Smithsonian.com

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

Possible postures of Triceratops. Image courtesy John Hutchinson.

For decades, paleontologists have been debating how Triceratops stood. Did old “three-horned face” hold its forelimbs straight up and down like other dinosaurs, or did the horned dinosaur waddle along with its elbows out to the side? The dinosaur’s skeleton has not delivered an unambiguous answer. The critical articulation of the upper arm and shoulder can be reconstructed in a range of positions, and so it’s little wonder that different researchers have arrived at disparate conclusions.

According to paleontologist John Hutchinson of The Royal Veterinary College in London, reconstructing how dinosaurs like Triceratops walked from bones alone is very tricky. “Bones themselves tell you only a bit about locomotion or posture,” Hutchinson said. “Soft tissues and the nervous system have a huge role in such behaviors, so paleontology has long struggled to get past those unknown soft tissues to tackle the cool questions about behavior.” The few known ceratopsid footprints haven’t helped that much—the identities of the trackmakers are often ambiguous, and it can be difficult to relate the pattern in the tracks with the anatomy of an unknown species. “To me,” Hutchinson said, “biomechanics is the best way to integrate all those data and test questions about behavior.”

In a paper published last week in the Proceedings of the Royal Society B, Hutchinson and Shin-ichi Fujiwara of the University of Tokyo proposed a new biomechanical technique to test some of the previously proposed ideas about Triceratops posture. Instead of using skeletal articulation alone as a guide, Hutchinson said, “basically we estimated the moment arms (leverages) of key elbow muscles in three dimensions, using landmarks on the bones.” This method, he explained, allowed the researchers to “determine how the elbow is mechanically supported against gravity.” Fujiwara and Hutchinson then measured a variety of modern animals and determined that the moment arms reflected particular postures. This relationship, they conclude, could be used to study prehistoric creatures. “That gave us extra confidence that we could apply the method to extinct animals, so off we went to study some nicely preserved fossils that could illuminate controversial forelimb postures,” Hutchinson said.

Fujiwara and Hutchinson incorporated several different sorts of extinct creature in their study, including Triceratops. They found that the dinosaur probably had upright forelimbs that were held close to the body—a conclusion also supported by evidence from the dinosaur’s anatomy, scaling patterns and rare footprints attributed to horned dinosaurs. Nevertheless, Hutchinson explained that other evidence might indicate a semi-erect, sprawling forelimb posture. “I don’t think the controversy is over by any means,” he said, “but our method tips the scales closer to the upright end of the spectrum.”

Triceratops wasn’t the only dinosaur in the study. Fujiwara and Hutchinson also studied Protoceratops—a much smaller ceratopsian from Cretaceous Mongolia—to see how the forelimbs of horned dinosaurs might have changed with size. The results were ambiguous, Hutchinson says, but Protoceratops may have “had fairly upright forelimbs, albeit maybe no so much as Triceratops did.” This small ceratopsian, therefore, “would be a reasonable approximation of what the distant, smaller ancestor of Triceratops may have stood or moved like,” although Hutchinson stressed the need to obtain additional details from a wider range of horned dinosaurs.

Hutchinson also noted that the technique utilized in the study is “a new tool in the arsenal of techniques for reconstructing limb postures in land tetrapods.” The method can be extended to a variety of extinct animals with controversial limb postures. In addition to the dinosaurs, Hutchinson explained:

[W]e applied our method to desmostylians (giant hippo/pig-like aquatic mammals), whose forelimb poses have been the subject of a controversy similar to that for ceratopsids. We found quite similar results for 2 genera of desmostylians as for Triceratops—they too seem to have been more upright on land. Similarly, the pterodactyloid Anhanguera emerged as having upright forelimbs, although our analysis cannot address the controversy over whether it was a biped or quadruped, so these results need to be taken with a grain of salt. As a reality check, we also applied the method to a recently extinct thylacine, which video and photos tell us was upright, and obtained that result, which was reassuring.

Perhaps, by combing this technique with other lines of evidence, paleontologists will eventually solve the mystery of the Triceratops slouch.

References:

Fujiwara, S., & Hutchinson, J. (2012). Elbow joint adductor moment arm as an indicator of forelimb posture in extinct quadrupedal tetrapods Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2012.0190

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