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Bones from the foot of a hadrosaur attributed to Edmontosaurus annectens. Modified from Zheng et al., 2011.

Sometimes, hadrosaurs can be a real pain. Even though they are some of the most abundant dinosaurs among Late Cretaceous fossil sites, and are therefore an excellent resource for investigating the biology of dinosaurs, the fact is that there are far more isolated bits and pieces of them than complete skeletons. Properly identifying and cataloging these single bones can be difficult—you need a comprehensive knowledge of dinosaur anatomy to know what a lonely bone once belonged to. Now students Rachel Zheng and Gy-Su Kim from southern California’s Webb Schools and paleontologist Andy Farke have taken a step towards offering their colleagues a way to recognize isolated bones from hadrosaurine dinosaurs.

Zheng, Farke, and Kim have just published a hadrosaur atlas in PalArch’s Journal of Vertebrate Palaeontology. Their aim was to fill in a gap in the literature. Even though lots of hadrosaurs have previously been described, seemingly no one had published a detailed, illustrated guide to the hadrosaur foot. To remedy this, the researchers decided to compose a detailed description of the well-preserved foot of a specimen tentatively attributed to the common Late Cretaceous hadrosaur Edmontosaurus annectens. With this atlas to the hadrosaur foot, they propose, other researchers and collections managers may be better able to properly identify hadrosaur foot bones, especially if those researchers don’t already have a reference collection to make comparisons with.

Frustratingly, the precise identity of the dinosaur used to create the atlas is uncertain. Hadrosaurs are notoriously difficult to identify without their skulls, and the specimen in question was missing one. Nevertheless, a combination of anatomical and geological detail allow Zheng, Farke and Kim to hypothesize that the dinosaur in their atlas is an Edmontosaurus annectens. Along with the foot and other bones, part of the right hip (the ischium) of the dinosaur was found. The distal tip of this hip bone is narrow, and this feature identifies the dinosaur as belonging to the hadrosaurine lineage of hadrosaurs. (The other major hadrosaur lineage—the ornately-crested lambeosaurines—had a flared ischium tip.) Since Edmontosaurus annectens is the only hadrosaurine dinosaur known from the Hell Creek strata where this specimen was uncovered, the identification is the most reasonable one on the basis of the material at hand.

The bulk of the paper consists of labeled color photographs of the hadrosaur’s foot from different angle. This is not the super-sexy kind of research that’s going to end up in Nature or Science. That’s a good thing. Some of the biggest gaps in our understanding about dinosaurs involve relatively simple things. There is a definite need for detailed descriptions and comprehensive atlases that will allow other researchers to easily compare and identify different dinosaurs. I love paleobiology and wondering about the lives of dinosaurs as much as anybody else, but in order to generate hypotheses we need a solid foundation of descriptive analysis. I certainly hope that other researchers take the time to go through their own collections, identify well-preserved specimens, and create similar guides so that various mystery bits scattered through museums can be better identified and cataloged.

References:

Zheng, R.; Farke, A.; Kim, G. (2011). A Photographic Atlas of the Pes from a Hadrosaurine Hadrosaurid Dinosaur PalArch’s Journal of Vertebrate Palaeontology, 8 (7), 1-12

The skulls of Late Cretaceous hadrosaurs from western North America. From Campione and Evans, 2011.

Hadrosaurs just can’t get any respect. In a new paper published in PLoS One, paleontologists Nicolás Campione and David Evans have proposed that the immense, Late Cretaceous hadrosaur Anatotitan was actually just the fully mature stage of the dinosaur Edmontosaurus. No one batted an eyelid: “Huh? Anato-what?” Compare the lack of reaction to the tizzy the public fell into last year when confused reporters mistakenly told readers that paleontologists were sinking the name Triceratops. As far as I know, no one has started up a “Save Anatotitan!” group to object to the conclusions of Campione and Evans.

The new hadrosaur paper is just the latest in a growing body of research on the changes Late Cretaceous dinosaurs underwent as they grew up. In 2009 Horner and co-author Mark Goodwin proposed that the dinosaurs Dracorex and Stygimoloch were juvenile and sub-adult stages of the dome-headed genus Pachycephalosaurus, and Horner and John Scannella proposed that the horned dinosaurs Nedoceratops and Torosaurus were more mature growth stages of Triceratops. (Regarding each case, the names Pachycephalosaurus and Triceratops would be preserved while the others would be sunk.) These papers have been very controversial among paleontologists. Have we really been naming too many dinosaurs, or are we now entering an age when we’re lumping too many together?

