Read Dinosaurs Without Bones Online
Authors: Anthony J. Martin
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:
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.
Paleontologists working the site were also excited to find six definite nest structures directly associated with egg clutches. Five of these six nests were adjacent to one another and less than a meter (3.3 ft) underneath a bed with abundant adult-sized sauropod tracks. All of this trace fossil evidence suggested that these nests might have been made at about the same time, and that adult sauropods came along later in the same place as the nests, perhaps to start another generation of titanosaurs.
Probably the most surprising trait of these titanosaur nests, though, was their size, which did not match the proportions of their tracemakers. Instead, some of these nests had about the same width and depth of the
Troodon
nests, measuring about 1 to 1.4 m (3.3–4.6 ft) wide and 10 to 18 cm (4–7 in) deep, respectively; a few were only half the size. Some of these nests—like those of
Troodon
—also had partially preserved rims, although these were made of sandstone, not micrite. The nests cut into an underlying well-stratified sandstone that was otherwise nearly identical to the sandstone composing the rims, implying that the rims were originally derived from soft sand below. These were shockingly delicate works for creatures of their size, like an elephant molding a peanut bowl out of clay.