SVP Dispatch: Dinosaurs and the Proofs of Evolution

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.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.