My Beloved Brontosaurus

by Brian Switek

  Through the ongoing investigations of embryos, infants, and juveniles, paleontologists have discovered a rule that is consistent across the non-avian dinosaur family tree: baby dinosaurs didn’t look just like miniature copies of their parents. Infant Maiasaura were big-eyed, short-snouted babies that would undoubtedly be a hit on websites like Cute Overload if they were around today, and the six-inch Massospondylus babies that toddled around the nesting grounds on all fours hardly looked anything like their twenty-foot-long, bipedal parents. Sweeping anatomical changes transformed gawky infant dinosaurs into impressive adults, and the alterations were so drastic that paleontologists have sometimes mistaken juvenile dinosaurs of an already known species for an entirely new kind of dinosaur. This problem has become a growing point of contention among paleontologists, and no beast represents this debate better than one of the greatest dinosaur ambassadors to the public—Triceratops.

  * * *

  In 2010, ill-informed news services wailed that Jack Horner and his PhD student John Scannella had banished Triceratops into the dustbin of discarded dinosaurs, where “Deinodon,” “Trachodon,” and many others had come to rest. Some journalists woefully misconstrued what was actually happening, missing the point that many dinosaurs went through elaborate growing pains.

  It all started in July of that year, when Scannella and Horner published a paper on how the appearance of Triceratops changed as the dinosaur aged. Previous fossil finds and research conducted by Horner and collaborator Mark Goodwin had documented how the dinosaur’s horns and frill transformed from infancy to adulthood, but Scannella and Horner found something else. They realized that the specimens they’d thought were adult Triceratops—big-headed herbivores with forward-pointing brow horns and a solid frill—hadn’t finished growing. All the large Triceratops were really young adults, and the fully mature adults had been misidentified as a different dinosaur genus.

  This tension between dinosaur biology and our efforts to name and classify dinosaurs is as old as paleontology itself. Triceratops is a classic case—our image of this dinosaur has been shifting ever since naturalists first discovered its remains in 1887, at the height of the Bone Wars between the dueling paleontologists E. D. Cope and O. C. Marsh. It was in that year that George Cannon, a high school teacher and amateur geologist, discovered a pair of large horns and part of a skull roof in an exposure near Denver, Colorado. Cannon sent the horns and a few additional fragments to Marsh in New Haven, and, after examining them, the Yale paleontologist believed that the weapons must have belonged to some enormous herbivore. The bones seemed to resemble the horns of America’s classic Western icon, the bison, and so Marsh dubbed the mysterious animal Bison alticornis.

  The skulls of Triceratops (A) and Torosaurus (B). Were these truly two different dinosaurs, or was Torosaurus the fully mature form of Triceratops? (Illustration from www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0032623)

  But there was something amiss about Marsh’s bison. Cannon had found definite dinosaur bones in the same layer as the horns. Why would a bison be found in the same stratum as Cretaceous dinosaurs? The puzzle wasn’t resolved until Marsh received a partial dinosaur skull from another fossil hunter in 1889. The horns of this new animal were similar to those of the giant “bison,” and so Marsh reasoned that Cannon’s creature must have also been a dinosaur. With this new data in hand, Marsh recast his “bison” as the dinosaur Triceratops horridus the same year, and drew from a different specimen to name a second species in 1890, which he called Triceratops prorsus.

  Now, as we saw during the “Brontosaurus” episode, Marsh had a habit of naming new species from partial remains that differed only slightly from other known fossils. A different curve of horn or an altered angle of armor plate was all that Marsh felt he needed to establish a new species or genus. No surprise, then, that when Marsh’s assistant John Bell Hatcher recovered a pair of unusual partial skulls from the Triceratops-bearing beds of Wyoming in 1891, Marsh thought the fossils represented a new ceratopsian genus he called Torosaurus. Torosaurus was another three-horned dinosaur, similar to Triceratops, but this big-headed herbivore had an elongated frill with two large circular holes in the parietal bones.