So far, the focus of the lumping/splitting debate has been on the Late Cretaceous dinosaurs of western North America. The work by Campione and Evans continues this trend with Edmontosaurus and closely related genera. Specifically, the paleontologists chose to investigate 23 edmontosaur skulls, ranging from Edmontosaurus regalis and Thespesius edmontoni from roughly 73-million-year-old deposits in Alberta, to the dinosaurs Edmontosaurus saskatchewanensis, Edmontosaurus annectens and Anatotitan copei from the time interval spanning roughly 70 to 65 million years ago. Just how many of these dinosaur genera and species are valid has been debated for some time, and the new research narrows down this list to just two species of Edmontosaurus.

Through comparisons of particular anatomical landmarks on each edmontosaur skull, Campione and Evans concluded that individual variation and anatomical changes due to growth had led other researchers to name too many hadrosaurs from the pocket of Late Cretaceous deposits they investigated. Hadrosaurs given the name Thespesius edmontoni simply appear to be small individuals of  Edmontosaurus regalis from the same deposits, while Edmontosaurus saskatchewanensis and Anatotitan copei seem to be younger and older growth stages, respectively, of Edmontosaurus annectens. Just like that, five different dinosaurs are reduced to two species of a single genus.

Further study and debate will test the hypothesis proposed by Campione and Evans. (For example, do changes in bone microstructure follow the proposed growth series for Edmontosaurus annectens?) Of one thing, though, there can be no doubt: just how many different dinosaurs existed in North America during the last ten million years of the Cretaceous has become a matter of major debate among paleontologists. How things shake out will undoubtedly influence our understanding of how and why dinosaurs became extinct on the continent. If some of the new studies are correct and the number of different dinosaurs in western North America at the end of the Cretaceous was lower then previously expected, then we are left with the question of why the drop in diversity occurred and whether the changes made dinosaurs more vulnerable to extinction. Then again, if genera like Torosaurus, Dracorex and Anatotitan are preserved, we must ask how so many similar dinosaurs evolved and co-existed alongside each other. Right now, it is too early to tell. We are only at the beginning of what may become an important and long-running debate about how dinosaurs grew up and why they disappeared.

References:

Campione, N., & Evans, D. (2011). Cranial Growth and Variation in Edmontosaurs (Dinosauria: Hadrosauridae): Implications for Latest Cretaceous Megaherbivore Diversity in North America PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0025186

A snapshot of the Tyrannosaurus known as "Stan" at the Institut Royal des Sciences Naturelles de Belgique, Bruxelles. Image from Wikimedia Commons.

Of all the organisms scientists have found in the fossil record, Tyrannosaurus rex is the most prominent ambassador for paleontology. No dinosaur hall is complete without at least some fragment of the tyrant dinosaur, and almost anything about the dinosaur is sure to get press coverage. We simply can’t get enough of old T. rex. It was no surprise, then, that a census of Tyrannosaurus specimens from Montana’s Hell Creek Formation published by Jack Horner, Mark Goodwin and Nathan Myhrvold in PLoS One gained wide media coverage, but there was a sub-story that many news outlets missed. Rather than overturning the image of Tyrannosaurus as a predator, as some reports claimed, the conclusions of the new study actually brought Horner’s stance on the iconic dinosaur close to what other experts thought.

The story behind the new PLoS One study began eighteen years ago. The film Jurassic Park had just triggered a wave of dinomania unlike any seen before, and paleontologists were quick to take advantage of the interest that the film had generated. Among them were Gary Rosenberg and Donald Wolberg, who organized 1994′s Dino Fest event at Indiana University-Purdue University at Indianapolis, and one of the invited speakers was Jack Horner. One of the chief consultants on Jurassic Park, Horner had helped bring the film’s terrifying Tyrannosaurus to life, but in his talk he presented a different picture of the dinosaur.

Horner’s lecture was titled “Steak Knives, Beady Eyes, and Tiny Little Arms (A Portrait of T. rex as a Scavenger),” and a transcript of it was printed in the collected proceedings of the conference. With Jurassic Park fresh in the audience’s mind, Horner explained that the real animal probably wasn’t as speedy or ferocious as the film made it seem. “In fact,” Horner said, “I think the only thing that Tyrannosaurus rex would have done in that movie is eat that lawyer.”

In Horner’s view, Tyrannosaurus was built for scavenging. Despite possessing a huge head full of serrated teeth the size of rail spikes, the tyrant dinosaur had puny, rigid arms, and Horner argued that strong arms would have been essential for an active predator to catch and subdue prey. Furthermore, Horner pointed to the apparently small eyes of Tyrannosaurus and the large olfactory lobe of the dinosaur’s brain. Horner asserted his uncertainty about these features—”I don’t know if it’s worth anything,” he said—but hinted that they might be consistent with the idea of Tyrannosaurus as a scavenger that was better at sniffing out carcasses than following live prey. Since the hadrosaurs and horned dinosaurs of the time lived in giant herds, Horner suggested that tyrannosaurs followed them to pick at the carcasses of those that died as the herds trod around the landscape. Horner concluded:

Picture Tyrannosaurus rex. He has no arms, can’t run fast, appears to have a large olfactory lobe and he’s big. Interestingly enough if you think about it, one of the best things to be if you are a scavenger is big so you can chase away anything else around the carcass.

