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The reconstructed skeleton of Olorotitan, from Godefroit et al., 2012.

Olorotitan was one of the most elegant dinosaurs of all time. The 26-foot-long hadrosaur, found in the Late Cretaceous rocks of eastern Russia, had the typical deep tail, beefy legs and slender arms of its kin, but a fan-shaped crest jutting out of the back of the dinosaur’s skull gave it a striking profile. As with its North American cousins Corythosaurus and Lambeosaurus, the hollow head ornament is what makes this dinosaur stand out.

Paleontologist Pascal Godefroit of the Royal Belgian Institute of Natural Sciences and colleagues initially described Olorotitan in 2003. Now, in Acta Palaeontologica Polonica, Godefroit joins co-authors Yuri Bolotsky of the Russian Academy of Sciences and Ivan Bolotsky of Jilin University in China in a thorough assessment of the hadrosaur’s osteology and relationships. The study is based on a mostly complete skull and skeleton–the dinosaur is primarily missing its hands and feet, perhaps because scavengers consumed them before the Olorotitan was buried, but much of the rest of the skeleton was found in articulation.

The hadrosaur’s crest is the most distinct part of its skeleton. As the researchers write, “The large crest dominates the skull.” While crushed and not entirely complete, the preserved part of the crest nevertheless shows that the ornament curved up high over the skull. According to the paper’s reconstruction of the missing skull parts, the front spire of the crest supported a backwards-pointing fan of bone.

This crest was hollow, just as in North American lambeosaurine hadrosaurs such as Parasaurolophus. Indeed, these ornaments were not just for show, but probably allowed adorned dinosaurs to allow them to bellow low-frequency calls over long distances. Each species had their own call based on the shape of the nasal passage inside their skull. Frustratingly, though, the relevant portions of the crest in the Olorotitan skull are either fragmentary or crushed, so no one knows the route its nasal passage took. We need another skull to find out.

There are a few other curious things about Olorotitan. The dinosaur’s skeleton has 18 neck vertebrae–several more than other hadrosaurs. While certainly not in the sauropod class of magnificent necks, Olorotitan had a relatively elongated neck compared with its closest relatives, which is fitting for a creature’s whose name translates to “gigantic swan.”

Further along the spine, the dinosaur’s skeleton seemed to have 15 sacral vertebrae (the fused vertebrae that run through the upper blades of the hips). But, as Godefroit and collaborators point out, the actual number of sacral vertebrae is probably slightly lower. The principal, mostly-complete Olorotitan skeleton used in the study was apparently an old individual in which extra bones of the lower back and tail fused to those at the sacrum.

But, in comparison with another specimen, the estimated age of the mostly-complete Olorotitan shows how size can be a deceiving factor in determining how old a dinosaur was. Godefroit and colleagues point out that various aspects of the old animal’s skeleton were fused, and that the dinosaur shows evidence of many repaired fractures. But there’s another partial Olorotitan skeleton–principally a portion of the lower back, hip and part of the tail–that appears to be of “equivalent size” that doesn’t show these age-related characteristics. If this is accurate, it’s a reminder that dinosaurs varied in terms of size at any particular age–just like us. That’s a simple fact, but something worth keeping in mind as researchers continue to debate how dinosaurs grew up. Skeletal indicators of age, such as bone fusion and the microstructure of skeletal elements, are more important than size alone.

Reference:

Godefroit, P., Bolotsky, Y., Alifanov, V. (2003). A remarkable hollow-crested hadrosaur from Russia: an Asian origin for lambeosaurines Comptes Rendus Palevol, 2, 143-151 DOI: 10.1016/S1631-0683(03)00017-4

Godefroit, P., Bolotsky, Y.L., and Bolotsky, I.Y. (2012). Olorotitan arharensis, a hollow-crested hadrosaurid dinosaur from the latest Cretaceous of Far Eastern Russia. Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0051

A sculpture of Pentaceratops outside the New Mexico Museum of Natural History and Science. Could sexual selection account for the prominent ornaments of this dinosaur? Photo by the author.

Non-avian dinosaurs were weird. That’s one of the reasons we love them so much. There’s nothing quite like a slender-necked Barosaurus, a beautifully-crested Dilophosaurus or lavishly-ornamented Pentaceratops alive today. If such dinosaurs were anything, they were bizarre, but why were they so strange? Each case demands its own explanation, and paleontologists have continuously tussled over whether particular ornaments were weapons, sexual displays or something else.

