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.
The soil piles defining the nest exterior also likely required its maker(s) to pat these down and otherwise mold the rim of the nest, firming it so that it later hardened, perhaps cemented by precipitating minerals. A firm exterior would have been essential, as loose mounds of dirt would not have offered much protection to the eggs: one hard rain or windstorm would have easily eroded such inept attempts to shield their brood. The gentle slope on the interior of the nest also suggests this area was somewhat compacted, perhaps by trampling in a grape-stomping sort of way. However, this might have been the final action of nesting before actual brooding, happening after the eggs were laid. Because the eggs were oriented vertically, they must have been stuck into soft sediment first; a parent Troodon then may have compressed the surrounding soil to reinforce and stabilize them. Regardless of when the nest interior was finished, the nest rim must have been sculpted—like hand-building with wet clay—an action that could have been done from inside or outside the nest.
How long did it take a Troodon (or pair) to make a nest? The answer depends on whether just one or both parents were making it. If both were involved, the time might have been halved, but as many human couples know all too well, this would have depended on their level of cooperation (or lack thereof). Nevertheless, one can easily imagine cooperative digging and molding, with one parent on the inside and the other on the outside, making a nest together, and thus decreasing their labor. Just to compare, though, crocodilians and sea turtles normally take just a few hours to dig a hole nest. However, this isn’t the best model for how dinosaurs constructed a ground nest, as these hole nests are less complex structurally than what the dinosaurs made.
A more realistic estimate might be gained by looking at times required by alligators and mound-nesting birds to make their egg-protecting structures. For alligators, constructing a nest normally takes about 10 to 15 days. On the other hand, mound-nesting birds may need months to build the massive, vegetation-laden structures they use for burying and incubating their eggs. Some of these mounds, such as nests of the Australian brush turkey (Alectura lathami), can be 2 m (6.6 ft) tall and 20 m (66 ft) wide. Based on the comparatively modest proportions of Troodon nests, the bowl-like structure itself could not have taken more than a few weeks to dig. These nests have no evidence of having been topped by vegetation, but if they were, their construction would have taken a
bit longer.
Troodon egg-laying itself probably took a couple of weeks, assuming that each pair of eggs took at least a day to emerge from the mother’s two oviducts. These anatomical traits of Troodon mothers are interpreted on the basis of how eggs are paired. Mentioned earlier as “statistically significant,” you actually don’t need statistics to see the pairing, though, as it is obvious in every nearly complete Troodon egg clutch studied thus far. As noted before, the eggs also must have been aligned vertically and stuck in the ground by one or both of the Troodon parents. The most parsimonious way to accomplish this sort of interior decorating would have been to:
Lay two eggs;
Dig two shallow holes next to each other, each slightly more than the width of and half the length of an egg;
Place the eggs in their respective holes, pointy ends down;
Compact the soil around the eggs;
Repeat with each new pair of eggs until done;
Sit above eggs until hatched.
Another intriguing trait of the eggs, when oriented like this, is that they had a greater concentration of pores toward their exposed tops. More pores were there so the rest of the embryo living underneath the eggshell could breathe easier in that position. This, along with all of the other trace and body fossil evidence, implies that the eggs were not buried under a thick pile of vegetation or soil, but more likely were protected by a Troodon parent sitting above the egg clutch. This is a reasonable assumption, as the relatively small size of these dinosaurs meant they would not have crushed the eggs. Additional evidence from body fossils of related theropods also suggests that Troodon and its close kin had feathers, which would have provided some insulation for the eggs, too. Even better evidence of brooding behavior is the adult itself, as is seen with Citipati and Oviraptor: the bones of an adult Troodon, found directly in contact with a partial clutch of Troodon eggs.
Further study showed that this Troodon, as well as Citipati and Oviraptor, were probably male, providing an important clue related to how dinosaur parents took care of an egg clutch. For instance, in modern birds, large egg-clutch sizes correlate with paternal care, whereas small clutches correspond with both parents helping with the brooding. Hence large clutches for Troodon, Citipati, and Oviraptor, along with their skeletons on top of these egg clutches, implied they were male. Also, large modern ground-nesting birds—such as emus, ostriches, and rheas—females develop medullary bone. This bone has a distinctive texture, imparted because the mother birds extract calcium from their bones to help build calcium carbonate in the eggshell microstructure. In contrast, male birds lack medullary bone, as did the bones of Troodon, Citipati, and Oviraptor. As a result, this absence suggests that the adult dinosaurs on top of each nest were not the mothers, but the fathers.
Not coincidentally, this same sort of paternal protection is a modern behavior likewise seen in nesting emus, ostriches, and rheas. Emu fathers, for instance, sit on or otherwise stay close to egg clutches laid by the mothers for 50 to 55 days, even eschewing food and water during that time. Once their babies hatch, they consume the leftover eggshells, which not only gives these new fathers much-needed sustenance but also decreases the likelihood that a predator will smell freshly hatched eggs and make its way to the nest for some easy snacks. Did Troodon have the same behaviors, cleaning up a newborn mess while also getting a long-awaited meal? So far we don’t know, and the only trace fossil evidence that could support this would be a nest structure with scraps of eggshells but also holding abundant footprints by at least one adult and hatchlings.
