by Brian Switek
A decade later, the Fayetteville State University paleontologist Phil Senter applied the same idea to sauropods. Senter argued that long sauropod necks did not give dinosaurs any appreciable vertical reach, but instead only helped the dinosaurs browse on low-lying plants. Elongated necks, in Senter’s view, also might have made dinosaurs conspicuously vulnerable to attack from Jurassic predators. All an Allosaurus or Torvosaurus would have to do, he suggested, was chomp into an exposed Apatosaurus neck to gain many tons of succulent sauropod meat. Since sauropod necks seemed to have little utility in helping their owners survive and may have even come at a high cost, Senter concluded, the long necks were probably visual signals that had somehow become established through generations of mating.
But the sauropod experts Mike Taylor, Dave Hone, Matt Wedel, and Darren Naish rebutted Senter’s sexual selection argument. Sauropods probably kept their necks elevated much of the time, the paleontologists pointed out, and this gave the dinosaurs a distinct feeding advantage—an Apatosaurus could stand in one place and browse up, down, and from side to side without having to move. (Giraffes, too, have been shown to gain a feeding advantage from their neck, overturning the “necks for sex” hypothesis.) And the graceful necks of sauropods—supported by ligaments, tendons, and bone—were actually quite sturdy parts of the body that an attacking Allosaurus would have a hard time biting into. The four argued that, contrary to Senter’s assessment, sauropod necks were sturdy organs essential to the success of the giants.
That’s not to say that the necks of Apatosaurus and kin had nothing to do with mating. Sauropod necks likely evolved to allow the dinosaurs to reach a wide range of food, after all, but their necks could have been co-opted for sexier purposes. During the mating season, sauropod necks might have acted as huge fleshy billboards advertising individuals’ fitness. Since many of these giants were too big to worry about hiding—predatory dinosaurs probably targeted juvenile and small individuals over the biggest behemoths—immense sauropods could dispense with camouflage. Might the necks of dinosaurs like Brachiosaurus have been especially colorful during the mating season, ornamented with striking color patterns to gain the attention of potential mates and show off that they were the healthiest, most desirable dinosaurs around? These are the kinds of questions that can keep a paleontologist up at night.
Other dinosaurs may have shown off a bit, too. Maybe the spiky dinosaur Kentrosaurus found the plates and spikes of the opposite sex arousing, and perhaps females of the sauropod Amargasaurus looked for males with the longest neck spines. We don’t know for sure, but such prominent visual signals probably had some utility in the mating game.
Ornamentation can be a way for potential breeding partners to detect if there’s truth in advertising. Magnificence costs energy, which the individual flaunting it must have to spare. A good mate will look healthy and especially flashy. And there was probably a lot of stomping, hooting, and showing off during the Mesozoic when dinosaurs tried to impress each other. But this only leads us right back to that most persistent of dinosaur sex mysteries: after all the posturing and showing off, how did they do it?
Let’s assume that male dinosaurs didn’t have monstrous penises. (My apologies to Mr. Rex.) Mating dinosaurs would have had to bring their cloacae into close contact for sex to work. How this actually happened depends on what dinosaurs were physically capable of, and on that score we can only speculate. In 1993, the American Museum of Natural History unveiled a stunning Jurassic vignette: a mother Barosaurus rearing up to defend her offspring from an Allosaurus about to pounce on the youngster. The museum made it clear that the scene was speculative—a vision of what dinosaurs might be able to do, rather than what we know they did—but the reconstruction caused a stir among paleontologists. How could an animal so immense rear back without snapping its hind limbs like toothpicks? And how did the dinosaur’s heart manage to pump enough blood up to the sauropod’s head? Traditionalists saw the mount as unbridled, sensationalistic speculation, while advocates of dinosaurs as agile, highly active animals viewed the display as a perfect example of this new view.
