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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

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

Virtual, fleshed-out models of the Tyrannosaurus specimens "Sue" (left) and "Jane" (center) compared to a human. Image by Julia Molnar.

I think science writer Alexandra Witze put it best in her succinct summary on Twitter yesterday: “T. rex = thunderthighs.” No, she wasn’t suggesting that the mighty sauropod Brontomerus (translated: “thunder thighs”) is synonymous with Tyrannosaurus—the current trend of lumping multiple dinosaur genera and species into growth series of single taxa has not gone that far yet—but was instead referring to some of the conclusions presented in a new PLoS One paper by paleontologists John Hutchinson, Karl Bates, Julia Molnar, Vivian Allen and  Peter Makovicky.

As Hutchinson and colleagues point out, Tyrannosaurus has rapidly become “an exemplar taxon for palaeobiological studies” because the Cretaceous predator is big, popular and known from a number of relatively complete specimens from various growth stages. We have a better fossil dataset for Tyrannosaurus than many other giant theropod dinosaurs. In the case of this study, the abundance of specimens allowed Hutchinson and co-authors to virtually reconstruct the body of the animal during juvenile and adult growth stages. Based on skeletons alone, paleontologists have recognized that Tyrannosaurus was rapidly transformed from a lanky juvenile to a bulky adult, but the point of the new study was to investigate how these changes affected the way the dinosaur moved as its body proportions changed.

Through examination of the reconstructed skeletons and virtual models, Hutchinson and collaborators found that as they grew from juveniles to adults, Tyrannosaurus‘ arms became lighter; the animal’s torso increased in length and weight, and the dinosaur’s thighs became heavier despite the hindlimbs becoming lighter overall. This doesn’t mean that the dinosaurs were svelte, though—in the models, the four adult Tyrannosaurus specimens were estimated to weigh more than six metric tons, with the famous specimen “Sue” being more than nine tons. Tyrannosaurus was a heavy-hitter. The authors recognized that subjective differences and the incomplete nature of some of the specimens might have produced some errors in their models, but overall, the trends seen in the skeletons and the virtual, fleshy dinosaurs were in accord.

The changes in body shape and mass would have undoubtedly influenced the way Tyrannosaurus moved, but exactly how these changes would have manifested themselves in terms of locomotion is unclear. A few inferences about how fast Tyrannosaurus could move can be made, though. For example, Hutchinson and co-authors were not able to confirm a recently-published study suggesting that one of the large muscles that attached from the dinosaur’s tail to the upper part of the femur increased in size as Tyrannosaurus grew. The opposite actually appeared to be the case, and a decrease in the size of this muscle may have adversely affected the running ability of Tyrannosaurus. Juveniles were likely more agile animals. At the same time, the virtually restored Tyrannosaurus specimens had massive hip and thigh muscles that were as large as, if not larger than, those in any living animal. So even if the dinosaur had less “junk in the trunk” we could still call it “thunder thighs.” Just make sure you’re at a safe distance when you do that, should you ever happen across a Tyrannosaurus.

For additional details, see the official press release from the Field Museum and the open-access paper, referenced below.

References:

Hutchinson, J., Bates, K., Molnar, J., Allen, V., & Makovicky, P. (2011). A Computational Analysis of Limb and Body Dimensions in Tyrannosaurus rex with Implications for Locomotion, Ontogeny, and Growth PLoS ONE, 6 (10) DOI: 10.1371/journal.pone.0026037

Are there too many dinosaurs? Paleontologist Jack Horner thinks so, and he explained his reasoning in a short TED talk last month in Vancouver, Canada.

Over the past several years, Horner has been picking over the skeletons of Late Cretaceous dinosaurs from North America in an attempt to figure out whether some of the dinosaurs labeled as distinct species are actually growth stages of a single species. In 2009, for starters, Horner and Mark Goodwin proposed that the dome-headed dinosaurs Dracorex and Stygimoloch were actually immature representatives of the larger Pachycephalosaurus. Last year, Horner and colleague John Scannella made a bigger splash when they published a Journal of Vertebrate Paleontology paper suggesting that the broad-frilled, horned dinosaur Torosaurus was the adult stage of Triceratops (though this hypothesis has been contested). In the video, Horner also suggests that the hadrosaur Edmontosaurus was the subadult stage of the larger Anatotitan.

This kind of revision isn’t new. Many dinosaurs specimens that were once thought to be pygmies or oddly-proportioned adults of new species have turned out to be juveniles, such as the diminutive sauropodomorph Mussasaurus, hadrosaur specimens previously assigned to “Procheneosaurus,” and the ever-contentious Nanotyrannus. What’s different now is that paleontologists have more powerful techniques to investigate and compare specimens from well-sampled areas. Scientists can now look into the bone itself to estimate age, for example, allowing researchers to see if a seemingly small form was truly an adult or still had a bit left to grow.

I wouldn’t say that we have too many dinosaurs, though. Many new species are coming from areas that have not been previously explored or are poorly understood. Given how little we know about the past and how few paleontologists there are,many, many dinosaurs are undoubtedly yet to be discovered. These new species will be subjected to in-depth scientific investigations and in time, paleontologists will gain a deeper understanding of how dinosaurs grew up.

For another take on the same video, check out Love in the Time of Chasmosaurs.