Smithsonian.com

Archosaur skull changes (juveniles on the left, adults on the right). While there was a significant amount of change between the juvenile and adult skulls of alligators (top) and the non-avian dinosaur Coelophysis (middle), there was little change between the juvenile and adult skulls of early birds such as Archaeopteryx (bottom) and their closest dinosaur relatives. From Bhullar et al., 2012.

Birds are dinosaurs. That much is certain. That deep connection, bolstered by fossil finds and theoretical frameworks, has made dinosaurs seem more bird-like than ever expected. From feathers to nesting behavior, many aspects of avian natural history are now known to have originated among non-avian dinosaurs.

But non-avian dinosaurs were not just like birds in every respect. The way many dinosaurs grew is vastly different from the way avian hatchlings mature. Take Triceratops, for example. Even if we ignore the controversial “Toroceratops” hypothesis—which suggests drastic skull transformation late in the life of the horned titan—the reconstructed growth trajectory for this dinosaur involves major skull changes. The horns of young Triceratops curved backward but reoriented as the animal grew to point forward. And the little ornaments around the fringe of the dinosaur’s frill, called the epiossifications, transformed from high, pointy spikes to flattened structures.

Granted, Triceratops was about as far from being an ancestor of birds as is possible while still being a dinosaur. But major transformations seem to have been the norm among dinosaurs, from Tyrannosaurus to Diplodocus to Edmontosaurus. Juvenile dinosaurs had significantly different skull shapes from adults of the same species, and in some cases, it seems that juvenile dinosaurs were occupying different habitats or consuming different food than more mature individuals. (This would be a prehistoric example of what ecologists call niche partitioning.)

Despite the fact that birds represent the only surviving dinosaurian lineage, though, their growth pattern is different. Instead of going through a period of protracted change, as with non-avian dinosaurs, the skulls of young birds are anatomically almost identical to those of adults. And birds take a much faster, more direct route to maturation—many bird species grow to adult size in a year or less. As a new Nature study by Bhart-Anjan Bhullar and collaborators suggests, this feature of bird life can be traced back to ancient transformations that effectively locked bird skulls into a permanent juvenile anatomy.

Bhullar and co-authors used a technique called geometric morphometrics to survey the degree of skull change among birds, various non-avian theropods, the archaic archosaur Euparkeria and the modern American alligator. By tracking landmarks on the skulls in virtual models, the researchers were able to quantify how much the skulls of particular creatures changed. As expected, most non-avian dinosaurs retained the ancestral growth pattern—juvenile skulls were significantly different from adult skulls, regardless of how big those dinosaurs were.

The dinosaurs most closely related to birds showed a different pattern. The eumaniraptoran dinosaurs—the group that contains the sickle-clawed, feathery deinonychosaurs as well as birds—had skulls that looked more juvenile in form, and there was less change in shape between youngsters and adults. A juvenile skull form was undergoing little modification through maturity. Biologists know this as paedomorphosis, when descendent species resemble the juvenile stages of their ancestors.

What could trigger this kind of change? That’s difficult to say. Paedomorphosis is a result of modifications to growth, a developmental phenomenon regulated by particular genes. Bhullar and collaborators suggest that something happened to truncate the development of eumaniraptoran dinosaurs, which included the ancestors of birds. Quirks of development caused these dinosaurs to mature in a juvenile form. And while birds continued this trend in their evolution, its first glimmerings can be traced back to their non-avian ancestors. Non-avian dinosaurs are the key to understanding how birds came to be.

Reference:

Bhullar, B., Marugán-Lobón, J., Racimo, F., Bever, G., Rowe, T., Norell, M., & Abzhanov, A. (2012). Birds have paedomorphic dinosaur skulls Nature DOI: 10.1038/nature11146

A sculpture of Torosaurus—or, according to some, a mature Triceratops—outside Yale's Peabody Museum of Natural History. Photo by the author.