According to an in-press paper at Trends in Ecology & Evolution, at least some weird dinosaur features may best be understood in the context of mate competition, mate choice and sexual signalling. The paper, by entomologist Robert Knell and colleagues, is the latest in a long-running debate over whether sexual selection had any influence on dinosaur lives and how to detect the hallmark of such pressures.

The debate has been going on for years but only recently increased in intensity. In a 2010 study, paleontologists Kevin Padian and Jack Horner rightly noted that sexual dimorphism–or a significant anatomical difference between the sexes–has never been conclusively demonstrated among non-avian dinosaurs. The idea had been proposed for a variety of dinosaurs using a number of skeletal landmarks, but none of the hypotheses have stuck. Even if sexual dimorphism existed among dinosaurs, we lack the sample size to identify the phenomenon. More than that, Padian and Horner cited the lack of sexual dimorphism as a sign that sexual selection probably wasn’t an important facet in the origin and modification of bizarre dinosaur features. Instead, the researchers hypothesized, the various horns, crests, plates and other ornaments evolved because of species recognition–the ability for dinosaurs to quickly and easily identify members of their own species.

Other researchers disagreed. Knell and Scott Sampson had a brief exchange in the journal pages with Padian and Horner. This was followed by a paper by Dave Hone and co-authors that suggested that mutual sexual selection might explain the mystery of why dinosaurs had bizarre ornaments but don’t seem to exhibit sexual dimorphism. Under this hypothesis, both males and females may prefer mates with elaborate visual signals, and therefore the same prominent structures would be expressed in both sexes. This kind of sexual selection has been documented in modern avian dinosaurs, but, until now, hasn’t been considered as an explanation for the ornamentation of non-avian dinosaurs. Even though mutual sexual selection has not been proven as an evolutionary driver among extinct dinosaurs, it’s a possibility worth considering.

The new paper by Knell and co-authors also draws on modern examples to investigate how we might identify examples of sexual selection among prehistoric species. The paper covers a wide variety of creatures, from ammonites to birds, but, since this is the “Dinosaur Tracking” blog, I’ll focus on how the argument applies to the ever-controversial adornments of non-avian dinosaurs.

As the researchers state, there’s no simple, tell-tale way to identify sexual selection. This is partly because many strange structures are multifunctional, and structures may be co-opted for different functions during the course of their evolution. Think of sauropods. The elongated necks of these dinosaurs allowed them to feed over a wide swath of greenery, but they could have also been used as visual displays. A big fleshy neck is prime advertising space. In this case, a feeding advantage appears to have preceded any signalling function, but the mosaic nature of evolution hinders our efforts to tease apart the influence of different, interacting pressures.

All the same, there are a few clues that can help paleontologists identify possible cases where sexual selection was at play in the deep past. One possible line of investigation is sexual dimorphism, although, as I said above, this has yet to be conclusively demonstrated in dinosaurs. (And, as Knell and co-authors argue, sometimes the sexes might differ for reasons other than sexual selection.) The way prominent displays grew is another phenomenon worth looking into. We would expect that features that make a difference in mating would only appear as the dinosaur approached sexual maturity. Juvenile, and presumably sexually-immature, Lambeosaurus don’t have the full-blown crests of older individuals. Perhaps this is because the crests are sexual signals that only grow as the dinosaurs approach mating age, although it’s possible that the development of crests are related to the overall growth of the dinosaur’s skeleton.

The diversity–or disparity–of ornament shapes among closely-related species may also be important. Even closely-related species of ceratopsid dinosaurs, Knell and collaborators note, had very different horn shapes and arrangements. This could be a sign of sexual selection by way of competition and mate choice, but, as Padian and Horner pointed out, the same evolutionary pattern could be the result of selection for distinct-looking species. Finally, Knell and co-authors cite “costliness” as another potential indicator–if a trait is flashy, requires a good deal of energy to grow and comes at a cost to the organism’s survival potential, then it may be a sexually-selected trait.

Obviously, each line of evidence comes with caveats. Sexual selection can be difficult to identify even among living species, much less extinct ones. It would be strange if sexual selection played no role in dinosaur evolution, but we’re left with the question of how to detect and test the hypothesis of sexual selection. Paleontologists will have to very carefully test hypotheses about bizarre structures, paying careful attention to distinguish between competing alternatives. Ultimately, paleontologists may only be able to identify possible scenarios for the origin and evolution of bizarre features, but studies of modern species can at least provide guidelines for what researchers should look out for.

