Dinosaurs Without Bones (15 page)

Take away a parent dinosaur’s skeleton, though, and discerning a dinosaur nest becomes a little more challenging. For instance, a cluster of identical dinosaur eggs is sometimes considered as indirect evidence of nests. However, unless these eggs are inside a bowl-like structure, show some sort of post-laying arrangement, or have remains of a parent dinosaur on top of the eggs, a collection of eggs may not be a nest after all.

This doubt is cast because the eggs may have been moved after laying. As anyone who has done an Easter-egg roll can attest, or had an egg fall from a kitchen counter to the floor while trying to make an omelet, well-rounded eggs can move easily from one place to another under the influence of gravity. In nature, eggs can be rolled or floated by currents, then deposited in low-lying areas with other debris that, to an untrained eye, might look like someone’s idea of a nest. Indeed, the discovery of a dinosaur egg—containing embryonic bones, no less—in Late Cretaceous shallow-marine deposits in Alabama shows that at least one dinosaur egg was capable of floating a long way from land. As we learned previously, dinosaurs may have occasionally swum, but like modern birds or reptiles, were not inclined to make underwater nests.

In order to better understand what constitutes a dinosaur nest as a trace fossil, a good starting place is to look at the traces associated with modern vertebrates that lay eggs and make nests, such as those of reptiles and birds. If talking just about reptile nests, these can be summarized into two broad categories: ground nests and hole nests, with ground nests made on the ground surface and hole nests below the surface. From there, each type can vary. For example, ground nests can range from simple hastily scraped depressions made by some lizards, to large mounds of vegetation amassed by alligators.
Hole nests are also diverse, ranging from the blunt, shallow holes of freshwater turtles and tortoises, to the more elaborate excavations of sea turtles and crocodiles.

On the other hand, bird nests are as crazily diverse as birds themselves, almost defying facile categorization. Bird nests range from simple scrapes in the ground, to better defined excavations, to tunnels in the ground or in vertical bluffs, to elaborately woven and architecturally complex structures made of a wide variety of natural and man-made materials. Some birds resemble reptiles in their nesting behavior by constructing nests on the ground or as underground burrows, but flight has also made it easier for birds to build nests well above ground surfaces, such as in trees, cliff faces, or buildings. Furthermore, by “in trees,” this is sometimes literal for woodpeckers that actually bore into tree trunks with their beaks, hollowing out areas to lay their eggs and raise hatchlings inside trees.

How did dinosaurs fit in “reptile vs. bird” models for nesting behavior? It turns out they were somewhere in between, although definitely leaning toward behaviors we observe today in some reptiles and ground-nesting birds. For one thing, every dinosaur nest structure recognized thus far is a ground nest. Hole nests like those of sea turtles or crocodilians, or more fancy structures such as those of some tree-dwelling birds, aren’t yet known for dinosaurs. Of course, this current lack of evidence does not necessarily mean that no dinosaurs made either underground or arboreal nests. For instance, one good candidate for underground nesting would have been the small Cretaceous ornithopod
Oryctodromeus cubicularis
, which made dens for raising its young (discussed more in the next chapter). For arboreal nests, a few small feathered tree-climbing, gliding, and flying non-avian dinosaurs are known from Early Cretaceous rocks of China. So these dinosaurs feasibly could have made their nests in trees, just like modern birds. However, until we have some evidence for these, we can only speculate.

For those people who might wonder how to tell a hole in the ground from, well, other things, how would someone recognize a
dinosaur nest in the fossil record? Fortunately, paleontologists who have interpreted the few indisputable dinosaur nests in the geologic record made a nice little checklist for the rest of us to follow, which helps considerably. Here is a summary of that list, but posed as questions to ask when encountering a depression in a Mesozoic rock that might be a dinosaur nest:

1. Does the depression cut through any layered sedimentary rock below it? This is an especially good question to ask when investigating whether the rock that preserves a potential nest has different colors and grain sizes than the sediment above it. For example, the sediment below the depression might be a green rock mostly made of mud (mudstone), whereas the sediment filling it might be a red rock composed of sand (sandstone).
   