I wish I could take dinosaurs away from the media for a while. Someone certainly should. Lazy journalists and unscrupulous documentary creators have amply demonstrated that they just can’t play nice with Tyrannosaurus, Triceratops and kin.

In the past month and a half, we’ve seen aquatic dinosaur nonsense resurface in shoddy news reports, a brief media invasion of hyperintelligent alien dinosaurs and stinky stories about dinosaur farts, not to mention the bizarre creationist/extraterrestrial conspiracy-theory mashup on Ancient Aliens. I’m almost surprised that this glut of utter dreck wasn’t followed by reports of dinosaurs who used their noxious flatulence to propel themselves through space. To paraphrase the immortal words of Ozzy Osbourne, it would seem that dinosaur news has gone off the rails on a crazy train.

And the distortions keep coming. The sci-fi and science news aggregator io9 just drew on a ceratopsid misunderstanding I thought had been left behind two years ago. Yesterday afternoon Ed Grabianowski posted an article titled “Everything you need to know about the scientific controversy that could destroy Triceratops.” The article was meant as a quick survey of recent, and quite controversial, research about whether the horned dinosaurs called Torosaurus and Nedoceratops are really more mature forms of ol’ three-horned face, Triceratops. The general idea is that the solid, rounded frill of Triceratops changed shape and developed two large holes, called parietal fenestrae, relatively late in life, when the dinosaur hit skeletal maturity. What were previously considered to be three different dinosaurs might actually just be three growth stages of the same genus.

Whether this was truly the case is a matter of debate. And while Grabianowski produced a fair review of the research, the post repeated the hyped—and entirely wrong—idea that paleontologists might soon be sinking Triceratops. “If you cried over the sick Triceratops in Jurassic Park, or just loved this horned dinosaur as a kid,” Grabianowski wrote, “there’s one scientific controversy you need to understand right now—it’s the one that may wind up demonstrating that Triceratops never existed.”

Here’s the thing. Triceratops is totally safe. It’s only the dinosaur die-hards who adore Torosaurus and Nedoceratops who have anything to worry about. I covered this two years ago, when the publication of the first paper in this ongoing debate kicked off a wave of ill-informed hysteria. (Although I must say that the Triceratops-Pluto shirt design was pretty cool.)

If—and I emphasize if—Triceratops, Nedoceratops and Torosaurus turn out to be growth stages of a single dinosaur, then the name Triceratops has priority. Paleontologist O.C. Marsh named Triceratops in 1889, and he followed that with the first description of Torosaurus in 1891. Nedoceratops is a newcomer name for a single skull that has been given many monikers over the past century; A.S. Ukrainsky coined the name in 2007. Given the taxonomic arcana that govern the proper scientific names for organisms, Triceratops would remain the proper name for the dinosaur since it was established first.

(“Brontosaurus” was put to bed for the similar reasons. Brontosaurus is a synonym for a dinosaur O.C. Marsh named earlier—Apatosaurus—and paleontologist Elmer Riggs recognized this state of affairs more than a century ago. But Brontosaurus still has a great deal of cultural cachet because museums, books, documentaries, writers and paleontologists keep reminding everyone of the name change. Brontosaurus still lives because we keep reminding people that it didn’t really exist.)

No matter what happens, Triceratops isn’t going anywhere. What we think we know about the dinosaur’s biology might change, but the classic name will stay. I pointed this out on Twitter shortly after the io9 post appeared, and, to io9′s credit, science editor Analee Newitz quickly changed the headline and introduction. I appreciated the speedy edit. The body of the post was a good summary of the argument as it presently stands, but it was painful to see the same “ZOMG, THEY’RE TAKING AWAY TRICERATOPS!” myth used to frame the article.