If we’re truly going to understand the visual signals of dinosaurs, though, we need better sample sizes. We need to know how individuals of the same species varied from one life stage to the next. Without this anatomical foundation, researchers will be left to argue from a typological standpoint that may misconstrue how certain features changed with age and evolved over time. Recall the “Toroceratops” debate–if Triceratops changed into a Torosaurus-form late in life, most likely beyond the onset of sexual maturity, that is certainly going to influence how paleontologists investigate and discuss dinosaur visual signals.

The influence of sexual selection, or lack thereof, will undoubtedly be debated for some time to come. But, as Knell and colleagues conclude, investigating the possible influence of sexual selection in prehistory “is neither a forlorn nor impossible task.” We may yet find out what’s sexy to a dinosaur.

For more on this study, see this post by Dave Hone, one of the paper’s authors.

[My thanks to Darren Naish, another of the paper's authors, for sending me the new study.]

Reference:

Knell, R., Naish, D., Tomkins, J., Hone, D. (2012) Sexual selection in prehistoric animals: detection and implications, Trends in Ecology & Evolution DOI: 10.1016/j.tree.2012.07.015.

The AMNH skeleton of Styracosaurus, one of the dinosaurs from the upper zone of the Dinosaur Park Formation. Image from Brown and Schlaikjer, 1937 via Wikipedia.

Dinosaurs didn’t all live at the same time. Not counting the avian species that have thrived during the last 65 million years, dinosaurs proliferated throughout the world during a span of over 160 million years. As I’ve pointed out before, it’s amazing to think that less time separates us from Tyrannosaurus than separated Tyrannosaurus from Stegosaurus.

Even within specific geologic formations, not all the dinosaurs found in those layers lived side by side. Dinosaur-bearing strata accumulated over millions and millions of years and record both ecological and evolutionary changes. Look closely enough, and you can even see particular communities of dinosaurs give way to different assemblages. In an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper, Jordan Mallon and colleagues have done just that.

Canada’s Dinosaur Park Formation is one of the most spectacular slices of Late Cretaceous time found anywhere in the world. Spanning approximately 76.5 to 74.8 million years ago, the formation has yielded lovely specimens of dinosaurs such as the crested hadrosaur Corythosaurus, the spiky ceratopsid Styracosaurus, the lithe tyrannosaur Gorgosaurus, the heavy-armored ankylosaur Euplocephalus and many others. Not all of these dinosaurs were neighbors, though. Since 1950, at least, paleontologists have recognized that some kinds of dinosaurs are restricted to certain slices of the formation, and the dinosaur community changed over time. Mallon and co-authors decided to have another look at the dinosaur turnover, focusing on the large herbivores and investigating what might have shook up the dinosaur populations during the time the Dinosaur Park Formation was being laid down.

The paleontologists identified two broad divisions in the Dinosaur Park Formation, which they call “megaherbivore assemblage zones.” Each zone lasted roughly 600,000 years each. There are a lot of names here, so bear with me. In the lower zone, the horned dinosaur Centrosaurus and the crested hadrosaur Corythosaurus are found throughout; other dinosaurs restricted to this half of the formation include the ceratopsid Chasmosaurus russelli, the hadrosaurs Gryposaurus and Parasaurolophus, and the ankylosaur Dyoplosaurus.

Yet there are some dinosaurs that first appear in the lower zone and persist into next one. The ceratopsid Chasmosaurus belli, the ankylosaur Euoplocephalus and the hadrosaurs Lambeosaurus clavinitialis and Lambeosaurus lambei show up in the lower zone but pass through into the second zone as well. And, as with the lower swath, there were dinosaurs that were only found in the second zone. The hadrosaurs Prosaurolophus and Lambeosaurus magnicristatus, as well as the horned dinosaurs Styracosaurus, Vagaceratops and a pachyrhinosaur, are only found in the upper zone.

So the big picture is that the lower zone is characterized by Centrosaurus and Corythosaurus, the upper zone is distinguished by Styracosaurus and Prosaurolophus, and there are some dinosaurs–such as Lambeosaurus and Chasmosaurus–that are smeared across the two. As the researchers note, it’s even possible to break down the two halves into even smaller subsets, although the picture gets a little muddier at these levels.

