Dinosaurs Without Bones (13 page)

Fortunately for everyone, I recalled how the tour guide had used a portable PA (public address) system during the tour. I quickly commandeered this unit, cranked it up to its highest volume, and
commenced hooting and hollering into the microphone. This commotion managed to attract the attention of the tour guide, who was about to leave for Winton but luckily was still talking with a visitor outside the building. When she opened the door with a bemused (but amused) expression, I thanked her, introduced myself to her and the other visitor who had stayed behind, and we all had a good laugh about how this crazy Yank paleontologist got himself in such a predicament. I had been saved. Hallelujah! So however appealing it might have been in a spiritually enriching way to spend a night with these much-adored tracks, I was also happy to say “good-bye for now” and hope that I would see them another day.

As mentioned before, I did see them again, twice more in fact. The next time was with my wife Ruth in 2010, when we passed through while doing a grand tour of the paleontological and cultural riches of the Queensland outback, and the most recent time in 2011 was with students from that year’s study-abroad program. It was a proud educational moment to be there with my students at the tracksite and to discuss its then-refreshed notoriety with them. Adding to their educational experience, a graduate student working with Romilio and Salisbury was there, and he graciously volunteered to talk with my students about other research he had been doing in the area. Even better, my students later witnessed him engage in a disagreeable discussion (one might term it an argument) with our tour guide about the “old” versus “new” stories of Lark Quarry and its tracks. This squabble served as a living example for my students of how one step forward in scientific investigations can cause two steps back in public relations.

Perhaps the most important perspective gained from this last visit, though, came when one of my students later related an epiphany he had while we stood there on the walkway, looking at the tracks. At some point he realized, “Wait a minute. There were dinosaurs
right here!
” With that stunning insight, he felt the primordial and compelling power of dinosaur tracks, behavioral remnants showing us exactly where dinosaurs had lived, breathed, walked, run, or swum, pulling us back into those freeze-frame moments
of time yet still leaving us with unanswered questions. Was there a stampede? Who left the big tracks? Did they all go for a swim? What else can we learn from Lark Quarry and its tracks as a place where dinosaurs were right there and then? One thing is for certain, though: without these footprints there would be no such questions, and our lives would be considerably duller.

Parenthood, Dinosaur-style

Dinosaurs were good parents. Although such a statement is imbued with anthropomorphism and hints at unrealistic expectations of “family values,” we now know that at least a few species of dinosaurs protected and nurtured their young. Even better, we know that those same species of dinosaurs had evolved the instinct to change their surroundings in a way that protected their potential offspring before a single egg left a dinosaur mother’s body. Both trace and body fossils for such sophisticated behaviors show that foresight and planning for the next generation was naturally selected in these dinosaurs.

Yet dinosaurs did not always have the winning reputation of the Cleaver family. Instead, their parenting skills were thought of as more along the lines of
The Addams Family
or
The Simpsons
. Previously viewed as “lay ’em and leave ’em” parents, they were assumed to have been like lizards, snakes, or turtles, depositing their eggs in
a nest, then waving good-bye to them, never to find out whether they hatched or not, or watch them grow up. We imagined dinosaur mothers thinking “You’re on your own, developing embryos!” as they walked away from a clutch of eggs, possibly laid out in the open for ravenous insects, mammals, and other dinosaurs to enjoy as meals. Continuing such a dire state of affairs, dinosaur fathers were regarded as ne’er-do-wells, long gone after performing their (very) brief service to dinosaur reproduction.

Fortunately for dinosaurs, though, these unseemly character flaws have undergone a massive makeover during the past thirty years or so. The pivotal shift for this still-new perspective on dinosaurs came about from a combination of evidence: from dinosaur eggs and embryos that have been connected to adult dinosaurs (their parents); to dinosaur babies; and, yes, trace fossils made by juveniles and parents.