I’m thrilled that dinosaurs are so popular. Discoveries are coming so fast and furious that it is almost impossible to keep up, and a new wave of cultural dinomania seems to be swelling. That’s why I get so frustrated by misrepresentations of what we’re actually learning about dinosaur lives. We don’t need embellishment—whether it’s teasing us with the false threat of a beloved dinosaur’s disappearance or the idea that sauropods farted themselves into extinction. The wonderful resolution we’re gaining into dinosaur biology and evolution is best communicated simply, directly, and without having to find some snarky or inaccurately comedic hook that ends up distorting what we actually know. And I would be remiss if I didn’t point out that scientists sometimes play this game, too. Chemist Ronald Breslow tried to use armchair speculation of space dinosaurs to give an otherwise mundane paper a little spice—a ham-fisted and ill-executed grab for attention that I sadly saw some other writers agree with. There’s no such thing as bad publicity, right?

Of course, I realize that simply shaking my fist in the air and frustratedly growling “Do better!” isn’t going to fix the problem. There are seemingly innumerable internet news outlets, and never enough professional science writers, so it’s all too easy for dumbed-down churnalism and other misconstrued reports to richochet around the web. Maybe we’ve seen the last of dinosaur farts and space tyrannosaurs, at least for a while, but places like the Daily Mail, FOX News, and the various outlets that pass off barely modified press releases as news will undoubtedly come up with another headache-inducing hook in the not-too-distant future. If it’s not too much to ask, though, I’d like it if the usual suspects gave it a rest. Dinosaurs are amazing enough without the sensationalism.

Did egg-laying spell doom for non-avian dinosaurs, such as this crispy Troodon at the San Diego Natural History Museum? Photo by the author.

How did dinosaurs come to rule the Mesozoic world? No one knows for sure, but the way dinosaurs reproduced probably had something to do with it. Dinosaurs grew fast, started mating before they hit skeletal maturity, and laid clutches of multiple eggs—a life history that may have allowed dinosaurs to rapidly proliferate and diversify. And egg laying itself may have been critical to why many dinosaurs were able to attain gigantic sizes. By laying clutches of small eggs, dinosaurs may have been able to sidestep biological constraints that have limited the size of mammals.

But there was a catch. Consider a large dinosaur, such as Diplodocus. Infant Diplodocus hatched out of eggs roughly the size of a large grapefruit, and if they were lucky, the dinosaurs grew to be more than 80 feet long as adults. And the little sauropods were not just small copies of adults. Like many other dinosaurs, individual Diplodocus changed drastically during their lives, and young dinosaurs may have preferred different habitats and food sources from those of more mature individuals. As outlined by Daryl Codron and co-authors in a new Biology Letters paper, this peculiar life history may have been a consequence of laying eggs.

Codron’s group created a virtual dinosaur assemblage to see how intensely dinosaurs might have competed with one another as they grew. If all dinosaurs started off relatively small, then the largest species had to pass through a series of size classes and change their ecological role as they matured. This ramped up the pressure on young dinosaurs. Juvenile dinosaurs had to contend with other juveniles as well as dinosaurs that topped out at smaller sizes. In a diverse Late Jurassic ecosystem, for example, young Allosaurus, Torvosaurus and Ceratosaurus not only had to compete with one another, but also with smaller carnivores like Ornitholestes, Coelurus, Marshosaurus and Stokesosaurus. Dinosaurs would have faced the most competition at small size classes, and this may have driven some dinosaur lineages to become large.

The new paper also suggests that dinosaur life history may have played a role in the demise of the non-avian species. Competition at smaller size classes, Codron and colleagues suggest, drove dinosaurs to become bigger and bigger, and this created a lack of species that were small at maturity. Mammals and avian dinosaurs occupied those niches. This could have made dinosaurs more vulnerable to the intense pressures of the end-Cretaceous extinction. If the catastrophe targeted large animals, but was less severe among small animals, then non-avian dinosaurs would have been doomed. The big dinosaurs disappeared, and there were no small non-avian dinosaurs left to quickly proliferate in the aftermath.