What does all this evolutionary dinosaur shuffling mean? Other researchers have proposed that the Dinosaur Park Formation represents a series of turnover pulses–after a period of stability, rapid ecological change wiped out some dinosaurs while creating opportunities for a new community. The now-vanished Western Interior Seaway has been invoked as a possible mechanism for this. As this shallow sea, which once split North America in two, expanded and encroached further inland, the area of the Dinosaur Park Formation became a mostly coastal, muddy, swampy habitat. This may have put pressure on some forms of dinosaur while providing opportunities for others. As the seaway fluctuated, the attendant changes would have altered the environment and therefore affected dinosaur populations.

According to Mallon and collaborators, though, there’s no strong evidence for the turnover pulse hypothesis. We simply don’t have the resolution to tell how closely certain dinosaurs were tied to particular habitats or niches, and shifts in ecology would have influenced dinosaur evolution. Other possible influences–such as dinosaurs migrating to the area from elsewhere, or the evolution of one species into another within the formation–are also frustratingly unclear. As the researchers state, “Whether the appearance and disappearance of the megaherbivorous taxa of the [Dinosaur Park Formation] was due to evolution, migration, or to a combination of these factors, is difficult to determine.” We don’t yet know what drove the alterations in the formation’s dinosaur communities.

Aside from the ongoing mystery about what caused the changes between the two zones, the revised look at the Dinosaur Park Formation also raises a few questions about dinosaur ecology. Despite the shifts in dinosaur communities, the paleontologists note, there were about six to eight different megaherbivorous dinosaur species living alongside each other. That’s a lot of big herbivores on the landscape, especially since the hadrosaurs and ceratopsids may have formed huge herds. Such vast, hefty dinosaur communities would have required a large amount of vegetation, and the disparate megaherbivores were in competition with each other for food. In order to live alongside one another, then, we can assume that there was some kind of niche partitioning–the dinosaurs were adapted to have restricted diets or live in particular habitats as a result of their competition for resources. How exactly this happened, though, requires further study into the ecology and evolution of these dinosaurs.

And there was something else that caught my eye. The new study focused on the megaherbivores, but what about the large carnivores? The large tyrannosaur Gorgosaurus was also present in the Dinosaur Park Formation and was rejected by the researchers as a zone marker because this theropod ranges throughout the formation. Think about that for a moment. We can see a significant amount of change and turnover among the big herbivores, but one of the large carnivores stays the same throughout the entirety of the formation. Why should this be so? Perhaps it has something to do with the fact that the ornamentation and headgear of hadrosaurs and ceratopsids changed quite a bit, but their general body plans were conservative–a Gorgosaurus could take down a Corythosaurus just as well as a Lambeosaurus.

Likewise, I wonder if the same pattern might hold true elsewhere. The Kaiparowits formation of southern Utah, laid down around the time of the Dinosaur Park Formation further north, also hosts an array of hadrosaurs, ceratopsids and ankylosaurs, but there seems to be just one large dinosaurian predator, the tyrannosaur Teratophoneus. (The giant alligator cousin Deinosuchus was another megacarnivore in the Kaiparowits.) We need more fossils to be sure, but perhaps, like Gorgosaurus, the short-snouted Teratophoneus remained the same as different large herbivores came and went. If this turns out to be the case, the lack of an arms race between predator and prey would be further evidence that the ornamentation of ceratopsids and other dinosaurs had more to do with decoration and combat among each other than defense.

Indeed, the new study of the Dinosaur Park Formation lays some important groundwork for future studies. Paleontologists are currently investigating and debating why the roughly 75-million-year-old dinosaurs from Alberta are different from the roughly 75-million-year-old dinosaurs from southern Utah. What factors drove the diversity and disparity of these dinosaurs across the latitudes, and who really lived alongside whom? So far, the Dinosaur Park Formation is the best-sampled slice we have, and there is much work to be done. With any luck, and a few more decades of careful sampling, we’ll be able to put together an intricate picture of how dinosaurs lived and evolved during this brief span of Late Cretaceous time.

Reference:

Mallon, Jordan C., Evans, David C., Ryan, Michael J., Anderson,, & Jason S. (2012). Megaherbivorous dinosaur turnover in the Dinosaur Park Formation
(upper Campanian) of Alberta, Canada Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2012.06.024

The skull of Edmontosaurus, a Cretaceous hadrosaur from North America. The vandalized dinosaur wasn’t an Edmontosaurus, but belonged to the same evolutionary group. Photo from Ballista, image from Wikipedia.

