Read Dinosaurs Without Bones Online
Authors: Anthony J. Martin
However, where these nests diverged the most from the
Troodon
nests was in their overall shape. Whereas the best-preserved
Troodon
nest was nearly circular when viewed from above, the titanosaur nests were longer than they were wide, giving them
oblong or kidney-bean profiles. This made the titanosaur nests look less like kiddie pools and more like troughs. Moreover,
Troodon
egg clutches were tightly packed in a relatively small central area of the nest, whereas the titanosaur eggs were spread throughout their nests, filling most of the interior space. Complicating matters further, the expansion and contraction of soils around the eggs fractured and moved them after burial. This meant that some eggs laid by different sauropod mothers at different times were mixed together. The egg arrangements of
Troodon
and these titanosaurs were as different as the rooming habits of Felix and Oscar (respectively) in
The Odd Couple
.
So although these titanosaur nests shared some of the same traits as the
Troodon
nests, they also differed enough to raise more questions about how they were made. For instance, one can more easily imagine a small theropod using its hands, rear feet, and maybe its snout to dig into loose sediment, neatly composing a ground nest. This same creative exercise becomes more challenging with massive sauropods. For one, their small heads don’t look as if they would have been much help in excavating a nest, and their snouts seemed ill suited for doing much more than grabbing plants. Looking elsewhere on a sauropod body, a quick cursory glance at sauropod feet also disappoints, as these do not have any obvious adaptations for scratching out a nest. Or do they?
Let’s take a closer look at sauropod feet. One might initially think that the feet of sauropods and elephants were similar, considering that, just like elephants, sauropods were large terrestrial herbivores that greatly outweighed all of their contemporaries. Yet when their feet are compared, striking differences emerge, with the most obvious being their toes. Elephants tend to have blunt
unguals
(“nails”), whereas most sauropods have pointed unguals, which are especially apparent on their rear feet. Even more odd is how the ungual on the first digit of the rear foot of a sauropod—homologous with our “big toe”—is significantly larger than all of the others. Then the unguals in that same foot are all oriented so that they point away from the center of the body, instead of straight forward.
This same skewed orientation shows up in well-preserved sauropod tracks, so we know that this was a real trait in living sauropods and not something inferred mistakenly from putting together their foot bones the wrong way. All of this evidence thus led paleontologists to ask a simple question: What’s with the weird feet?
Interpretations of these pedal oddities, which paleontologists had noted since the 19th century, have been quite varied and include: (1) better traction on uncertain terrain, kind of like how snow tires or treads on running shoes are used for the same function; (2) use in defense, in which a kick from a massive leg with huge curved claws would devastate a theropod attacker or sauropod rival; (3) use in mating (more about that later); and (4) digging out nests. Fortunately, with regard to digging, no one has seriously offered the idea that sauropods burrowed, in which their long necks might have served as periscopes. Instead, the phrase “digging sauropods” implies that they merely scratched a surface and never got too deep.
To test the “scratch-digging” hypothesis, two paleontologists, Denver Fowler and Lee Hall, followed up on Jackson’s and others’ discovery of sauropod nests in Argentina by more critically examining the rear feet of sauropods, as well as how living animals make ground nests. In their paper published in 2009, they proposed that many sauropod feet were perfectly adapted for digging out nests. A creative aspect of their study was to completely reject the “elephant foot” comparison, and instead look at the feet of much smaller modern tortoises. Once size is discounted, the foot anatomy of some sauropods and tortoise species are remarkably similar. This was an intriguing clue, because some tortoises, such as the gopher tortoise of the southeastern U.S. (
Gopherus polyphemus
), are magnificent burrowers and can make twisting 5 to 10 m (16–33 ft) long burrows. More important, though, females use their rear feet for scratching out their ground nests in which they lay their eggs.
To get a better idea of how sauropods might have dug out their nests, Fowler and Hall read many accounts of tortoises digging nests and watched videos of tortoises making nests, noting the following sequence of motions:
What they found out was that tortoises produced nests in a way that would explain sauropod nest building. Although the sizes and overall shapes of the tortoise and sauropod nests are quite different—flask versus trough, respectively—their origins are both explainable by scratch-digging. However, the whole sequence of events just described for tortoise nesting is not a perfect fit for nesting sauropods, either. For example, paleontologists who studied the Argentine sauropod nests do not think these eggs were actively buried by the sauropods. Still, other sauropod eggshells have the right kind of pore structure for having been buried and incubated
under a layer of sediment, just like in tortoises. But we also presently have no trace fossil evidence about whether any sauropods urinated on the ground first before nest building, even though one sauropod urination trace fossil has been interpreted from Late Jurassic rocks in Colorado, explained in a later chapter.
The scratch-digging hypothesis was then further tested with sauropod eggs and nests in Late Cretaceous rocks in Spain. Bernat Vila, Frankie Jackson, and others looked at nine stratigraphic
horizons with fossil eggs, which they classified as megaloolithid, an egg type affiliated with sauropods. Using some computer-aided 3-D modeling, they were able to reconstruct the original undistorted forms of the egg clutches, which also outlined the forms of the nests. These nests, which on average held clutches of 25 eggs, apparently lacked the rims of the Argentine nests, but their shapes had the same sort of asymmetry: shallow, oblong, wider at one end, and a little curved, like a kidney bean. These paleontologists also attributed such shapes to scratch-digging, in which the wider end was where the sauropod started digging with the rear feet, and the narrower end was about where the foot pulled out. They further proposed that the size and form of the enlarged unguals on the rear feet of sauropods were very likely responsible for making the widest and deepest part of the nest, in which the majority of eggs were laid.