As John Hutchinson pointed out in a Nature news story about this research, however, we’re going to need a lot more testing to see if this hypothesis holds up. The conclusion is based on a virtual model of ecosystems that we can’t study directly, and mass extinctions are frustratingly complicated phenomena.

Of course, a new dinosaur extinction scenario is irresistible journalist bait. Various news sources picked up the extinction hook (promoted in the paper’s press release) and pointed to the fact that dinosaurs laid eggs as the seeds of their undoing. But this isn’t quite right. After all, turtles, crocodylians and birds all laid eggs, too, and they survived. And mammals did not survive the end-Cretaceous extinction unscathed—several mammalian lineages disappeared or took major hits during the catastrophe. Likewise, not all dinosaurs alive during the final days of the Cretaceous were huge. Titans like Tyrannosaurus, Triceratops and Edmontosaurus are the most famous end-Cretaceous dinosaurs, but in western North America alone, there were also relatively small ceratopians, oviraptorosaurs and troodontid dinosaurs that topped out at about six feet in length. Were these dinosaurs still too big to survive? Was the threshold even lower? If it was, then the reason why medium-sized animals such as crocodylians survived, and why some mammals disappeared, becomes even more complicated. Why non-avian dinosaurs perished, and why so many other lineages survived, remains a mystery.

References:

Codron, D., Carbone, C., Muller, D., & Clauss, M. (2012). Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates Biology Letters DOI: 10.1098/rsbl.2012.0240

A partial Tenontosaurus skeleton on display at the Museum of the Rockies in Bozeman, Montana. Photo by the author.

Tenontosaurus is a difficult dinosaur to describe. This beaked herbivore—a distant, roughly 110-million-year-old cousin of the more famous Iguanodon—didn’t have any spectacular spikes, horns, plates, or claws. In short, Tenontosaurus was a vanilla dinosaur, and is probably most famous for being the prey of the “terrible claw” Deinonychus. But there is something very important about the unassuming plant-eater: Paleontologists have collected a lot of them. There are at least 30 complete or partial Tenontosaurus skeletons in museums across the country, including everything from very young dinosaurs to adults. With such a sample size, paleontologists can compare skeletons to dig into the dinosaur’s biology, and University of California at Berkeley paleontologist Sarah Werning has done just that. In a paper just published in PLoS One, Werning details how Tenontosaurus grew up.

The secret to Tenontosaurus growth is in the bones themselves. The microscopic structure of dinosaur bone contains clues to how rapidly the dinosaurs grew and what was happening to them at the time of death. For this study, Werning created slides from sections of Tenontosaurus long bones—the humerus, ulna, femur, tibia and fibula—to tease out the history of each animal and the larger pattern of how the dinosaur changed with age.

During early life, Tenontosaurus grew quickly. “Throughout early ontogeny and into subadulthood,” Werning writes, “Tenontosaurus tilletti is characterized by bone tissues associated with fast growth.” But the dinosaur didn’t maintain this quick pace during its entire life. Sometime in its adolescence, perhaps around the time Tenontosaurus began reproducing, the dinosaur’s growth rate slowed. (Working with colleague Andrew Lee, Werning previously found that Tenontosaurus and other dinosaurs started having sex before they reached full size.) The dinosaur kept growing, but at a much slower rate, until it eventually reached skeletal maturity and its growth all but ceased.

This kind of growth pattern wasn’t unique to Tenontosaurus. Similar and closely related dinosaurs, such as Rhabdodon and Zalmoxes, appear to have grown quickly in their youth before slowing down sometime in their subadult lives. But not all ornithopod dinosaurs grew this way.

Tenontosaurus, Rhabdodon, Zalmoxes and similar dinosaurs were all on branches near the base of a major dinosaur group called the Iguanodontia. This group also contains Iguanodon itself and the full swath of hadrosaurs (think Edmontosaurus and Parasaurolophus). And, as Werning points out, hadrosaurs and the closer kin of Iguanodon grew extremely rapidly. These dinosaurs grew faster than Tenontosaurus and sustained the high growth rates until their skeletons were fully developed—there was no extended period of slow growth as the dinosaurs approached skeletal maturity.