  For more than a century, paleontologists followed what Marsh had determined. When Scannella and Horner reexamined the horned dinosaurs, though, they found Torosaurus wasn’t really a distinct dinosaur. Marsh’s Torosaurus latus lived in the same place and at the same time as Triceratops, and other paleontologists had noted how the two dinosaurs could be told apart only on the basis of a few skull characteristics. These two lines of evidence revealed that Torosaurus, rather than being a unique dinosaur, was actually the fully mature form of Triceratops. Scannella and Horner argued that late in the dinosaur’s life, the frill expanded, large holes opened up in the parietal bones, and triangular ornaments (or epiossifications) skirting the edge of the structure split and flattened.

  * * *

  This dramatic osteological transformation is laid out in full at the Museum of the Rockies in Bozeman, Montana. I had the chance to stop by during a break from fieldwork in the summer of 2011. A prowling Tyrannosaurus nicknamed Big Mike greets visitors to the primarily paleontological institution, where authentic fossils, sculptures, and casts all find their place among the dimly lit corridors. As I stroll through, I stop to admire the skull of an Allosaurus named “Big Al” and examine an exploded Tenontosaurus skull that shows each individual bone sitting in a glass case near a life-size re-creation of the herbivorous dinosaur suffering under the switchblade claws of a feathery Deinonychus. Both dinosaurs are cleft in two—fully fleshed-out restoration on one side, bare bones on the other.

  The huge ceratopsian in the last room is displayed the same way. Head down and horns angled forward, the enormous dinosaur is only half dressed; he has skin on one side and an X-ray view on the other. I want to call the dinosaur Torosaurus because of the gaping hole in the frill, but the neighboring display places this aged specimen at the very end of the Triceratops growth series—an array of authentic fossils and casts that record the dinosaur’s life from infancy to old age. In this case, Triceratops skulls, all in a row, display almost the entire life of the dinosaur. A tiny, short-frilled infant with only nubs for horns starts the parade, followed by increasingly larger skulls with long brow horns that curve backward and then come forward again as the dinosaur ages. The biggest skull, a copy of the one on the full sculpture, seems to slot right into the series. With a blunted nose horn, forward-curved brow horns, small epiossifications, and two holes in the frill, this creature would seem to be the elusive final form of Triceratops. I consider the displays at length, and they’re lovely, but why did Triceratops change so much? And where are the specimens that document that last, important change between the solid-frilled type and the one—exemplified by Torosaurus specimens—with perforated headgear? I decide to call on Mark Goodwin, the University of California, Berkeley, paleontologist who had worked with Horner to describe how baby Triceratops grew into burly adults.

  * * *

  Berkeley’s Valley Life Sciences Building, where Goodwin works, houses the university’s museum. This is a research institution rather than a gallery of public displays, but there are still a few reconstructions along the building’s corridors. A cast of our prehistoric forerunner “Lucy”—the famous Australopithecus afarensis skeleton described by Berkeley’s anthropologist Tim White—lies prostrate beneath a tombstone reading “R.I.P. Lucy, 3,200,00,” and an impressive cast of a Tyrannosaurus nicknamed “Wankel rex” snarls at undergrads as they pass him on the staircase to the building’s library.

  If you visit the library, you’ll be greeted by a pair of Triceratops skulls. The larger of the two looks like the typical image of the dinosaur—long nose, three horns, solid frill—but the smaller specimen, held up on a little stand, isn’t any bigger than my own skull. The cast is a reconstruction of the only known baby Triceratops, the same one that kicked off the age series at t
he Museum of the Rockies. I wouldn’t say that baby Triceratops were ugly. Not exactly. The little three-horned dinosaurs had an unconventional charm—the sort of so-homely-it’s-kind-of-adorable look shared by manatees and shar-peis. Goodwin, the museum’s assistant director, keeps the original in a file drawer downstairs.

  Goodwin’s office is a typical paleontologist’s workspace. The dull-white room, washed with the irritating glare of fluorescent bulbs, is festooned with journal reprints, specimen casts, books, and fossil specimens. I try to suppress my fanboy enthusiasm as Goodwin picks up part of the disarticulated Triceratops skull he happens to have lying on his desk. The piece is one of the brow horns, but it’s not quite like the decorations on the skull in the library. These horns are curved slightly backward—a feature Goodwin and Horner had determined was an indicator of adolescence.

