My Beloved Brontosaurus

by Brian Switek

  Yet, as I learned during my Triassic road trips, dinosaurs did not immediately take over the world. The first ones were rare, and Eoraptor may have been more of a plant-focused omnivore than the terrible little blighter it was originally characterized as. The image change has infiltrated that common purveyor of paleontology products—the CGI-filled basic-cable dinosaur documentary. In the overhyped miniseries Dinosaur Revolution, flocks of brightly colored Eoraptor are shown as timid little things that have to contend with an especially acrobatic relation of Postosuchus, the razor-jawed Saurosuchus. Dinosaurs didn’t rule the Triassic Valley of the Moon. They eked out an existence alongside more powerful creatures.

  During Triassic time, the dinosaur subplot was just one small part of the greater archosaur story. If you traveled back 230 million years, with no knowledge that dinosaurs would eventually reign, you’d probably consider the dinosaurs to be cute creatures of little importance. There was no clue of what was to come. The same could be said of our own kin that lived during the same time. Synapsids didn’t entirely disappear. After all, we’re the descendants of such proto-mammals that survived into the Triassic.

  We don’t know why the archosaurs took over the world from our synapsid precursors. Not for certain. But a large-scale analysis of archosaur and synapsid fossils gives us a clue. When researchers tracked the fates of the two lineages after the Permian, they didn’t find many signs of direct competition between our primordial relatives and the archosaurs. It was not as if synapsids and archosaurs directly battled each other for control of habitats. Mammals and their forerunners, for the most part, remained relatively small, while the archosaurs diversified into a variety of body sizes—from creatures the size of a pigeon to the largest of the sauropod dinosaurs, reaching one hundred feet in length or more. Biological constraints might have made all the difference. From what paleontologists have been able to tell from bone microstructure, the ancestors and cousins of both crocodiles and dinosaurs grew faster and began reproducing earlier in their lives than proto-mammals. This shorter generation time allowed them to proliferate and be molded by natural selection faster, and they simply outgrew their proto-mammal competition. Mammals didn’t have a chance to get big because the faster-growing archosaurs had already taken up that ecological space.

  As I learned about the rise of the archosaurs, though, I still had trouble understanding why dinosaurs prospered when similar animals—like Effigia and Postosuchus—perished at the end of the Triassic. Why didn’t the other kinds of archosaurs become the basis for a different, sadly unseen proliferation of extraordinary forms? They were already the dominant form of archosaur on the planet during the Triassic. Dinosaurs were the underdogs.

  As the Triassic expert Randall Irmis once explained to me, dinosaurs were few and far between in North America, and no Triassic ornithischians have yet been found on the continent. Even in places they have been found, such as South Africa, ornithischians were small, rare creatures. The only dinosaurs that got big in the Triassic were the primordial precursors of Apatosaurus, which are called sauropodomorphs. These dinosaurs were a prominent presence in parts of Late Triassic Europe, but apparently nowhere else. The big picture is that dinosaurs diversified into a variety of forms soon after their origin but didn’t really start to rule on a global scale until the Jurassic.

  Another extinction was the essential event. It’s not as infamous as the Permian extinction, and certainly less so than the mass die-off at the end of the Cretaceous around 66 million years ago, but there was a distinct drop in the world’s faunas at the end of the Triassic. The usual suspects of climate change, asteroid impact, and intense volcanic activity have all been implicated, without much resolution, but, whatever happened, about half of the world’s biodiversity abruptly disappeared. Many croc-line archosaurs—such as the phytosaurs, aetosaurs, and rauisuchians—were among the losses on land, but the disaster doesn’t appear to have affected dinosaurs in the least.

  Exactly why dinosaurs outlasted many other archosaur lineages is a question that still confounds paleontologists. What was it about dinosaurs that let them survive and then thrive while their cousins perished? A one-size-fits-all hypothesis for the demise of other archosaurs and the rise of the dinosaurs doesn’t exist.

  * * *

  Paleontologists thought they had cracked the puzzle of dinosaur identity and success. Now we know differently. Serendipity, more than anything else, gave dinosaurs room to become the creatures we adore. The celebrated story of dinosaurs fighting their way to the top, crushing all competition underfoot, doesn’t work anymore. “The more new archosaurs we find in the Triassic,” according to Sterling, “the less special dinosaurs seem to be.”