The reconstructed skeleton of the Jurassic stegosaur Kentrosaurus. Did these dinosaurs use their spikes as sexy signals? Even if not, how did they go about the delicate business of mating? (Photograph by H. Zell, from Wikipedia: en.wikipedia.org/wiki/File:Kentrosaurus_aethiopicus_01.jpg)
When the famous exhibit came up in my Paleontology 101 course at Rutgers University, my professor pointed out that some sauropods must have reared up in just such a position every now and then. How else would males be able to mate with females? The biomechanics expert R. McNeill Alexander made a similar point. Alexander imagined that dinosaurs mated just like today’s elephants and rhinos: females had to bear the extra weight of the mounting male. The main difference would be that dinosaurs had those big, relatively stiff tails. Working from the idea that a male dinosaur threw one of his forelegs over the back of the female, Alexander pointed out that the weight of the male would have rested on the female’s hindquarters. This would have been a massive load, but, Alexander noted, the stresses involved wouldn’t have been any worse than walking, since, during the step cycle, the dinosaur’s own weight would have been supported by just one hind leg as the other swung through the air. “If dinosaurs were strong enough to walk they were also strong enough to copulate,” Alexander wrote. “They were presumably strong enough to do both.”
All the same, what positions were in the dinosaur playbook has remained in the realm of speculation. When I described the problem to my wife, who thankfully doesn’t think it at all strange to wonder about dinosaur sex lives, Tracey replied, “You’d think the genitals of the females would be in a more convenient place. Maybe on the side, like a gas tank.” Clearly, dinosaurs are yet another example of unintelligent design.
However they did it, though, it’s clear that male dinosaurs had to somehow mount female dinosaurs. That’s the general idea the paleontologist Beverly Halstead ran with when he pontificated on dinosaur sex for live audiences. (He is a legend among paleontologists for regularly bringing his wife on stage during lectures to demonstrate positions from the Mesozoic Kama Sutra.) He believed that dinosaurs did as lizards and alligators do today. Male dinosaurs grasped or leaned on the female with their forelimbs and threw one hind leg over her back, Halstead surmised, and this would push the male’s hips beneath the tail of the female to bring their cloacae into contact. Longer-tailed species may have even intertwined their tails, just as some snakes twist their bodies around one another. Halstead’s version of dinosaur style, with a few variations, became the favored hypothesis.
I’ve never been satisfied by this standard explanation of dinosaur sex. It’s easy to draw a two-dimensional image of dinosaurs mating, but we don’t really know if their legs and tails could have bent and flexed enough to achieve the traditional position. And the mating habits of stegosaurs defy the imagination. Just thinking of how Kentrosaurus managed gives me a headache. This relative of the more famous Stegosaurus had small plates armoring its neck and upper back that transitioned rearward into paired sets of huge spikes, including large weapons over the hips. The leg-over-back position just wouldn’t work—I know because I asked my paleontologist friend Heinrich Mallison to check.
Using the Kentrosaurus mount at the Museum für Naturkunde in Berlin, Mallison had previously created a digital scan of the dinosaur’s skeleton to investigate just how flexible this spiky herbivore was. Among other things, Mallison discovered that Kentrosaurus could swing its tail in a seventy-five-degree arc at an estimated ten meters per second. “At this speed,” he wrote, “the spikes could penetrate deeply into soft tissues or between ribs and were able to shatter bones.” Not a dinosaur you’d want to piss off. But reading his papers made me think about the more tender moments in the life of Kentrosaurus. If Mallison had made flexible virtual models of the dinosaurs, then those models could be used to test dinosaur sex positions in three dimensions.
Sugges
ting that a friend and researcher make his carefully assembled virtual dinosaur models have pixelated sex is a delicate proposition, but thankfully Mallison was enthusiastic about the idea. And the traditional dinosaur sex position didn’t work. If a male Kentrosaurus tried to throw his leg over the back of a crouching female, he’d castrate himself on her sharp spikes. One hip spike, in particular, seemed to be positioned just right to strike fear into the hearts of stegosaur suitors. More than that, the tails and hips of these dinosaurs so strictly limited their mobility that the classic dinosaur-style position was impossible. These prickly dinosaurs must have done something else, and, with luck, making virtual dinosaurs fool around might help solve the problem.