2. Does the depression hold lots of entire (or nearly entire) eggs, along with skeletons of baby or otherwise young dinosaurs, and all of the same species? Be careful with this one, though. As mentioned before, eggs can be transported far away from where a mother dinosaur originally laid her eggs, meaning such a depression might simply be where these eggs and skeletal remains accumulated after being washed about by currents.
   
3. Does the depression also have a raised rim around it, accentuating its basin-like appearance? A related question to ask is, is this rim also made of sediment differing from what is underneath or outside of it? These rims would have served an important primary purpose, such as preventing rounded eggs from rolling away as a mother dinosaur laid them.
   
4. Does the rock filling in the depression and covering the rim have its own distinct traits, like different colors and textures (mentioned before), bedding, or insect burrows and cocoons? Such features might hint of a fossil soil (paleosol), which would tell paleontologists
   that the depression was exposed at the ground surface before being covered, as opposed to, say, at the bottom of a river.
   

However facile this checklist might seem, it represents a lot of previous work on dinosaur nests. Knowledge of some of the history behind it may provide a perspective on how what we know now about dinosaur nests also reminds paleontologists how much we still need to learn about these trace fossils.

The First Recognized Dinosaur Nests

Anyone who has not studied the history of dinosaur studies might be astonished to know that the first genuine dinosaur nests were not interpreted until 1979. At the time of this discovery, more than a hundred years had elapsed between paleontologists first linking fossil eggs to dinosaurs, which was in 1869. Furthermore, only a few of the previously mentioned criteria were applied to these nests, showing how far paleontologists have come since then in defining them.

In what became a revolutionary discovery, inspiring nearly everyone to reconsider what they thought they knew about dinosaur parenting, John (“Jack”) Horner and his friend Robert (“Bob”) Makela, while prospecting Late Cretaceous rocks in Montana, found depressions filled with eggs and partly grown juveniles of the large ornithopod dinosaur
Maiasaura peeblesorum
. Horner and Makela inferred the presence of nests on the basis of many nearly entire and identical eggs, some containing embryonic skeletons of
Maiasaura
, which were in the bottom of what looked like indentations to them. These former hollows were visible as differently colored sediment above and below the eggs; unfortunately, though, no sedimentary rims were present. So either these dinosaurs did not make such rims, the rims had been eroded and not preserved, or these researchers missed them because of too-subtle differences in the sediment.

Still, on the basis of localized assemblages of eggs and baby dinosaur bones in the same small areas, they hypothesized that
this part of Montana was a
Maiasaura
nesting ground about 75 to 80
mya
. The close spacing of the nests also hinted, for the first time, that dinosaurs nested communally. Subsequent field work done in the same area yielded more and different eggs, which at first were linked to the small ornithopod dinosaur
Orodromeus makelai
(yes, this species was named in honor of Makela), although later they were connected to
Troodon formosus
. Hence, for the first time paleontologists began thinking of dinosaurs and their nesting behaviors as more akin to those of ground-nesting birds and not reptiles. Even more exciting, the find of one nest with fifteen juvenile
Maiasaura
in it, all the same size and apparently in the same age range, was strong evidence favoring extended parental care in this species of dinosaur. Otherwise, why stay in a nest unless your parents are feeding you? (All of you parents with children who have graduated from college yet are still living at home, you may now wearily nod your heads in agreement.)

Despite this seminal work by Horner, Makela, and others, the full checklist for interpreting dinosaur nests as trace fossils would not come about until more than twenty-five years later, stemming from studies of nests of two dinosaurs very different from
Maiasaura
and from one another: the small Late Cretaceous theropod
Troodon formosus
of Montana, and massive Late Cretaceous titanosaurs (sauropods) of Argentina. Amazingly, their nests are not so different in their overall sizes, despite titanosaurs weighing about a thousand times greater than
Troodon
(50,000 vs. 50 kg, or 110,000 vs. 110 lbs). However, each of their nests have their own distinctive forms, leading to better understanding of how each type of nest was made.

First, let’s talk about
Troodon
nests. In a geological coincidence that was not a coincidence, paleontologists discovered these nests in the same location as the
Maiasaura
nests—a place that was nicknamed “Egg Mountain”—and in the same sedimentary rocks of the Late Cretaceous Two Medicine Formation. Recognized by David (“Dave”) Varricchio, who was a Ph.D. student of Horner’s in the early 1990s, these structures were the first dinosaur nests defined solely as trace fossils. Interestingly, until Varricchio and
his colleagues worked on these nests, Horner and Makela attributed
Troodon
eggs to the small ornithopod
Orodromeus
, a case of mistaken identity that was fairly common with dinosaur eggs until just recently.