This different pattern might explain why dinosaurs like Edmontosaurus were so much bigger than their archaic cousins. A really big, mature Edmontosaurus could reach more than 40 feet in length, but Tenontosaurus topped out at around 25 feet. Perhaps the rapid, sustained growth rate of the hadrosaurs and their close kin allowed them to attain huge sizes, while the more variable growth rates of Tenontosaurus constrained the dinosaur’s size to the middle range.

As paleontologists study other dinosaurs, perhaps the details of how iguanodontian growth rates shifted will become clearer. And Werning has set an excellent precedent for other researchers delving into dinosaur histology. Not only is her paper open-access, but Werning also uploaded multiple high-resolution images of the Tenontosaurus bone slides to the website MorphoBank. Other scientists can readily download the images and investigate the slides for themselves. I hope the Tenontosaurus images are just the start of what will become on online library of dinosaur histology—a resource that will undoubtedly help researchers further investigate the biology of these amazing animals.

References:

Werning, S. (2012). The Ontogenetic Osteohistology of Tenontosaurus tilletti PLoS ONE, 7 (3) DOI: 10.1371/journal.pone.0033539

The skull Gilmore described as "Gorgosaurus lancensis." From Gilmore, 1946.

The name “Nanotyrannus” is a polarizing one. Depending on who you ask, the remains attributed to the controversial dinosaur represent a small-bodied tyrannosaur distinct from Tyrannosaurus, the juvenile form of a previously unknown tyrannosaur genus, or the long-sought bones of young Tyrannosaurus. Even before the debate about dinosaur growth stages blew up last year with the suggestion that Torosaurus is a mature Triceratops, paleontologists were arguing over what, exactly, “Nanotyrannus” was.

I was reminded of the ongoing debate during the annual Society of Vertebrate Paleontology meeting last week when I happened upon a thin monograph tucked within a stack of old reprints. The 1946 paper was by the Smithsonian National Museum of Natural History paleontologist Charles Gilmore and was titled “A New Carnivorous Dinosaur From the Lance Formation of Montana.” I should have recognized the paper immediately, but I only realized what I was reading when I flipped to the illustrations at the back and saw the skull that would later carry the name “Nanotyrannus.”

Gilmore’s monograph didn’t mess around. After a quick note explaining that he doubted the validity of the dinosaur “Deinodon” because it was based on indistinguishable teeth, Gilmore jumped right into a description of a small tyrannosaur skull that had been found in the latest Cretaceous strata of Montana. The fossil was beaten up—a few bones were missing from the right side, and many of the teeth were damaged—but overall, the specimen was one of the most complete tyrannosaur skulls then yet found. He called the dinosaur Gorgosaurus lancensis, basing this decision on the large, rounded eye openings, the long and shallow jaws, and the small size of the specimen. The last plate in the monograph demonstrated how different the new dinosaur was. Compared to the skulls of an adult and juvenile Gorgosaurus, the new skull lacked the little ornamental flange of bone above the eye, and the profile of the snout had a slightly deeper and more rounded profile compared to the other small Gorgosaurus skull.

Gilmore also took the opportunity to clean house a little. As many as five different tyrannosaur species, called “deinodonts” at the time, had been named from the latest Cretaceous of North America. In addition to the species he just named, Gilmore found only one species, Tyrannosaurus rex, to be valid. Everything else had been named from teeth, skeletons without heads, or otherwise was difficult to diagnose. Gilmore concluded: “This brief review of the large Upper Cretaceous carnivorous Dinosauria focuses attention on the very unsatisfactory state of our knowledge concerning the nomenclatural status of many of the included forms.” Funny that Gilmore should say that—years later, his “Gorgosaurus lancensis” would play a role in the debate of just how many species of tyrannosaurs were stalking Late Cretaceous Montana.