Horner’s book “The Complete T. rex“, published that year with science writer Don Lessem, presented the “obligate scavenger” hypothesis to a wider audience. Similar ideas had been proposed before, but Horner’s public suggestion that Tyrannosaurus was a lazy scavenger stirred immediate controversy. This was not so much an academic debate as a tug-of-war over who would mold the image of Tyrannosaurus.

Among the early responses to Horner’s ideas was a 1997 lecture delivered by Theagarten Lingham-Soliar to the British Association for the Advancement of science (later printed in Geology Today) titled “Guess who’s coming to dinner: A portrait of Tyrannosaurus as a predator.” Tyrannosaurus would have scavenged when the opportunity arose, Lingham-Soliar said, but the reinforced skull and impact-resistant teeth of the dinosaur were clearly well-suited to handling struggling prey. Even juveniles had these features, and given their small size it was probable that they were actively hunting smaller fare instead of relying on scraps from carcasses already obliterated by adults.

Responses like Lingham-Soliar’s did little to quell the debate. The scavenger hypothesis was popularized in books, news reports and documentaries. Horner’s influence even turned Tyrannosaurus into a scavenger during an early scene of Jurassic Park III. Horner hinted that part of his motivation for proposing the obligate scavenger idea was to get scientists and dinosaur fans to think critically about commonly accepted ideas. Despite the amount of attention the idea received, other paleontologists were not convinced.

The ultimate take-down of Horner’s hypothesis was published by tyrannosaur expert Thomas Holtz in the 2008 book “Tyrannosaurus rex: The Tyrant King.” Right at the outset, Holtz pointed out that flesh-eating animals do not break down into neat categories of “scavenger” and “predator.” Spotted hyenas—traditionally believed to be almost pure scavengers—have been found to be active hunters, and even lions, iconic hunters, obtain a significant part of their food through scavenging. Large carnivorous animals both hunt and scavenge food. Tyrannosaurus would not have been different.

Holtz’s paper was the first comprehensive and scientific critique of Horner’s idea. The ideas had been batted around in talks, documentaries and popular books, but Holtz actually put in the scientific legwork to see if the traits Horner associated with scavenging truly indicated that Tyrannosaurus relied almost entirely on carrion.

Holtz’s analysis dismantled what Horner had proposed. The eyes of Tyrannosaurus were not atypically small; the proportions of its legs would have allowed it to run faster than other large theropods (and, more importantly, potential prey species); it had deeply rooted teeth that would have been able to cope with the stresses generated by struggling prey; and its small forelimbs would not have prohibited it from hunting and killing other dinosaurs. Oddly enough, some of the best evidence of tyrannosaur hunting come from two animals that escaped attacks by the dinosaur: an Edmontosaurus with a partially healed bite along its tail and a Triceratops skull showing a similar type of damage. Since Tyrannosaurus was the only gigantic predator known from the habitats in which the injured herbivores were found, it is probable that the dinosaurs were survivors of Tyrannosaurus attacks.

Tyrannosaurus almost certainly scavenged—something that has been supported by the recent discovery of cannibalism and an instance of scavenging by the related Tarbosaurus—but there was nothing about the dinosaur that barred it from being a formidable hunter. “[T]here is no evidence to suggest that tyrannosaurs were radically different in diet from living large-bodied carnivores, which obtain food [by] both predation and scavenging,” Holtz said.

As reconstructed by Holtz, Tyrannosaurus may have been the spotted hyena of its day. The hyenas do not have large claws or muscular arms like lions. Instead, they primarily catch, kill and consume prey with their robust jaws, which is what the tyrant dinosaur would have done as well. Especially after Holtz’s paper, the idea that Tyrannosaurus hunted and scavenged should not have surprised anyone. So why did so many media sources act with astonishment at the statements by Horner and his team in reference to their new PLoS One paper?

The answer — after the jump (More…)

The partial skeleton of an oviraptorid dinosaur brooding over a nest. From Clark et al., 1999.

What caused the end-Cretaceous mass extinction is one of the greatest mysteries of all time. Paleontologists have racked up a long list of victims—including the non-avian dinosaurs—and geologists have confirmed that a massive asteroid that struck the earth near the modern-day Yucatan peninsula was probably the extinction trigger, but just how that impact translated into a global extinction crisis is still being figured out. Of course, dinosaurs were the most charismatic creatures to perish during the event, and for years Sherman Silber has been forwarding his own peculiar proposal.