When paleontologists uncover a dinosaur, they have plenty of reason to worry. In some parts of the world, such as Mongolia, black market thieves often steal and smuggle dinosaurs that wind up bringing in hefty sums at auction houses. Sometimes, paleontologists have returned to field sites to find skeletons stolen right out from under their noses. But, even closer to home, vandals regularly damage and destroy dinosaurs. Earlier this month, an “irreplaceable” dinosaur skeleton discovered near Grande Prairie, Canada was destroyed by persons unknown.

According t0 the CBC, the destroyed skeleton was a hadrosaur being excavated by paleontologist Phil Bell and a University of Alberta field team. The dinosaur was discovered on June 15th, and was complete enough that Bell intended the dinosaur to eventually go up on exhibit. When Bell returned to the site this month, however, the dinosaur was turned into a cascade of broken bone fragments. Even worse, this isn’t the first time dinosaurs at this site have been vandalized. Since May, the report says, three other fossils have been stolen or damaged.

There’s no clear motive for why the criminals smashed the site. But they vandals left a clue behind. At a campsite near the dinosaur excavation, the CBC reports, investigators found a liquor store receipt that may help track down the people who so callously pulverized the hadrosaur.

I’m completely baffled as to why anyone would want to destroy a dinosaur. The fantastic animal beat the odds against preservation and remained locked in stone for tens of millions of years, and can tell us about a world that we can never see ourselves. What sort of stupid, selfish person would even think of turning a wonderful fossil into a pile of rubble? It is truly sad that paleontologists have to worry about this kind of destruction. Dinosaurs belong to everyone, and it’s heartbreaking to see one stolen from us by ignorant despoilers.

A reconstruction of the Edmontosaurus skull LACM 23502, with a beak based on a natural mold. From Morris, 1970.

I’ve never liked the term “duck-billed dinosaur.” I know it’s part of the accepted dinosaur lexicon, just like “raptor” is, but every time I hear the phrase I think of a sluggish, swamp-bound Edmontosaurus dabbling in the water for soft water plants and algae. Paleontologists tossed out this imagery decades ago—hadrosaurs were terrestrial creatures with jaws specially adapted to grinding down tough vegetation.

I admit that the skull of Edmontosaurus looks superficially duck-like. Much like a mallard’s, the Late Cretaceous hadrosaur’s mouth is long, low and generally bill-shaped. The resemblance between these very, very distant relatives helped inspire images of wading hadrosaurs. But most Edmontosaurus skulls you see in museums present only the bony framework of the skull. The tough keratinous beak that tipped the skull typically decayed during the fossilization process, but in 1970, paleontologist William Morris described a rare Edmontosaurus skull with a beak trace.

You can see the specimen on display at the Natural History Museum of Los Angeles today. Designated LACM 23502, this Edmontosaurus skull was collected by Harley Garbani near Montana’s Ft. Peck Reservoir. Other Edmontosaurus have been found here, but this fossil included a natural mold of the dinosaur’s beak. (While the beak itself was not preserved, the mold showed what the internal surface looked like. In life, the actual beak sat on top of the fossilized mold.) The structure was not shaped just like a duck’s bill. On the bottom jaw, the beak surface curved slightly upward, and the upper half of the beak created a vertical, fluted surface that hung over the tip of the lower jaw. Maybe the term isn’t the most apt—and I’m open to suggestions—but Edmontosaurus seemed to be a shovel-beaked dinosaur rather than a duck-billed one.

At the time Morris described the skull, though, hadrosaurs were still thought to be semi-aquatic dinosaurs. Morris believed that the bill traces he described supported this idea and imagined that ridges on the interior part of the mold helped the dinosaurs strain plants and small invertebrates from the water. “A filtering device would be very important in assuring that these large animals could ingest large amounts of concentrated food relatively free of water in a manner similar to that of the dabbler ducks,” Morris wrote, which made the term “duck-bill” seem all the more apt for these dinosaurs.

Despite Morris’ insistence that hadrosaurs nourished themselves by slurping plant-heavy Cretaceous soup, though, we now know that Edmontosaurus and kin were terrestrial animals capable of breaking down tougher plant materials. Exactly how the beak of Edmontosaurus contributed to feeding is not entirely clear—perhaps the beak cropped vegetation that was broken down by the rows of small teeth lining the jaws. One thing is for sure, though. The duck-bills weren’t really so duck-like after all.

Reference:

Morris, William J. (1970). “Hadrosaurian dinosaur bills — morphology and function“. Contributions in Science (Los Angeles County Museum of Natural History) 193: 1–14.