  The end-Permian extinction cleared the way for archosaurs to diversify thanks to biological quirks such as early reproduction and fast growth, and the end-Triassic extinction sank other archosaurs that dominated local habitats. As Irmis, Nesbitt, and other Triassic experts have explained, contingency and opportunism may have allowed dinosaurs to dominate. Dinosaurs were awfully lucky. “[T]here was nothing predestined or superior about dinosaurs when they first arose,” Irmis and his colleagues concluded, “and without the contingency of various Earth-history events during the early Mesozoic, the Age of Dinosaurs might have never happened.”

  What about the evolutionary routes left untraveled? What would the world be like if the end-Permian extinction or the end-Triassic catastrophe had been canceled? Dinosaurs wouldn’t have evolved. We probably wouldn’t have either, our mammalian forebears forced into the shadows by a different cast of fearsome archosaurs. I like to think of these major events as what fantasy satirist Terry Pratchett once characterized as bifurcations in the trousers of time. The history we know went down one leg, but there was another possible outcome. The constrained limits of our imaginations are the only ways to visit those alternate histories. We’ll never know all the unrealized evolutionary possibilities, but the survival of the dinosaurs built the foundation for an ongoing dynasty so beautiful and fantastic that I doubt we could have dreamed them if we didn’t already know they existed. Even monsters of fantasy—wyverns, the Cyclops, chimeras, and the whole mythological lot—pale in comparison to the real animals paleontologists are continuing to excavate.

  For all those stunning forms to exist, though, dinosaurs had to have sex. For vertebrates, at least, that’s the process that provides natural selection with raw material to work with—genetic combinations that produce variations which then die off or proliferate according to the constant scrutiny of natural selection. But how do we even approach the question of how Apatosauri made the earth move for each other? We need to look at the intimate details of dinosaur fossils to find out.

  Three

  Big Bang Theory

  There is a dinosaur in Concourse B of United Airlines Terminal One at Chicago’s O’Hare International Airport. The first time I saw the skeleton, I thought it was a mirage created by my flight-addled brain.

  I hate air travel. I wish I could just opt for the King Kong package—knock me out with sedatives, put me in a crate, and unload me when I get to my destination. My aversion to flying at 30,000 feet isn’t helped by my habits. When I travel alone, I am so focused on getting to Point B that I don’t stop to eat. Running on the can of soda and tiny pretzel packet from my first flight from the tiny Billings, Montana airport, the first leg on my trip back to my former home in New Jersey after a fossil-hunting trip in Wyoming, I shuffled through the crowded halls to find a seat in the sweltering waiting area. And that’s when I saw the dinosaur. I focused on the mirage for a moment, waiting for it to evaporate, but it stayed there—a magnificent, towering skeleton of Brachiosaurus.

  The fossil cast in Concourse B used to stand in Stanley Field Hall at Chicago’s Field Museum, until Sue, the eight-million-dollar Tyrannosaurus, came on the scene and bumped the sauropod from its pedestal. In 2000, the sauropod was reassembled among the airport’s kiosks and ads. On the day I shuffled through the concourse
, the dinosaur peered over the top of a banner for the airport’s Wi-Fi as if it was looking to the tarmac beyond to check the latest departures and arrivals.

  This Brachiosaurus definitely wasn’t the bored slouch I saw in my elementary school’s faded textbooks. The heavy shoulders and columnar forelimbs gave the dinosaur a dignified air, only enhanced by the long swerve of neck bones up to a boxy, round-muzzled skull. That skull—and a few other parts—were borrowed from a different, closely related dinosaur from the Jurassic of Tanzania called Giraffatitan, but the amalgamation of facsimile bones still reflected the imposing stature of one of the largest dinosaurs ever found. Brachiosaurus may not hold the heavyweight title anymore, but for me, an eighty-five-foot-long sauropod is just as impressive as a hundred-foot one. I can only guess how my tiny mammalian ancestors must have seen these giants.