One of the most essential aspects of dinosaur lives remains modestly cloaked in mystery. Still, Kentrosaurus and kin obviously figured it out well enough to continue their kind. In fact, the products of their prehistoric unions—eggs and baby dinosaurs—are beginning to show paleontologists just how dramatically dinosaurs changed as they grew up.
Four
The Dinosaurs, They Are a-Changin’
A long time ago, in elementary school science class, my peers and I watched a baby chick peck its way out of an egg. At the time, I had no idea that I was seeing a dinosaur being born. To my younger self, birds and dinosaurs were entirely separate branches on the tree of life. Now I know better. Looking back on that little theropod, it’s clear that the chick was carrying on the dinosaur legacy. Pick any dinosaur you like—all the cute infants greeted the world by breaking their way out of an egg.
The simple fact that dinosaurs laid eggs took decades to confirm. For a full century after the first dinosaurs were discovered, paleontologists didn’t really know how the creatures gave birth to the next generation. Some, such as William Diller Matthew, entertained the idea that dinosaurs were capable of live birth; perhaps pregnant females carried one large offspring at a time, like elephants. Others thought dinosaurs were more like modern reptiles, packing nests with eggs. It wasn’t until the 1920s, when an American Museum of Natural History expedition to Mongolia returned to the United States with undeniable dinosaur eggs, that paleontologists confirmed that baby dinosaurs hatched in carefully constructed nests. This raised additional questions. Did little dinosaurs start off their lives as miniature copies of adults, and did their parents care for them at all? These questions have surrounded subsequent discoveries of dinosaur eggs, nests, babies, and adults fossilized in brooding positions. By probing these intricate specimens, paleontologists found something unexpected. Dinosaurs didn’t just look weird—they grew up in a unique way.
In fact, the eggs the AMNH team discovered in Mongolia were even more important than paleontologists understood at the time. The oval fossils were not only evidence that dinosaurs constructed nests. Paired with a later, fortuitous find, those eggs have revealed a dramatic dinosaurian irony that further sets the scene for how baby dinosaurs started life.
When the AMNH expedition collected the eggs from the Gobi Desert, paleontologists thought they belonged to a small ceratopsian dinosaur named Protoceratops that they found nearby. But the same crew also found another dinosaur associated with a Cretaceous nest—a graceful, birdlike theropod dinosaur with a toothless beak. When he described the animal in 1924, Henry Fairfield Osborn called it Oviraptor philoceratops, an “egg seizer” with a “fondness for ceratopsian eggs,” because the dinosaur’s skull was found right on top of a clutch of them. The close juxtaposition of Oviraptor and the nest, Osborn wrote, “put the animal under suspicion of having been overtaken by a sandstorm in the very act of robbing the dinosaur egg nest.” The supposed predator’s red-handed shame was immortalized in its moniker.
Only, the Oviraptor wasn’t stealing the eggs. We know thanks to a beautiful baby dinosaur that was uncovered much later, in 1994. The AMNH paleontologist Mark Norell and his colleagues described an astounding, delicate fossil they discovered the year before at another fossil-rich site in the Gobi Desert. Curled up inside an egg was the embryo of an oviraptorid dinosaur—the poor baby was so well developed, in fact, that the paleontologists expected that it was close to hatching when it was entombed in sediment. And the anatomy of the egg that encompassed the little guy was the same as all the so-called Protoceratops eggs the original Mongolian expedition had collected so many years before. Astonishingly, Osborn’s Oviraptor was really protecting her own nest. Due to the rules of taxonomy, though, her kind will always be saddled with the “egg seizer” name.
At first, the idea of a nurturing Oviraptor parent was based on the circumstantial evidence of corresponding egg shapes. But soon after the embryonic dinosaur was found, Norell and other paleontologists documented oviraptorid skeletons preserved brooding on top of their nests. These dinosaurs, which were covered in gorgeous plumage, spread their feathery arms over clutches of eggs.