However, probably the most important trace fossils that prompted paleontologists and the rest of the world to change their minds about dinosaur parental behavior are dinosaur nests. Although dinosaur eggs had been known since the mid-19th century, actual nests—structures deliberately built by dinosaurs that held and protected their eggs, and were also used to raise dinosaur hatchlings into responsible members of Mesozoic ecosystems—were not described until 1979, and definitive examples remained unrecognized until the 1990s. Still relatively rare compared to dinosaur tracks and some other trace fossils, these nests and what they contained nonetheless propelled and supported the revolutionary idea that dinosaurs cared for their young, behaving less like lizards and more like modern crocodilians and birds. Also, now that paleontologists know what to look for, I have every confidence that more dinosaur nests will be found in upcoming years, expanding our understanding of this formerly enigmatic dimension on how dinosaur lives began.

Mom, Dad: Where Did Baby Dinosaurs Come From?

Before talking about dinosaur eggs, babies, and nests, a discussion of the potentially weighty subject of dinosaur sex is warranted, if for
no other reason than that these behaviors were a necessary prelude to fertilization, but mostly because such banter is also great fun. For example, whenever dinosaur sex comes up in conversation, its mysteries are summarized succinctly by the question “How did they do it?” The incredulity behind such a question might reflect the wonder inspired from a trip to a zoo or on a safari, where if one was timely enough to witness elephants, giraffes, and rhinoceroses in an amorous moment, it also provoked ideas on how other big animals, like dinosaurs, mated. Alas, these couplings can never quite satisfy, no matter how much we might like to watch. After all, the largest bipedal dinosaurs (theropods) may have been close to 8 tons, and the largest quadrupedal ones (sauropods) were 4 to 5 times that weight, dwarfing the largest of mammals and birds we have today. There simply are no modern equivalents for dinosaur mating.

So how did two-legged male and female dinosaurs manage to keep their respective balances while mating? How did four-legged males pick their front feet up from the ground? How did male dinosaurs get past the thick tails of some females to reach their goals? More intriguing, how did male dinosaurs get past the spiky or clubbed tails of some females? For the larger dinosaurs, how did a female dinosaur support the weight of a male without him breaking her back? How did the male get off, or rather, disengage from the female? Sure, we have a few ideas that approach these questions from watching what we consider today as “charismatic megafauna.” But elephants, giraffes, and rhinoceroses are inadequately sized and inappropriately equipped when compared to the largest of dinosaurs. It would be like comparing the mating habits of kangaroo rats to kangaroos, which is not just about size but also anatomy and physiology.

So knowing that modern sex surrogates and zoo-based voyeurism just aren’t up to the task of giving us proper analogs, we must look at the fossil record. The first and most obvious question would be: Have we ever found a male and female dinosaur skeleton of the same species together in what might be construed as a compromising position, depicting the bawdy aphorism “the beast
with two backs”? Frustratingly, as far as we know no body fossils have been found of a closely joined male and female dinosaur of the same species, together forever. This disappointment is not surprising, as such a pair of fossils would have required an extraordinary set of circumstances to have preserved them: mating, then both suddenly dying while mating, an example of
la petite mort
extended a bit too literally. For ideal fossilization conditions, this sudden death of both partners would have been followed by a quick burial, one that managed to keep them in close contact.

Still, paleontologists hold out hope that such fossils will be found some day and are encouraged by an incredible find, which was of the “fighting dinosaurs” found in Late Cretaceous rocks of Mongolia in 1971. This discovery was of a small ceratopsian dinosaur,
Protoceratops
, on top of a dromaeosaur,
Velociraptor
, with both locked in an embrace but a pointedly deadly one. The right forearm of the
Velociraptor
was in the mouth of the
Protoceratops
, which had apparently clamped down on it, fracturing the arm. Meanwhile, the formidable raised claw on the
Velociraptor
’s left foot was positioned just below the
Protoceratops
’ head shield, close to its neck; its left hand was grasping the head shield. Rest assured, this was not like a kitten and puppy wrestling with one another, nor was it like any other hint of interspecies play, but more emblematic of Tennyson’s “nature, red in tooth and claw.” Paleontologists who studied these fossils agree that both dinosaurs must have been buried instantaneously, which probably happened from the collapse of a nearby sand dune, a geological referee abruptly and permanently stopping the fight.

