November 2, 2011
What can dinosaurs teach us about evolution? Charles Darwin mostly ignored them during his career, and evolutionary patterns are often easier to study in creatures that left more numerous fossils, such as trilobites and the tiny, armored plankton called foraminiferans. Yet, as paleontologist Jack Horner explained during a lecture at the 71st annual meeting of the Society of Vertebrate Paleontology last night, what we have come to know about dinosaurs can illustrate big-picture evolutionary facts.
Despite the fact that Horner was addressing an audience of scientifically minded peers, his talk was very simple. I wouldn’t be surprised if it became a regular lecture on Horner’s speaking circuit to schools and public venues. There were no technical graphs of data points or tables of measured variables. Instead, Horner began with the nuts and bolts of how to find a dinosaur in the Montana badlands. Many people have the impression that paleontologists just walk out into the badlands and dig holes, but as Horner pointed out, simply digging random holes won’t help you find anything. Dinosaurs are gifts of erosion—we find dinosaurs when they are already coming out of the ground. From there, Horner explained, he typically tasks a cadre of graduate students with the back-breaking parts of the excavation and soon whatever there is of the dinosaur skeleton becomes exposed.
Once those bones are out of the ground and cleaned up, all the fun technical nitpicking can start. Horner used dinosaur color as an example. Although I was disappointed that he didn’t mention our recently gained ability to detect the colors of some dinosaurs from fossil feathers, Horner pointed out that we don’t really know anything for sure about the color patterns of most dinosaurs. Horner also mentioned his own work on some evolutionary patterns among Cretaceous dinosaurs in the Two Medicine Formation, specifically whether the horned dinosaur Rubeosaurus was gradually modified into Pachyrhinosaurus in a straight line of descent through several other transitional types within the geologic formation or whether the different dinosaurs in question represent a branching evolutionary pattern. “We paleontologists love to argue about this,” he said, and pointed out that the assembled group had come to the conference to argue, after all. But, Horner quickly added, we don’t argue about the fact of evolution. We can go back and forth indefinitely about the minutiae of paleobiology and the patterns of evolutionary change, but vertebrate paleontologists agree that evolution is a fact.
So what do dinosaurs have to do with the fact of evolution? Horner outlined five different proofs of evolution: three proofs that Darwin cited, a “test” proof, and what Horner called the ultimate proof. The first on the list was simply descent with modification. Horner cited the many strange breeds of dogs and chickens as an analog for how organisms can become drastically modified over the course of history. Humans specifically selected for those changes in the domesticated animals, but as Darwin illustrated in On the Origin of Species and other works, the changes that dogs, chickens and other animals have undergone underscores the fact that the same thing is happening due to entirely natural causes every second and every day. To greater or lesser extents, lineages of organisms change over time, and the fossil record demonstrates this beautifully.
Next on the list were rudimentary features: structures that once served a particular function but became vestigial organs that don’t carry out that same function anymore. (Keep in mind, though, that “vestigial” does not mean “useless.”) Horner cited the modified wings of flightless birds and the remnants of hind limbs in whales as modern day examples, and identified the small forelimbs of Tyrannosaurus as another. Since the time the tyrant dinosaur was discovered, paleontologists have been asking, “What did it use those arms for?” Horner concluded that Tyrannosaurus probably didn’t do more than scratch its belly after a big meal with them. That point is debatable, but we do know that tyrannosaur forelimbs did become greatly reduced in size during the evolutionary history of their lineage. Horner’s hypothetical “chickenosaurus” even made a cameo here. Tweaks in the genetics and development of chickens can cause the reappearance of long-lost traits, such as teeth, and by carrying out these experiments Horner hopes to understand which genes and developmental quirks were key in the evolution of birds from non-avian dinosaurs.
In a phrasing that sounded appropriately Victorian, Horner then moved on to evolutionary proof from the “geological succession of organic beings.” Simply put, we find fossils in layers, in successions of strata that together span hundreds of millions of years. Fossils are not all together in one big clump (as would be expected if the entire fossil record were attributable to the biblical flood as many young earth creationists claim). You’re not going to find a prehistoric horse in the 150-million-year-old Jurassic limestone quarries of Germany, and you’re certainly not going to find a dinosaur in the 505-million-year-old rock of the Burgess Shale. But Horner said that he encourages creationists who want to believe in alternate histories to go looking for the out-of-place fossils they think they’re going to find. “I encourage people who don’t believe in evolution to look for horses in Jurassic Solenhofen limestone,” Horner said, especially since those searches may be much more useful in turning up new specimens of the feathered dinosaur and archaic bird Archaeopteryx.