An infertility specialist at St. Luke’s Hospital in St. Louis, Missouri, Silber believes that dinosaurs died out because there were not enough females. In a 2004 paper written with David Miller and Jonathan Summers, Silber suggested that dinosaurs had a crocodile-like reproductive strategy in which temperature determined the sex of developing embryos inside their eggs. In American alligators, for instance, lower nest temperatures produce more females, whereas nests with higher temperatures produce more males. If dinosaurs development was also temperature-dependent, Silber and co-authors proposed, then the climatic changes created in the wake of the asteroid strike may have caused many more male dinosaurs to be born than females. Incapable of finding enough mates to ensure their survival, the dinosaurs were wiped out.

Of course, this idea was entirely based on inference. Most birds—the direct descendants of small, feathered dinosaurs—have their sex determined genetically, and there is no clear indication that the sex of developing dinosaurs was determined by variations in temperature. (In fact, the recent discovery that some sauropod dinosaurs repeatedly laid eggs in nurseries heated by geothermal activity would appear to argue against this point.) Furthermore, this hypothesis did not make sense of the fact that turtles and crocodylians—groups in which the sex of offspring is dependent on temperature—survived the extinction and all the subsequent temperature fluctuations the planet has undergone over the past 65 million years. Paleontologists panned Silber’s hypothesis, particularly since it had been previously proposed by other scientists and set aside, but news services ate up the story. “Fried eggs may have wiped out dinosaurs” announced Discovery News, and the BBC reported “Fewer females wiped out dinosaurs.”

Not to be discouraged, Silber has an article in-press with the Middle East Fertility Society Journal which retreads the argument he made in 2004. In it, Silber does not provide details about what scientists have learned about the reproductive habits of dinosaurs, what might have happened after the asteroid impact, or even the range of animals wiped out in the extinction event. Instead, Silber assumes that the sex of dinosaurs was temperature-dependent and that some sort of climate shift would have resulted in an over-abundance of males. As before, he provides no satisfying explanation as to why species with known temperature-dependent sexes survived, merely stating “Crocodilians and other TSD species (but not dinosaurs) survived because they could adapt successfully to the changing environment.” This statement tells us nothing about why these animals did not follow Silber’s predictions.

In fact, Silber was a co-author on another analysis which directly contradicted what he expected. Printed last year in Biology Letters with co-authors Jonathan Geisler and Minjin Bolortsetseg, the study looked at the survival of vertebrates found in Montana’s Hell Creek Formation, representing the very end of the Cretaceous just before the mass extinction, and those in the Tullock Formation, representing environments just after the catastrophe. Since sex determination in dinosaurs is unknown, they were left out, but the scientists found that the performance of Silber’s hypothesis among the other animals was “dismal.”

Of 32 extinctions and 30 survivals, the overwhelming majority of cases were inconsistent with what was expected under Silber’s hypothesis. In fact, most of the species with temperature-determined sex survived whereas 61 percent of species with genetically-determined sex went extinct, with small mammals being the hardest hit. The authors could not explain why this was the case—why turtles and crocodiles survived unscathed while so many other species perished remains an open question—but it was clear that Silber’s hypothesis failed. Being that he was listed as the first author on this paper, it is strange that he has returned to his original hypothesis in the forthcoming Middle East Fertility Society Journal paper.

Whether the sex of dinosaurs was determined by temperature or genetics remains unknown, but we cannot assume that they were just like alligators and crocodiles. After all, birds are modern dinosaurs and most have genetically determined sexes, and recent discoveries have shown dinosaurs to be extraordinarily bird-like. Perhaps dinosaurs were also like birds in having genetically determined sexes. Further research is required to figure this out.

Even then, though, reproduction is just one small part of the global extinction puzzle. Following the intense volcanic eruptions of India’s Deccan Traps, the asteroid impact 65.5 million years ago initiated intense ecological changes on a global scale. Scientists are still trying to find ways to detect how this major event forever changed life on earth.

References:

Clark, J.M., Norell, M.A., & Chiappe, L.M. (1999). An oviraptorid skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest American Museum Novitates, 3265, 1-36

Miller, D. (2004). Environmental versus genetic sex determination: a possible factor in dinosaur extinction? Fertility and Sterility, 81 (4), 954-964 DOI: 10.1016/j.fertnstert.2003.09.051

Silber, S. (2011). Human male infertility, the Y chromosome, and dinosaur extinction Middle East Fertility Society Journal DOI: 10.1016/j.mefs.2011.01.001

Silber, S., Geisler, J., & Bolortsetseg, M. (2010). Unexpected resilience of species with temperature-dependent sex determination at the Cretaceous-Palaeogene boundary Biology Letters DOI: 10.1098/rsbl.2010.0882