Full disclosure: Dave and I overlapped academically when we both attended the same graduate school (University of Georgia, Athens) in the late 1980s. In between our respective research projects, as well as going to local music clubs to watch melodic toe-tapping performances by bands such as Agent Orange, Black Flag, and other colorfully named troubadours, we both took a class from Robert (“Bob”) Frey, simply titled
Trace Fossils
. At the time, Frey was one of the foremost experts in ichnology, so we felt privileged to take a class from him. But it also resulted in Dave and me sharing an ichnological worldview that stuck with both of us. Thus I was not surprised to later read about his recognition of the
Troodon
nests, which he and his colleagues described quite properly as dinosaur trace fossils. Dave also dedicated the first paper about the
Troodon
nests to Frey, an acknowledgment of how good mentorship can later contribute to discovery.

The first
Troodon
nest structure noted at Egg Mountain surrounded a clutch of 24 asymmetrical eggs, which, like chicken eggs, were wider at one end than the other, but more elongated and voluminous, having held about 0.3 liters (1.2 cups) each. Eggs in the clutch were oriented almost vertically, narrow ends down but also pointing toward the center. They were unhatched and clustered in an oval space smaller than most serving trays, measuring only 45
3
56 cm (18
3
22 in). The eggs defined a ring-like pattern, with what Dave and the other paleontologists called an “egg-free space” in the middle. This clutch was surrounded by a semi-circular structure that was about

1.6 to 1.7 m (5.3–5.5 ft.) wide, about the diameter of a small kiddy pool. The “egg-free space,” egg clutch, and rim shared a north-south orientation, making concentric oval rings; this orientation was also apparent in another
Troodon
egg clutch at the same site.

The rim of this structure sloped abruptly, and then more gradually inward to a flattish area about a meter (3.3 ft) wide, where the
egg clutch was centrally located. The exterior of the nest was defined by a rim that was 10 cm (4 in) tall and about twice as wide. The rim was composed of micrite, a hard, resistant, fine-grained limestone; it and the egg clutch were buried under a mudstone. Unhappily, in recovering the egg clutch, the field crew had to destroy part of the nest to extract the eggs. Still, they uncovered enough of the nest to realize what it was before taking out the egg clutch, and surely stood back and gazed in awe at it when it was fully exposed. The bulls-eye pattern caused by the egg clutch, dead center in the structure and circumscribed by the rim, left little doubt of its significance. This was a nearly perfect example of a dinosaur ground nest, and must have been made by one or both
Troodon
parents before the eggs were laid.

Before making a nest,
Troodon
parents must have first scouted their surroundings to find suitable locations. For example, bad places to nest would have included underwater (eggs drowned), in places with abundant scavengers or predators (eggs eaten), areas with high traffic from other dinosaurs strolling through the area (eggs smashed), or on solid rock (eggs exposed, perhaps fried). Consequently, these parents probably sought out areas with well-drained and crumbly soils that were also well above the local water table, and away from other animals that might have accidentally or purposefully harmed the eggs. The fortuitous fossilization of the eggs and nest must have happened because a nearby river overflowed its banks and buried both before the eggs hatched. Thus even though the dinosaurs had made a nest rim high enough to prevent most rising waters from enveloping the eggs, it wasn’t high enough in this instance. In other words, what was unlucky for the expectant
Troodon
parents turned out to be fortunate for the paleontologists about 75 million years later.

Although we still don’t have sufficient information to say for sure,
Troodon
nest building probably involved their using a combination of rear feet, hands, and snouts to excavate underlying soft sediments. Making a circular nest probably required digging in a circular pattern, and the easiest way to make a doughnut-shaped pile of soil outside a depression would have been by moving either
clockwise or counter-clockwise from a central point inside of the nest. The width of the nest, rim included, was about half the total body length of an adult
Troodon
(3 m, about 10 ft), implying that only one adult at a time could have fit comfortably in the center to dig. Two at a time would have gotten in each other’s way, but both parents also might have taken turns.