  I kept staring at the dinosaur, following the complicated flanges of bone along each vertebra and the rough patches on the limbs where immense muscles would have attached. I let my mind’s eye linger and start to fit internal organs into the framework and pull muscle over the whole thing before wrapping the dinosaur in a dappled pattern of large brown spots and white lines—not unlike a giraffe. Not very original, but a little more colorful than the gray-and-olive-green standard I grew up seeing plastered on the dinosaur’s hide. And then a strange thought bubbled up from the back of my mind: how did such a gargantuan animal have sex?

  Giddy and tired, I envisioned a pair of amorous Brachiosaurus standing in a clearing within a Jurassic conifer forest, each waiting for the other to make the first move. I couldn’t quite figure out the mechanics of what should come next. I kept looking at the skeleton, as if the fake bones might offer some clue, but all I could think of was that sex must have been damn difficult with that big tail in the way. Alas, my flight started boarding, so I trudged over to take my place among the other tired and impatient passengers. At least the dinosaur mating mystery gave me something to focus on during my last flight home, and the question has kept nagging at me ever since.

  * * *

  As the British paleontologist Derek Ager once wrote, “After eating, the most widespread habits among modern animals are those concerned with sex, and there is no reason to suppose that this did not raise its allegedly ugly head millions of years before Freud.” Indeed, sex was the inheritance of dinosaurs and every other amniote descended from lizard-like creatures that left the life aquatic behind. As vertebrates took up an entirely terrestrial lifestyle more than 312 million years ago, sperm could no longer be squirted onto soft eggs suspended in safe little pools. Tougher-shelled eggs, harboring an internal pond to nourish developing embryos, required internal fertilization away from the water’s edge, and so the invention of sex was passed down the generations. From the tiny, feathery Anchiornis all the way up to Brachiosaurus and other gigantic sauropods, dinosaurs were part of this reproductive legacy. “Clearly if we are to regard our fossils as once-living creatures,” Ager wrote, “considerations of sex must arise.”

  One of the earliest considerations of passionate dinosaur encounters was put forward a century ago. In 1906, the American Museum of Natural History paleontologist Henry Fairfield Osborn pointed to affectionate occasions between fearsome Tyrannosaurus rex to explain the dinosaur’s oft-ridiculed arms. A pair of Tyrannosaurus specimens collected by fossil hunter Barnum Brown unmistakably showed that this dinosaur had short but heavily muscled forelimbs. Osborn couldn’t believe that such small arms played any role in grappling with big game like Edmontosaurus or Triceratops, but perhaps the arm of Tyrannosaurus was “a grasping organ in copulation.” Just imagine two immense predators, one atop the other and holding on to his mate with those beefy miniaturized appendages. Sadly, Osborn didn’t commission a drawing of the behavior from the skilled illustrators he often tapped to restore prehistoric creatures.

  Osborn didn’t give any serious consideration to dinosaur sex, though. Nor did many other paleontologists of his generation. Dinosaur copulation was seen as a silly subject and beyond the reach of investigation. Plus, it seemed to make dignified researchers feel rather squicky. Sex, in natural history, is a perfectly acceptable subject when considering flashy courting behavior or when boiled down into quantitative surveys of gene pools, but the sordid details of sex itself have often made researchers feel awkward. Not long after Osborn briefly mused about Tyrannosaurus sex, George Murray Levick—a naturalist with the 1910–13 Scott Antarctic Expedition—was shocked and disgusted by the “sexual depravity” of Adélie penguins (which, you’ll recall, we now know are living dinosaurs). He was especially horrified by a young male penguin that tried to mate with a dead female. Levick wrote notes in Greek so that only classically educated scientists like himself would be able to read what he observed, and when he prepared a monograph on the penguins, the passages on sexual behavior were considered so sensational and disgusting that the section was cut and only circulated among a small cadre of scientists. (It wasn’t until 2012 that Levick’s observations—which were unique for their time—were rediscovered and made publicly available.) Sexual behavior, even among living species, was a taboo subject, and speculating in unseemly detail about the mating habits of dinosaurs would surely highlight a scientist as a pervert. Whatever dinosaurs did on hot Jurassic nights was kept behind the shroud of prehistory.