  Goodwin explains that no two Triceratops skulls are exactly alike. If all the Triceratops skulls were assembled in one place, for example, we’d be able to see that the shapes of the frills and horn curvatures differed from one animal to another. This variation tricked paleontologists for many years, as evidenced by the fact that different authorities named twelve different Triceratops species. During the 1980s and 1990s, however, paleontologists figured out that many of these species were invalid. Most of the anatomical differences were due to individual variations and changes in the course of growth. Ultimately, the list was narrowed down to the only two Triceratops species we recognize today—Marsh’s original Triceratops horridus and Triceratops prorsus. Triceratops changed dramatically as they grew, and the infant skull in Mark Goodwin’s office underscores to me just how mind-bogglingly dramatic the transformation must have been.

  Goodwin walks around his desk to a white set of metallic drawers pushed against the wall behind me. When he gently pulls out the appropriate shelf, I’m not entirely sure what I’m seeing. Set in boxes on a bed of foam are various tan-colored bones, many of which have obviously been carefully reassembled from a cracked and broken specimen. But three large pieces right in the middle offer a clue to what they are. The long, slightly curved bones on the left and right were the sides of a tiny frill, and the center portion made up the ornament’s midline. The isolated pieces are the original parts of the reconstructed Triceratops skull I had seen upstairs.

  I feel a little embarrassed that I didn’t immediately recognize the baby Triceratops sitting right in front of me, but I wasn’t the only one to miss the connection. Goodwin explains how the bones were originally thought to belong to a pachycephalosaur—one of the thick-skulled bipedal dinosaurs that roamed North America at around the same time. It wasn’t until 2006 that Goodwin and his colleagues recognized the pieces for what they really were. I want to pick up a parietal—a bone decorated with tiny, arrow-shaped ornaments that makes up the middle part of the frill—but I try to hold myself back from asking to do so. I’ve had nightmares about dropping important specimens, so maybe it’s best to let resting dinosaurs lie.

  Goodwin takes me back up to the library before I can drum up the courage to ask. There’s something about the authentic Triceratops skull on display that he wants me to see. Despite its size and seemingly mature anatomy, the bigger Triceratops skull still shows signs of change. The clues are in the frill. While the dinosaur’s frill is still solid, Goodwin points out, there are two indentations on either side of the midline ridge. If Scannella and Horner are right, these were areas where the frill was thinning to create the holes seen in the skull of Torosaurus. The bone here was rapidly being resorbed and reshaped to create a very different look.

  Such a late-life transformation is awfully strange. If dinosaurs started reproducing early, as research by Berkeley’s own graduate students Sarah Werning and Andrew Lee has shown, then why would they develop flashy “Hey, look at me!” ornamentation at such a late age? Wouldn’t such characteristics—perhaps useful for indicating sexual maturity and dominance—develop earlier? I ask Goodwin if any modern animals show such late-life ornamental change. “Hornbills and cassowaries,” he replies, referring to birds with prominent crests on their heads. Perhaps, as in these birds, the ornaments of Triceratops continued to change throughout the animals’ life.

  Indeed, a different dinosaur at another museum might throw additional support to Scannella and Horner’s hypothesis that Triceratops was always changing. Inside a glass case at the Smithsonian’s National Museum of Natural History rests the enigmatic skull of a dinosaur named Nedoceratops. The skull looks more or less like a Triceratops, but with a few important differences. It lacks a nasal horn and has several holes in its frill, including a parenthesis-shaped fenestra in one of its parietal bones. Paleontologists have often treated these features as pathologies or unique characteristics that distinguish this dinosaur from its contemporaries, but according to Scannella and Horner, some of the features illustrate a major skull revamp. The hole in the parietal bone, for example, could just be a transitional stage between the solid frill of younger Triceratops and the gaps in the frill of older forms.

  If Scannella and Horner are correct, the Triceratops at Berkeley and the Nedoceratops at the Smithsonian are the critical specimens that show how the classic Triceratops skull transformed into what we’ve previously called Torosaurus. In their 2010 paper, the paleontologists concluded that these dinosaurs didn’t just change between birth and the time they started reproducing, but kept changing almost to the end of their lives.

  Leave it to sensational news items to misconstrue the findings. Even though Scannella and Horner affirmed that Torosaurus and Nedoceratops would be sunk, and specimens given those names would now go by Triceratops, many bewildered journalists reached the opposite conclusion. The dinosaur name game distracted from the major conclusion of Scannella and Horner’s study, and the confusion stemmed from the revised growth series. Since Torosaurus represented the fully mature form of the dinosaur, with the biggest Triceratops specimens being younger animals, some journalists assumed that Triceratops had been erased, and the name of the mature form was the right one. “The Triceratops Never Existed,” mourned Gizmodo, as did various other sources, from CBS News to the San Francisco Chronicle.