So to make a 70-plus-million-year-long story short, the discovery of sauropod nests as trace fossils helped paleontologists test the hypothesis that sauropod feet were not just meant for walking, but also for digging. Nonetheless, perhaps alert readers noticed a wee problem with this hypothesis, which is that it does not take into account gender differences. In other words, did male or female sauropods dig nests, or did they do this cooperatively? If done either by males or cooperatively, this would have been a radical departure from reptilian behavior, in which only female tortoises, turtles, lizards, and crocodilians dig nests. Alternatively, many male–female pairs of birds share the burden of building a nest, so why not dinosaurs?
Another question remains in thinking about scratch-digging sauropods: Why would both genders have the same scratch-digging traits in
their feet? One might also ask why both male and female mammals have nipples, and yet these are only functional in one gender (although I have not thoroughly nor personally tested this). Regardless, it is not such a big deal to have a similar anatomical trait related to reproduction in both genders of the same species.
All the same, Fowler and Hall thought about this problem too, and conjectured that large unguals in the front feet of sauropods may have been used for getting a grip—on their mates, that is. This certainly would have applied to male sauropods, in that they needed some way to hold on to a female while copulating. After all, the negative consequences of a 20-to-30-ton sauropod falling off its mate in mid-coitus would have caused much more damage than just humiliation in front of sauropod peers.
This is where the claws of a male sauropod’s front feet, such as the first on each foot that pointed in toward their midlines, would have come in handy, acting as grappling hooks to hold on to the forward sides of a female when mating. However painful this action might have been for the female, it would have prevented both of them from toppling over, a mishap that would have had far worse consequences for both dinosaurs. (Nevertheless, this would have made for a magnificent trace fossil, perhaps combined with the body fossils of both sauropods that died on impact.) Trace fossil evidence that might test the “get a grip” hypothesis (also known as the “hang on, it’s going to be a bumpy ride” hypothesis) would be broken or otherwise damaged front-foot unguals, caused by struggles between male and female sauropods trying to get into the right position, or gouge marks in bones near the front of a sauropod skeleton, such as on its scapula, ribs, or vertebrae just behind the neck. However, titanosaurs lack these front-foot claws, an absence that calls into question the “get a grip” hypothesis for these sauropods.
These nests and their egg clutches inspire yet another question, which is how sauropod mothers laid their eggs without occasioning onomatopoeia such as “crunch” and “splat.” For instance, from a full standing position, a mother titanosaur’s cloaca might have been as high as 4 to 5 m (13–16 ft) above the ground. An egg that fell
from such a height would have become instantly scrambled upon impact—no matter how soft the substrate—and a very messy multi-egg pile-up would have resulted from nonstop laying. Clearly these sauropods had to reduce the height from which their eggs dropped, or at least slow down their descent.
This could have been done in several ways. One would have been for the sauropods to squat low enough that eggs had less distance to fall and hence generated less force. Alternatively, these mothers may have had an anatomical attribute, such as an extendable tube that projected from the cloaca and thus brought the eggs closer to the ground without the sauropod mother having to squat. Another method would have been to use mucus, which would have acted like a slimy envelope to keep each egg from going into freefall, instead gently lowering it to the nest. Sadly, sauropod soft tissues or mucus produced by these animals are extremely unlikely to have been preserved in the fossil record, so these are very difficult hypotheses to test properly.
In any case, an idealized suite of trace fossils documenting sauropod egg-laying would show her hind-foot impressions on either side of the nest structure, pointing forward and perhaps overlapping as she laid eggs from the rear of the nest to the front. If all things were equal in the mud or sand preserving the tracks, though, evidence of sauropod squatting should have generated more depth or deformational structures in and around her tracks, which would also help to explain whether they got close to the ground or stood up high. Also, however thrilling it would be to find a trace fossil of an egg impact, complete with splatter pattern caused by the explosion of yolk and eggshell, this is rather doubtful when one considers how quickly such an inept reproductive strategy would be lost from a sauropod lineage.
Based on what we now know from a blend of body fossils and trace fossils, nesting titanosaurs should have had the following sequence take place, from doing the deed to making the nursery, so to speak:
We can be relatively sure that mother or father sauropods did not sit on their egg clutches afterwards, or if they did, they did not pass on those genes. (See previous caveats about dropping eggs from great heights.)
Knowing about the nesting behavior of these titanosaurs and
Troodon
also makes one wonder if they stayed in the same place after laying their eggs, however temporarily that might have been. In other words, did they have any post-hatching care, like that seen in modern crocodilians and birds, or other dinosaurs like
Maiasaura
? We don’t know an answer for sure yet, although some sauropod tracksites show a wide range of sizes in their tracks—from very small to extra large—suggesting that young sauropods and parents may have been in the same place and time. A few instances of adult sauropod skeletons preserved with much smaller, juvenile ones, such as in a few instances with
Camarasaurus
, also argue for young sauropods sticking close to their parents for a while after hatching. One thing is for sure: This question could be better answered by studying a combination of trace fossils, such as nests and tracks together.