Horner covered his last two points very quickly. The “test proof” for evolution, he proposed, comes through testing genetic relationships. We don’t yet have genetic material from Mesozoic dinosaurs, and we may never have it, so paleontologists will have to continue to rely on anatomy as they strive to sort out the dinosaur family tree. But the ultimate proof has nothing to do with the animals themselves. The ultimate proof of evolution, Horner quipped, is “ego.” Scientists are constantly arguing with each out about the patterns and processes of evolution, and scientists love to disprove ideas. Anyone who managed to show, beyond a shadow of a doubt, that evolution doesn’t happen would be the most famous scientist of all time, yet no one has been able to do this. Despite the best efforts of scientists to disprove ideas and their penchant for arguing over the nature of nature, the evidence for the fact of evolution keeps getting stronger and stronger.
October 15, 2010
The 70th annual Society of Vertebrate Paleontology meeting ended on Wednesday, but before returning to more regular coverage of all things dinosaur I wanted to share a few snapshots from the meeting’s welcome reception in Pittsburgh’s famous Carnegie Museum of Natural History.
For more on SVP, see these posts:
October 14, 2010
One of the biggest dinosaur stories of 2009 was the discovery of a pint-sized tyrant called Raptorex. Described by a team of paleontologists led by Paul Sereno and dated to about 126 million years ago, the dinosaur showed that many definitive tyrannosaur characteristics—such a puny forearms—evolved when the predators were still small. But a story published in Nature‘s news section this week highlights some of the uncertainty about the specimen.
Despite becoming something of an instant dinosaurian celebrity, there have been two aspects of Raptorex that have caused paleontologists some degree of unease. The first is that it looks like a juvenile form of later, bigger tyrannosaurs, particularly the 70-million-or-so-year-old Tarbosaurus. In fact, this was how the fossil was unofficially diagnosed when it was purchased—more on that in a moment—although Sereno and co-authors cite the fusion of the sutures on the skull of the animal as an indication that it was a young adult animal. (Comparison with complete, juvenile Tarbosaurus skeletons could also help resolve this issue.) Likewise, it would be expected that juveniles of later tyrannosaurs would be similar in form to earlier species—such as Raptorex—with definitive, advanced tyrannosaurs traits only appearing much later during the growth of later species. If juvenile Tarbosaurus roughly looked like the adult stage of their ancestors, in other words, then it would be easy to confuse the two when viewed outside of their geologic context.
As with the debate over the suggestion that Torosaurus was the adult form of Triceratops, however, not all paleontologists agree that Raptorex is really the juvenile form of another dinosaur. Both cases are part of a larger effort to find out how dinosaurs changed as they grew and what this might mean for the identification of new species. As for Raptorex, though, anatomy alone can’t solve the problem, especially since the most important issue yet to be resolved involves the dinosaur’s geological age.
Rather than being found an excavated by scientists, the dinosaur is said to have been collected in the vicinity of Liaoning Province, China, by amateurs. After being dug up, it was later sold to a private collector who then contacted Sereno after having other scientists appraise the specimen. Frustratingly, whoever uncovered the fossil did not collect data about the place where the dinosaur was found, and most of what we know about geological context of the dinosaur comes from the rock which still clung to parts of its skeleton.
In addition to the type of rock it was found in, fossil shells and fish bones would appear to place Raptorex at about 126 million years ago in the Yixian Formation. Given that fish bones and shells of the kind found alongside Raptorex are seen in many fossil localities, however, more rigorous geological testing will be needed to determine where it came from and how old it was. Nailing down a date and locality for Raptorex is important. If Raptorex really is 126 million years old, then it could not be a juvenile of a known, giant tyrannosaur such as Tarbosaurus since it would have preceded it by about 50 million years. If Raptorex turns out to be the same geologic age as Tarbosaurus, however, then paleontologists will have to reexamine the skeleton in detail to determine whether it could be a juvenile form of a larger dinosaur.
These problems with Raptorex have been known to paleontologists since the time of its description, but the Nature News story brought it to the forefront. According to the report, Peter Larson and Jørn Hurum will be publishing a critical assessment of Raptorex which will identify the dinosaur as a juvenile Tarbosaurus. When and where that paper will be published is unknown, and there was no presentation of poster about the topic at the 70th annual SVP meeting.
Since this story broke during SVP, however, a few scientists did acknowledge the debate over Raptorex. In some of the tyrannosaur presentations given on Wednesday paleontologists pointed out that Raptorex was found to be distinct from Tarbosaurus in their independent analyses of tyrannosaur relationships, and a presentation about testing tyrannosaur growth by paleontologist Thomas Carr will likely provide a template for other scientists to test whether certain tyrannosaurs are juveniles of other forms.
In general, though, conference attendees I spoke to were frustrated by the Nature news coverage of the event—since no formal critique of Raptorex was published or presented, there was nothing new to talk about outside issues already known to exist. The ongoing discussion over Torosaurus and Triceratops seemed to be a more prominent topic at this year’s conference, and the scientific debate over Raptorex awaits the publication of more data. Even when Hurum and Larson publish their paper, however, it will be unlikely to definitively close the case on Raptorex. Determining the true identity of this dinosaur will require multiple lines of evidence—from geology to bone histology—and this discussion will likely drag out through the literature for some time to come.