Four decades after Gilmore’s initial description, the little tyrannosaur skull was re-cast as a different sort of predator. In 1988 paleontologists Robert Bakker, Phil Currie and Michael Williams hypothesized that the skull actually belonged to a unique genus of small tyrannosaur which shared the environment preserved in the Lance and Hell Creek formations with Tyrannosaurus. The primary line of evidence was the fusion of the skull bones. As animals age, the various bones that make up their skulls fuse along sutures, and the degree to which the bones have fused can sometimes be used to roughly determine age. Since all the skull bones in the Gilmore skull appeared to be fused, Bakker and colleagues stated, the tyrannosaur must have been a small adult and therefore distinct from the bigger, bulkier Tyrannosaurus rex. Appropriately, they called the hypothesized animal Nanotyrannus.

Here’s where things get tricky, though. The timing of when sutures between skull bones fuse in dinosaurs varies among individuals and may not be a good indicator of growth stage. And in a 1999 study of growth changes in tyrannosaurid skulls, paleontologist Thomas Carr found that none of the bone fusions claimed by Gilmore or Bakker and colleagues were actually visible. That, in addition to typical characteristics of immature animals such as large, round orbits and the texture of the bone, identified the skull as a juvenile tyrannosaurid, most likely a young Tyrannosaurus rex. This wasn’t the only time young tyrannosaurs have led researchers astray. In 2004, Carr and Thomas Williamson sunk three proposed tyrannosaurs—Aublysodon mirandus, Stygivenator molnari, Dinotyrannus megagracilis—as young Tyrannosaurus rex specimens, and more recently Denver Fowler and colleagues proposed that the “tiny tyrant” Raptorex was probably a juvenile Tarbosaurus bataar. Given that tyrannosaurids were so variable and underwent such dramatic changes from small, gracile juveniles into bulky, deep-skulled adults, it is little wonder that the over-splitting that gave Gilmore a headache remains with us.

Nevertheless, hints and rumors abound that “Nanotyrannus” may make a comeback. Aside from rumors of yet-unpublished specimens, last year Larry Witmer and Ryan Ridgely published a new analysis of the skull Gilmore had found, often called the “Cleveland skull” since it is now kept at the Cleveland Museum of Natural History. Their results were inconclusive—pending the study and publication of other tyrannosaur specimens that will provide a greater context by which to compare the Cleveland skull—but they noted that the skull might have some unique  features which could be used to argue that it was different from Tyrannosaurus rex.

The Cleveland skull and other supposed “Nanotyrannus” specimens will undoubtedly remain in contention for some time. The features already examined and cited by Carr indicate that the specimen was probably not fully mature, and the best-supported hypothesis so far is that this animal—much like the specimen known as “Jane“—was a young Tyrannosaurus rex. Still, there remains the possibility that someone is going to describe the skeleton of a larger, more mature tyrannosaurid from the latest Cretaceous that significantly diverges in anatomy from Tyrannosaurus rex. That seems like a long shot, but we will have to wait for the description of many mysterious specimens to find out.

References:

Carr, T. (1999). Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria) Journal of Vertebrate Paleontology, 19 (3), 497-520

CARR, T.; WILLIAMSON, T. (2004). Diversity of late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America Zoological Journal of the Linnean Society, 142 (4), 479-523 DOI: 10.1111/j.1096-3642.2004.00130.x

Gilmore, C. 1946. A new carnivorous dinosaur from the Lance Formation of Montana.” Smithsonian Miscellaneous Collections, 106: 1–19.

Witmer, L.; Ridgely, R. (2010). THE CLEVELAND TYRANNOSAUR SKULL (NANOTYRANNUS OR TYRANNOSAURUS): NEW FINDINGS BASED ON CT SCANNING, WITH SPECIAL REFERENCE TO THE BRAINCASE Kirtlandia, 57, 61-81