, paleontologists assume that nearly all dinosaurs nested on or in the ground.

  Yet at some point in the Mesozoic Era—probably in the Early Cretaceous—nests began going up and into trees. This behavioral and evolutionary innovation likely happened in many places with multiple lineages of theropods. However, the proliferation of nesting must have been hosted in forested ecosystems, which provided plenty of opportunities for small theropods to go up and into trees, using these as safe havens from predators looking for eggs, baby theropods, or theropod parents. Also, just to show the plasticity of such behavior, some modern birds that we may normally think of as ground nesters—such as wild turkeys (Meleagris gallopavo)—may use trees on an ad hoc basis. In one instance, egg predation from feral hogs on a Georgia barrier island was so pervasive that wild turkeys began nesting in the trees there. In other words, trying circumstances can sometimes prompt birds to resort to behaviors that might have been deeply buried in their evolutionary histories.

  As one might imagine, nests consisting of collections of sticks, leaves, feathers, and mud—however artfully or systematically arranged—had poor fossilization potential. The possibility of winning the fossilization sweepstakes shrank even more if these nests were high up in trees, away from entombing sediments. Not surprisingly, then, stick nests attributable to non-avian or avian dinosaurs have not yet been interpreted from the Mesozoic Era. Much more likely to be preserved are massive nest mounds, like those made by modern mallee fowls or some of the extinct birds of New Caledonia.

  Nevertheless, even though trace fossils do not yet tell us exactly when non-avian or avian theropods first started nesting in trees, we do know when they bored into tree trunks to make cavity nests, like those made today by woodpeckers. At least one vase-shaped structure described from a petrified log from central Europe, dating from the Miocene Epoch (about 23 to 5 mya), is remarkably similar to woodpecker nests. Because this was well after the dinosaurs were gone, and no older ones have been found yet, this fossil cavity nest implies that the exploitation of tree trunks for nesting might have been a totally Cenozoic innovation in birds. Yet all we need to start a good argument on that point is a trace fossil of a Late Cretaceous cavity nest.

  Burrows: The Avian Underground

  Some birds are impressive burrowers. In my experience, this statement surprises many people until you say the words “burrowing owls” (Athene cunicularia), which elicits big smiles and vigorous head nodding in appreciation of these adorable birds. Yet these owls are not the only birds that use burrowing as part of their normal lifestyles. In North America, belted kingfishers (Megaceryle alcyon), bank swallows (Riparia riparia), and rough-winged swallows (Stelgidopteryx serripennis) burrow into soft, sandy bluffs adjacent to rivers or other bodies of water for their nesting and raising of young. In Europe, Africa, and elsewhere in the world, all species of bee-eaters make bank burrows, sometimes by the thousands at nesting sites. The Atlantic puffin (Fratercula arctica) of North America digs lengthy burrows, some of which are 3 m (10 ft) long and 1 m (3.3 ft) below the ground surface, a large burrow for a small bird. Some species of penguins, such as the little penguin (Eudyptula minor) of Australia and New Zealand, as well as the Magellanic penguin (Spheniscus magellanicus) of southern South America, also excavate burrows.

  In all instances, birds use these burrows for nesting. Nonetheless, burrows also have the great multi-purpose advantage of protecting chicks and adults from predators while providing a place that maintains a near-constant temperature year-round. The latter especially comes in handy in high-latitude environments, where trying to stay warm during wintertime without shelter could quickly sap energy reserves.

  Because birds do not have shovels, nor arms well adapted for holding shovels, they must use a combination of their beaks and rear legs to excavate a burrow. Imagine carrying out a mou
thful of sediment at a time or scratching with your feet to make a tunnel many times longer than your body: like The Shawshank Redemption, but without even the benefit of a little rock hammer. These burrows are not just tubular, either, but also expand at their ends to include a nesting chamber. These chambers must be large enough to accommodate eggs, chicks, and at least one adult to tend to the eggs and feed their offspring, which have the annoying habit of growing bigger with multiple feedings.

  An important point to know about burrowing birds, however, is that they may reuse or steal burrows, then—like squatters taking over a house—remodel and otherwise modify them to suit their own needs. For example, burrowing owls may take over appropriately sized mammal or gopher-tortoise burrows. Rough-winged swallows might usurp kingfisher burrows, and vice-versa. Some birds, though, may treat their burrows more like vacation homes, coming back to them at certain times each year. A male–female pair of Atlantic puffins, which normally mate for life, might return to the same burrow they used the previous year for nesting, then do some home improvement to prepare the nursery for their next brood. If the old burrow collapsed or otherwise is in bad shape, they may dig a new one. Because puffins are quite sociable, their togetherness results in huge colonies; this means they dig many burrows and many generations of burrows, which often intersect with one another, making for massive composite traces below ground. Complicating these underground neighborhoods is another bird, the Manx shearwater (Puffinus puffinus), which also nests in burrows among all of the puffin burrows. This implies that what might be classified as a “bird-burrow complex” in the geologic record might actually be the result of multiple species living in close relation to each other.