Oviraptor wasn’t the only dinosaur to watch over its shell-bound offspring. In fact, a major discovery that had been made years before and a continent away had revealed that dinosaurs were attentive to their nests, and even to hatchlings. During the 1970s, following a tip from fossil hunter Marion Brandvold, the paleontologists Jack Horner and Bob Makela discovered a vast nesting ground of hadrosaurs they called “Egg Mountain” in Montana. This was a fossil bonanza. Nests, eggs, and dinosaurs from infants to adults were all found in the same place. Even better, the anatomy of the youngest dinosaurs showed that they had hatched only a short while before but were not yet strong enough to leave the nest. The hadrosaur babies must have stayed in the nest for at least a short time, the researchers argued, and would have relied on their parents for food. No wonder Horner and Makela decided to name this hadrosaur Maiasaura—the “caring mother lizard.”
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Around the same time that Makela and Horner found Egg Mountain, yet another discovery of a nesting ground on another continent showed that the Maiasaura site wasn’t a fluke. Many different kinds of dinosaurs cared for their young. In 1976, the paleontologist James Kitching discovered a cache of dinosaur eggs in roughly 190-million-year-old strata of South Africa’s Golden Gate Highlands National Park. Kitching presumed that the eggs belonged to Massospondylus—a lanky, bipedal dinosaur with a long neck, small head, and short arms tipped in large claws, which had been found in rock of about the same age. (This was the archetype from which the later sauropods would evolve.) What Kitching didn’t know was that some of those eggs contained very rare preserved dinosaur embryos. In 2010, after the eggs had been fully cleaned and prepared, the paleontologist Robert Reisz and colleagues found that two of the eggs contained the bones of tiny sauropodomorphs. The nascent dinosaurs were awkward little things. With their short necks, stubby little legs, and big eyes, they were unabashedly cute, and, while different from that of the adults, their anatomy matched what Kitching had proposed.
And there were more eggs. When Reisz and colleagues relocated the fossil site, they not only found eggs, but nests and footprints spread through multiple rock layers. This was a place where mother Massospondylus returned year after year to nest, and the tracks of baby dinosaurs indicated that the infants remained at the nesting site after hatching—at least until they doubled in size. Since the babies stayed in the nest, chances are that one or both parents stayed to care for them. Birds protect their nests, and modern crocodylians guard their young for a short time after birth, so it’s reasonable to suppose (remember the principle of the extant phylogenetic bracket?) that Massospondylus did the same.
Dinosaurs didn’t just pick any spot to make their nests. Some species even looked for breeding grounds with special perks. One of the most fantastic dinosaur nesting grounds discovered, in the Early Cretaceous rock of Argentina, was a basin filled with hot springs and other geothermal features where sauropods made nests that were warmed by the natural heat emanating from the earth. Just imagine the long-necked leviathans walking through a prehistoric version of Yellowstone’s otherworldly geyser basins. Whether these dinosaurs cared for their eggs or ju
st left them to incubate in the heated nests is unknown. Some paleontologists think that sauropods may have employed a “lay ’em and leave ’em” strategy—that the huge dinosaurs excavated nests, deposited their offspring, and moved on about their business.
How much parental care dinosaurs provided seems to have differed from species to species. Indeed, for dinosaurs other than sauropods, there are tantalizing clues that adults and their young stayed together for extended periods of time. In southwestern Montana, for example, paleontologists have found burrows made by Oryctodromeus—a small, bipedal, beaked ornithischian dinosaur. We know this dinosaur species made the burrows because their skeletons have been found inside the holes they created, including remains of an adult and two juveniles preserved within a large burrow chamber. These unfortunate dinosaurs were buried together at the same time, suggesting that some fast-growing dinosaur kids stayed with their parents for months or even years after hatching.
Sooner or later, just like other species, most dinosaur young parted ways with their parents. According to David Varricchio of Montana State University, one of the researchers who studied the Oryctodromeus den, who has attempted to put together a generalized picture of dinosaur life history, multiple discoveries of dinosaur bonebeds—from Triceratops to the sauropod Alamosaurus and the superficially ostrich-like theropod Sinornithomimus—that contain juvenile animals of the same age suggest that young dinosaurs hung out together after leaving the nest. These immature dinosaurs were big enough to fend for themselves, but didn’t go off on their own or join more mature individuals of their own kind. Instead, many young dinosaurs led a separate existence from adults of their species, and often looked like the awkward teenagers they were.