For more SVP coverage, see these posts:
Sereno, P., Tan, L., Brusatte, S., Kriegstein, H., Zhao, X., & Cloward, K. (2009). Tyrannosaurid Skeletal Design First Evolved at Small Body Size Science, 326 (5951), 418-422 DOI: 10.1126/science.1177428
October 13, 2010
Paleontologists are constantly reminding themselves of the incompleteness of the fossil record. What has been preserved is only a small fraction of all the organisms and environments that have ever existed. This makes detecting evolutionary patterns a bit of a challenge. In a presentation given at this year’s Society of Vertebrate Paleontology conference, Smithsonian paleontologist Matt Carrano dug into the long-standing question of whether changes in sea level triggered changes in dinosaur diversity.
Over the past few decades, paleontologists have produced a number of graphs depicting dinosaur diversity through time. They show a general trend toward increasing diversity from the Late Triassic through the end of the Cretaceous, but with a few fluctuations in between. The rise and the fall of the seas has been proposed as one of the drivers of these changes. Perhaps, it has been hypothesized, high sea levels might have favored dinosaur diversity by fragmenting some terrestrial habitats or isolating one area from another while simultaneously creating more environments where dinosaurs might be preserved. Then again, it has also been suggested that dinosaur diversity might go up when sea levels are low since there would be a larger land area. In order to detect whether any such trends existed, the scientists looked at the occurrence of about 749 dinosaur species through time and space, noting where paleontologists have gone looking for their bones, as well.
What the Carrano and his colleagues found was that the fluctuations in sea level did not influence dinosaur diversity as we know it today. Our perspective of dinosaur diversity is significantly shaped by where paleontologists have gone looking for fossils, the amount of effort expended there, and also by places that have yet to be extensively studied. Dinosaurs might be more plentiful and easier to find in Cretaceous rocks than Triassic ones, for example, which would account for why dinosaur diversity differs between the two time periods. Any scientific work proposing to look at dinosaur diversity has to take these sampling biases into account.
This is not to say that sea level change did not or could not have influence dinosaur diversity, though. Rising sea levels could have created island chains and other geographical pockets that could have driven dinosaur speciation, or low sea levels might have allowed dinosaur species to range more widely. (We know, for example, that the Western Interior Seaway caused Cretaceous dinosaurs to evolve in different ways in the eastern and western parts of North America.) Detecting these signals from the fossil record, however, will require in-depth sampling and a recognition of the way in which our search for dinosaurs skews the picture of their diversity. As stated by the authors of the paper that was the basis for the SVP presentation: “Considerable future work is required to establish how sampling biases may affect proposed long-term diversity trends and mass extinction events in the terrestrial realm.” If paleontologists want to get at the big picture of dinosaur diversity, they need to look at these biases and get digging at places which are still poorly known.
Butler, R., Benson, R., Carrano, M., Mannion, P., & Upchurch, P. (2010). Sea level, dinosaur diversity and sampling biases: investigating the ‘common cause’ hypothesis in the terrestrial realm Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.1754
October 12, 2010
The first day of the 70th annual Society of Vertebrate Paleontology meeting was chock-full of dinosaur talks. Fans of ornithischian dinosaurs—the hadrosaurs, ankylosaurs, stegosaurs, pachycehpalosaurs, horned dinosaurs and their kin—had a lot to cheer about. There is a flood of new species, and new evolutionary comparisons are refining the relationships of some familiar species and, in some cases, are suggesting that there is much left to be discovered. Two researchers agreed to let me give you a sneak peek at research that is changing our understanding of dinosaur diversity and evolution.
From documentaries to technical papers, the armored dinosaur Euoplocephalus has often been taken as the quintessential ankylosaur. It seemed to occupy a long range of time and be represented by a wide array of skeletal material. Things are not as clean and neat as they seem. Just last year University of Alberta grad student Victoria Arbour and two others showed that some of the bones scientists had been calling Euoplocephalus really belonged to the distinct genus Dyoplosaurus, which had been named in 1924. This was not the only ankylosaur that was hiding within Euoplocephalus. At least one, and possibly two, other ankylosaurs have probably been mistakenly lumped into the genus. Arbour is continuing her efforts to tease apart the taxonomic mess in the hope that we will be able to get a clearer picture of ankylosaur diversity at the end of the Cretaceous in North America.
The year 2010 might as well be known as the “Year of the Ceratopsians.” From the Torosaurus = Triceratops debate to peculiar ceratopsian forms found in unexpected places, our understanding of these dinosaurs is rapidly changing. Paleontologist Andy Farke and colleagues will soon be adding another taxa to the mix. As he introduced it to colleagues Sunday morning, the new species looks like “the love child of Centrosaurus and Styracosaurus.“ The only thing more bizarre than its looks was the fact that the specimen sat virtually unnoticed on a museum shelf for about a century. Nor was it the only new ceratopsian introduced during the first two days of the conference, and by present indications there are still many new species waiting to be found.