So one would think by now that trace fossils would again come to the rescue with evidence of dinosaur mating, arising and proudly pointing to dinosaurian congress. Surely sediments somewhere, sometime, during the more than 160-million-year history of dinosaurs recorded visible signs of a male and female dinosaur conducting a Mesozoic version of “business time.” Alas, the answer thus far eludes us, a postponing of evidence that tries the already-stretched patience of intellectually lusty paleontologists who
desperately seek the warm afterglow of scientific gratification that comes from a sudden, ecstatic release of suspense.

Fortunately, ichnology, like all sciences, has predictive power, promising deeply satisfying scientific insights that happen multiple times and in rapid succession. Through the use of modern traces and animal behavior, we can paint pictures of what dinosaur mating traces should look like, and use those conjectured images as guides for detecting mating trace fossils preserved in the geologic record. Of course, when looking for such double-entendre traces, paleontologists will have to adjust their search images according to dinosaur anatomies, sizes, and behaviors that may have left their marks while making the horizontal bedrock.

For theropods, such traces would have varied with the size of the mating dinosaurs involved. For example, the mating traces of a pair of
Microraptor
—each weighing less than a kilogram (2.2 lbs.)—should have left much less of an impression compared to traces made by a pair of
Spinosaurus
, which together would have made for 8 to 15 tons of convivial society. Still, mating traces of these bipedal dinosaurs would show a basic pattern of one or two footprints adjacent to one another (male tracemaker) just behind but slightly offset to the right or left of two other paired footprints (female tracemaker). All four tracks should be nearly identical in size and form, but perhaps with slight differences accounting for gender differences (
sexual dimorphism
). Bipedal ornithopods would have had similar patterns, although distinguishing these from those of theropods feeds into the old “what made the three-toed track” dilemma discussed in the previous chapter. Furthermore, many of us sincerely hope that no mistaken attempts at interspecies mating took place and became registered in the fossil record, wherein a theropod found a same-sized ornithopod winsome, or vice-versa.

For quadrupedal dinosaurs, such as sauropods, ceratopsians, ankylosaurs, some ornithopods, and other dinosaurs that got around on all fours, mating traces would differ appropriately from those of bipedal dinosaurs by adding a few more tracks to the mix: two, specifically. This situation would have been a result of the
male, having approached his potential partner from behind and not being rebuffed—sexual selection or rejection writ large—then likely rearing up and putting his front feet onto the back of the female. This positioning also means that her rear legs would have supported at least part of his weight, which should have deepened her hind tracks relative to her front tracks. Hence the full sequence of trace fossils might be:

• Normal quadrupedal diagonal walking pattern by both dinosaurs, either subparallel to each other or perhaps showing a shortening of pace as the male approached from behind.
  
• Two sets of four (front–rear) tracks, one behind the other and perhaps offset to the right or left—accounting for bypassing the female’s tail on one side—denoting pre-mating positions. These tracks also might be accompanied by a central mark behind the male’s tracks, caused by his tail pressing against the ground as he adopted a tripod-like stance.
  
• Noticeably deeper rear tracks in both sets of tracks (mounting in progress), but maybe with one of the male’s legs draped over the female’s tail, and some evidence of shuffling forward if she needed adjusting to his weight or otherwise accommodating his anatomical attributes. Such shuffling also might have left an elongated drag mark or more intermittent bounce marks (from his tail, that is).
  
• The resumption of normal diagonal walking patterns, with hers showing a more rapid pace to get away from her partner, and his showing deeper front tracks as his weight was properly redistributed upon dismounting.
  
• A resting trace connected to his tracks, post-coitus.
  

Of course, for mating traces to get preserved in the first place, such activities should have taken place in environments with the
right sediments to mold every little (or not so little) parry, thrust, twitch, shudder, or shrug. Among the best environments for registering these traces would have been river floodplains or shorelines with firm mud or sand. On the other hand, the worst places to have made a lasting mark for future generations of humans to see would have been in trees, water bodies—especially if doing it while floating above a lake or river bottom—or burrows.