September 7, 2012
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.]
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.
August 2, 2012
There’s no shortage of dinosaur myths. Paleontologist Dave Hone recently compiled a list of eight persistent falsehoods over at the Guardian–from the misapprehension that all dinosaurs were huge to the untenable idea that Tyrannosaurus could only scavenge its meals–but there was one particular misunderstanding that caught my attention. For decades, popular articles and books claimed that the armor-plated Stegosaurus and the biggest of the sauropod dinosaurs had second brains in their rumps. These dinosaurs, it was said, could reason “a posteriori” thanks to the extra mass of tissue. It was a cute idea, but a totally wrong hypothesis that actually underscores a different dinosaur mystery.
Dinosaur brain expert Emily Buchholtz outlined the double brain issue in the newly-published second edition of The Complete Dinosaur. The idea stems from the work of 19th-century Yale paleontologist Othniel Charles Marsh. In an assessment of the sauropod Camarasaurus, Marsh noticed that the canal in the vertebrae over the dinosaur’s hips enlarged into an expanded canal that was larger than the cavity for the dinosaur’s brain. “This is a most suggestive fact,” he wrote, and, according to Buchholtz, in 1881 Marsh described a similar expansion in the neural canal of Stegosaurus as “a posterior braincase.”
Sauropods and stegosaurs seemed like the perfect candidates for butt brains. These huge dinosaurs seemed to have pitiful brain sizes compared to the rest of their body, and a second brain–or similar organ–could have helped coordinate their back legs and tails. Alternatively, the second brain was sometimes cast as a kind of junction box, speeding up signals from the back half of the body up to the primary brain. That is, if such an organ actually existed. As paleontologists now know, no dinosaur had a second brain.
There are two intertwined issues here. The first is that many dinosaurs had noticeable expansions of their spinal cords around their limbs–a feature that left its mark in the size of the neural canal in the vertebrae. This isn’t unusual. As biologists have discovered by studying living species, the enlargement of the spinal cord in the area around the limbs means that there was a greater amount of nervous system tissue in this area, and dinosaurs with larger expansions around the forelimb, for example, probably used their arms more often than dinosaurs without the same kind of enlargement. The expansion of the neural canal can give us some indication about dinosaur movement and behavior.
But the so-called “sacral brain” is something different. So far, this distinct kind of cavity is only seen in stegosaurs and sauropods and is different than the typical expansion of the neural canal. There was something else, other than nerves, filling that space. Frustratingly, though, we don’t really know what that something is.
At the moment, the most promising idea is that the space was similar to a feature in the hips of birds called the glycogen body. As sauropod expert Matt Wedel has pointed out, this space stores energy-rich glycogen in the hips. Perhaps this was true for the sauropods and stegosaurs, too. Again, though, we hit a snag. We don’t really know what the glycogen body does in birds–whether it helps with balance, is a storehouse for nutritious compounds that are drawn upon at specific times or something else. Even if we assume that the expansion in dinosaurs was a glycogen body, we don’t yet know what biological role the feature played. Dinosaurs didn’t have hindbrains, but the significant spaces in the hips of stegosaurs and sauropods still puzzle paleontologists.
July 23, 2012
All the non-avian dinosaurs are gone. The last of them died out 66 million years ago. All the same, living dinosaurs – birds – aren’t exactly a substitute for Apatosaurus, Tyrannosaurus, and Stegosaurus. We miss the truly spectacular, bizarre dinosaurs that lived and died so long ago. At least we can catch brief glimpses of our favorite prehistoric creatures in the ever-increasing list of dinosaur movies, and among the upcoming titles is a film that uses actual legends for its launching point.
When I was young, one of the first dinosaur movies I ever saw was Baby: Secret of the Lost Legend. Drawing from myths and unsubstantiated rumors, the film imagined what would happen if scientists discovered living sauropods in the Congo Basin. Indeed, this part of Africa has been the frequent focus of cryptozoologists and creationists who believe that some sort of swamp-dwelling brontosaur is hiding in the swamps and lakes of the region. There’s not even a single shred of evidence that there are sauropods or other dinosaurs in those wetlands, but that hasn’t stopped naive and self-styled explorers from trying to bring a prehistoric beast back alive.
We can still have a little fun with the idea of living sauropods in the realm of fiction, though. Now, almost 30 years after Baby debuted, The Dinosaur Project is taking a darker spin on the same legend.