What were ecosystems like in the same area of the Troodon nests? Much study on the sedimentary rocks there, including their geochemistry, revealed that this part of Montana was probably warm and semi-arid 75 to 80 million years ago. Furthermore, Maiasaura and Troodon nested on river floodplains that were cut by meandering rivers and dotted with small lakes. Relatively little vegetation was in the immediately surrounding landscape, based on how paleontologists have not found many fossil trees or plant-root trace fossils directly associated with the nesting grounds. Nonetheless, some root traces in nest rims show that at least a few plants may have taken root in these after having been built. Forests were farther away from the nesting grounds, as was an active volcano that occasionally erupted and helped to preserve both the trees of these forests and dinosaur bones. Some sedimentary deposits near, above, and below the Troodon and Maiasaura nests further indicate periodic flooding, which along with other geological evidence suggests monsoonal environments with annual dry and wet seasons. However, the area must have been suitable enough for nesting to motivate Troodon mothers to keep coming back, as paleontologists found at least ten egg clutches from three separate levels in the rock.
I was reminded of this previous observation during a visit at Egg Mountain on a field trip led by Varricchio and one of his colleagues, Frankie Jackson. On this visit, I noted how the distinctive light-brown micrite composing the rim of the undoubted Troodon nest was also apparent in at least three horizons of the hillside there. These may have been partially preserved nests, but ones not accompanied by eggshells or bones: trace fossils in the purest sense, unadulterated by body fossils. If so (although this requires more testing), this would be another way to find out whether Troodon had what modern biologists have called site fidelity. This is a behavioral trait shared with nesting sea turtles, crocodilians, and birds, in which mothers come back repeatedly to the same nesting site for brooding, or their offspring return to the same site where they hatched.
To better fill out this picture of the dinosaur nesting environments, and as an example of how dinosaur trace fossils are best interpreted with trace fossils made by other animals living with the dinosaurs, I later did a study with Varricchio on fossil insect nests near the Troodon nesting sites. Over years of field work there, he and other paleontologists in the area had noticed that several strata contained concentrated zones of fossil insect cocoons and burrows. I was astounded the first time I visited these sites, as the fossil cocoons were so abundant that handfuls of them could be scooped up from the ground where they eroded and fell out of the outcrops. The cocoons were also sometimes directly associated with dinosaur nests, having been reported previously by Horner, Varricchio, Jackson, and others. In fact, during several subsequent visits to this area, I noticed cocoons embedded in the rims of a few of the Troodon nests, showing that insects may have been nesting at the same time and same place as the dinosaurs.
The cocoons were exquisitely preserved, some with impressions of spiraled silk weaves still apparent, and a few burrows bore scratch marks from the legs of their makers. These trace fossils were apparently preserved through filling of the burrows and cocoons by fine-grained sediment, followed by microscopic precipitation of micrite. This process was further aided by a semi-arid climate, which would have caused rapid evaporation of water in between sediments, forming calcium-rich soils called calcisols.
What we also eventually figured out about these insect trace fossils was a little surprising. Based on the appearances of the cocoons and burrows, we surmised that burrowing wasps probably made most of them, with perhaps only a few made by beetles or other insects. Because modern ground-nesting wasps also burrow into and pupate in well-drained soils, we presumed that these trace fossils were made in similar soils. These insect trace fossils thus affirmed that Troodon probably picked its nesting locations for the same ecological reasons as the wasps and other insects. Horner originally conjectured that these cocoons, which were also near the Maiasaura nest sites, were those of carrion beetles, which he imagined fed on hatched eggs, or dead eggs and hatchlings. In the light of the new interpretation of the cocoons, his more gruesome scenario is now doubted. In contrast, dinosaurs and wasps may have peacefully co-existed, just like most people and wasps do today, with only a few sting-induced exceptions.
In summary, because of a wonderfully complete amount of both body and trace fossil evidence—but especially trace fossils—Troodon formosus
of Montana is arguably the best understood of nesting dinosaurs, only rivaled by its neighbors in the same field area, Maiasaura. This is not to say that we have stopped learning about Troodon and its nesting, but the holistic approach taken toward its study, including ichnological perspectives, gave paleontologists a superb model for comparing with other nesting dinosaurs.
Big Dinosaurs Scratching Out Big Nests
Thanks to Maiasaura and Troodon for their tracemaking abilities, as well as exceptionally good fortune in both fossil preservation and discoveries, paleontologists had a new search image for dinosaur ground nests as trace fossils going into the 21st century. Thus it was not surprising that one of the same people who studied the Troodon nests and eggs with Varricchio in Montana, Frankie Jackson, later noticed and defined similar structures in Late Cretaceous rocks of Patagonia, Argentina, in 2004. However, what was astonishing about these nests was that they were not from a mere 3-m (10 ft) long dinosaur like Troodon, but were made by titanosaur sauropods, which were among the largest animals to ever walk the earth.
The wealth of evidence supporting nest building in these sauropods included rimmed nest structures, but also complete egg clutches, embryonic titanosaur bones inside the eggs (some preserved with skin impressions), and at least four horizons containing nests or eggs. The latter showed that the titanosaurs returned to this nesting site over years and perhaps generations, suggesting site fidelity in these dinosaurs too. Further supporting this idea, paleontologists working at this site figured it held thousands of egg clutches, varying from eggshell fragments to entire eggs. The number of eggs was 15 to 35 per clutch, and each individual egg was big, about 13 to 15 cm (5–6 in) wide. Also, eggs were shaped like slightly squashed balloons: round ones, that is, not the cylindrical ones used to make funny balloon animals (including sauropods). Unlike the Troodon egg clutches, the clutches did not show pairing or any post-laying arrangements by the sauropods, and looked more haphazardly placed.
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