  The supposed demise of the iconic dinosaur even appeared as a riddle on NPR’s weekly quiz show Wait Wait … Don’t Tell Me! (“My third horn will go—yes, I swear it drops, the other two are a shy pair of flops. This dino’s not scary, I’m just ordinary, there’s no such thing as a…” “Triceratops,” Carl Kasell teased.) Triceratops fans were outraged, of course. A “Save the Triceratops” Facebook group bubbled up in pointless protest, and comment threads on dozens of websites roiled with the complaints of amateur dinosaur experts who weren’t about to let scientists take away their childhood memories. My favorite objection was a T-shirt design that depicted Triceratops alongside Pluto, the former planet. Triceratops is a classic dinosaur, they screamed, and it’s one of the first we encounter as children. To many who cherish dinosaurs, this felt like “Brontosaurus” all over again.

  The concerned dinosaur fans eventually understood that Triceratops wasn’t going anywhere. As Scannella and Horner’s study pointed out, Triceratops was named first, and so it had scientific priority over Torosaurus. Only the fans of Torosaurus (like myself) had reason to worry. (Almost no one gave a damn about Nedoceratops.)

  Of course, just publishing a new idea doesn’t mean that scientists must agree with the proposal. Scannella and Horner’s paper was not a statement of fact, but one hypothesis based on the current spread of data. Offering a contrary view, ceratopsian expert Andrew Farke pointed out that no one has ever documented the kinds of changes required to transform a solid-frilled Triceratops skull to a Torosaurus skull before. The enigmatic Nedoceratops aside, paleontologists haven’t uncovered the critical skulls that show how Triceratops could have morphed into Torosaurus. What’s more, Farke has noted that a Torosaurus specimen at Yale’s Peabody Museum is a candidate for a young individual of this animal. If the skull really does belong to a juvenile o
r sub-adult Torosaurus, then that means the dinosaur must have been a distinct, rare species that lived alongside Triceratops.

  * * *

  The debate over Triceratops is just warming up, and is just part of a bigger push to document how dinosaurs grew. It stems from the ongoing point of contention in dinosaur biology that for decades tangled up the unrecognized stages of dinosaurs’ growth and aging with the discovery of new dinosaur species. As we have started to realize how radically dinosaurs could change over their life cycles, dinosaur taxonomists have had to backpedal, folding many presumed species into others of which they turn out to be only older or younger forms.

  While “Brontosaurus” was unquestionably the most famous dinosaur to vanish, many others have disappeared, too. In 1975, for example, when Peter Dodson surveyed the diversity of crested hadrosaurs, he found that “Procheneosaurus” was just a juvenile of another hadrosaur species. The characteristics that appeared in adults—particularly the ornate crests—were relatively undeveloped in the juvenile form. The anatomical differences were due to growth, he realized, not membership in different species. The same was true of “Brachyceratops,” which are really juveniles of other horned dinosaurs, such as Rubeosaurus, that grew to much larger sizes. By reexamining the anatomy of many small dinosaurs, paleontologists have discovered that what we once thought were diminutive species were actually the youngsters of even bigger dinosaurs.

  In recent years, paleontologists have turned to additional lines of evidence—inside dinosaur bones—to investigate how dinosaurs aged. Researchers can examine whether certain bones were fused or not to estimate a specimen’s stage of life, and dark bands inside many fossil bones can help paleontologists determine how old dinosaurs were when they died. These prominent rings formed when dinosaurs slowed their growth—most often during harsh winters or dry seasons when food was scarce—and this pattern allows paleontologists to roughly measure a dinosaur’s age. Likewise, by looking at the kind of tissue between the bands, paleontologists can detect whether a dinosaur was still rapidly growing when it perished, or if its growth had slowed down due to maturity. The microscopic structure of dinosaur bone can tell us much about how these animals grew, and, by using these clues, paleontologists have found that Triceratops wasn’t the only dinosaur to undergo rapid, sweeping transformations.