  Not that paleontologists had much to go on. Signs of fossil sex are hard to find. Among the rare examples are 47-million-year-old turtles that died while copulating, and possibly a pair of much older, 320-million-year-old sharks that might have been courting when they were rapidly buried. Sadly, no dinosaur skeletons have ever been found articulated in the act, and not even the most beautifully preserved dinosaurs retain any sign of their reproductive organs. Even evidence of nonsexual organs is exceptionally rare, and sometimes fossils thought to represent internal dinosaur tissue have turned out to be something else.

  This was the case in 2000 when the veterinarian Paul Fisher, the paleontologist Dale Russell, and collaborators announced that they had found evidence of a dinosaur heart. The fossil was embedded in the chest of “Willo”—a small herbivorous dinosaur known as Thescelosaurus—and appeared to be a four-chambered heart with a single aorta. But in 2011 another team of paleontologists determined that the “heart” was really a blob of iron-rich rock that had cemented together in the position of the dinosaur’s chest. They found tidbits of what might have been fossilized dinosaur cells, which hinted that the concretion was created when the contents of the dinosaur’s heart were exposed and attracted iron-rich sediment, but the structure itself was not a heart of stone.

  Remnants of external dinosaur anatomy might not be of very much help in exploring dinosaur sex, either. Skin impressions are often found in patches, rather than wide swaths covering the whole animal, and even these would not preserve the internal plumbing of dinosaurs. Nevertheless, after reading a 2012 paper on soft-tissue traces preserved with the crested hadrosaur Saurolophus, I asked its author, the Philip J. Currie Dinosaur Museum paleontologist Phil Bell, if there might be any possibility of finding soft-tissue traces of the delicate dinosaur bits. After all, I knew that in 2009 paleontologists working on a much, much earlier organism—a 380-million-year-old armored fish called Incisoscutum ritchiei—had identified the oldest known fossil evidence of a penis. Maybe there is hope that some lucky paleontologist will have the amazing shock of finding preserved dinosaur copulatory equipment. (I would love to see such a discovery grace the cover of Science or Nature.)

  Bell, however, wasn’t optimistic. The trouble with soft tissues, he explained, is that there has to be some way for their presence to be recorded as a fossil. Bell cited cololites as an example. These are different from the infamous coprolites that paleontology professors seem to enjoy handing new students, asking “What do you think this is?” before revealing that the hard lumps are really fossilized dinosaur feces. A cololite is a premature coprolite—what Bell described as “fossil dung that hasn’t yet been excre
ted.” Gross, but highly informative. Cololites can preserve the form and extent of a dinosaur’s intestine, as in a little predatory dinosaur named Scipionyx that was found in Italy. In other cases, such as poor Willo, certain organs might be preserved as stains or concretions that formed when the dinosaur’s guts began to decay. “What we know of the internal organs,” Bell wrote to me, “comes from mineral-rich areas (like the liver) that somehow react favorably in certain conditions for them to be fossilized, or from secondary information (such as cololites) that is more prone to fossilization.”

  But Bell mentioned one extraordinary fossil that reveals something important about dinosaur reproductive organs. While it’s anyone’s guess what dinosaurian penises looked like—dinosaur dicks must have been as grotesquely fantastic and terrifying as the rest of their anatomy, right?—paleontologists actually know more about the reproductive organs of female dinosaurs, thanks to a very special pair of hips recovered from Cretaceous China.

  Inside the hips of an oviraptorosaur—a feathered, crested, beaked dinosaur that looked something like a terrestrial parrot—the Canadian Museum of Nature paleontologist Tamaki Sato and her colleagues found a pair of preserved eggs. This pregnant dinosaur died just before adding those eggs to her nest. Even better, the pair of eggs showed that this female dinosaur had a combination of traits seen in birds and crocodiles. The presence of two eggs at the same stage of development showed that dinosaurs had paired oviducts, which is a trait seen in crocodylians. But the fact that there were only two eggs indicated that, like birds, the dinosaur probably developed only one egg in each oviduct. This pattern matched what paleontologists saw in the preserved nests of these dinosaurs—the eggs were arranged in pairs, and the gravid dinosaur finally explained why this should be so. The eggs were paired because that’s how they were laid.