According to Empire, The Dinosaur Project is another found-footage horror flick that follows a television crew who ultimately stumble upon dinosaurs that were thought to have disappeared millions of years ago. The movie’s official website doesn’t reveal much – it’s just a fake landing page for the “British Cryptozoological Society” with a plea for any information about the missing expedition – although the film’s trailer offers a few glimpses at the various prehistoric creatures that will thin out the cast. Sadly, though, the dinosaurs and other prehistoric beasts look like stiff plastic toys come to life. This isn’t the awesome dinosaur movie we’ve been waiting for, but another piece of stinky movie cheese.
The Dinosaur Project debuts next month in the UK.
June 28, 2012
Compared to mounted dinosaur skeletons, fossil footprints might seem like mundane objects. They only record one small part of a fantastic creature, and it is harder to envision a whole dinosaur from the ground up than the wrap flesh around a skeletal frame. But we should not forget that dinosaur footprints are fossilized behavior—stone snapshots of an animal’s life. And sometimes, trackways record dramatic moments in dinosaur lives.
In 1938, American Museum of Natural History paleontologist Roland T. Bird traveled to Glen Rose, Texas to investigate rumors of huge dinosaur tracks found in the vicinity of the Paluxy River. Bird found them in abundance, but one site was especially intriguing. Set in 113-million-year-old rock were the footprints of a huge sauropod dinosaur—and it seemed that the long-necked giant was followed. The large, three-toed footprints of a predatory dinosaur, probably the ridge-backed Acrocanthosaurus or a similar theropod, paralleled and eventually converged on the footsteps of the sauropod. And at the point of overlap, the predator seemed to skip a step—a little hop that Bird took to mean that the carnivore had sunk its teeth into the herbivore and was lifted out of its tracks a short distance.
Bird excavated the trackway in 1940. About half of the long trail went to the AMNH and can now be seen stretching out behind the museum’s Apatosaurus mount, despite the fact that Apatosaurus lived millions of years before the tracks were made. The other portion is housed at the Texas Memorial Museum in Austin. Bird’s hypothesis about how the tracks were made has inspired exhibits at other museums, such as the Maryland Science Center and the North Carolina Museum of Natural Sciences. Yet not everyone agrees about what the tracks represent. Do they record an Acrocanthosaurus attack as it happened? Or could the trackway simply be a fortuitous association of tracks from dinosaurs that walked the same ground at different times?
Artist David Thomas and paleontologist James Farlow went back to Bird’s notes and the trackway evidence to reconstruct what might have transpired. The association between the sauropod and theropod tracks seemed too tight to just be coincidence. The predatory dinosaur very closely followed the pathway of the larger herbivore, both moving along a broad left curve. Near the end of the excavated area, both the theropod and sauropod turned abruptly to the right. If the two dinosaurs had passed at different times, then we’d expect that the sauropod or theropod would have continued on in the same trajectory and crossed another set of tracks preserved nearby. Based on the fully reconstructed image, the sauropod and theropod were interacting with each other.
And there’s something else. Just before the enigmatic double-right-footprints made by the theropod, there is a drag mark made by the sauropod’s right hind foot. This might be where the titan was attacked and faltered, or maybe the sauropod threw its weight to avoid being bitten. Frustratingly, we can’t know for sure. And the missing left theropod footprint isn’t a clear sign of an attack, either—all we know is that there’s a missing track right where the animals were in close proximity.
Whether or not the Paluxy River Trackway records a successful Acrocanthosaurus assault is uncertain. But the tight connection between the theropod and sauropod tracks suggests that the carnivore at least stalked the herbivore, and perhaps even took a swipe at it. Specimens like this test our ability to draw brief moments in time from stone. The task is made all the more complicated by the gradual loss of information contained within the rock. While they look sturdy, trackways are actually fragile fossils, and the half of the trackway at the Texas Memorial Museum has significantly deteriorated since it was put on display. The museum is trying to raise a million dollars to properly conserve and house this historically and scientifically significant fossil. If you wish to learn more about their campaign, you can find more information here.
June 18, 2012
On June 23rd, the Royal Ontario Museum is going to open a tribute to some of the largest and strangest dinosaurs ever found, in Ultimate Dinosaurs: Giants From Gondwana. The centerpiece of the celebration is a full-size mount of the huge sauropod Futalognkosaurus—a long-necked, 105-foot titan that was described in 2007. And as part of the lead-up to the exhibit’s debut, the Toronto Star is featuring a time-lapse video of how paleontologists put the dinosaur together. After just a few hours, an 87-million-year-old giant stands again.