Reconstructing Dinosaur Duels

Of all dinosaurian visions ever committed to canvas, few are as famous as Charles R. Knight’s depiction of a duel between Tyrannosaurus and Triceratops. In the mural, hanging on the wall of Chicago’s Field Museum, the titans stand poised to lash out at each other in a flurry of horns and teeth. And this is often how we think of dinosaur combat – ambushes and stand-downs between predator and prey. Often overlooked are the fights dinosaurs had among their own species.

In the ever-growing ranks of known dinosaurs, members of a major group called the Ornithischia are often cast as relatively peaceful herbivores. These are the tusk-toothed heterodontosaurids, spike-thumbed iguanodonts, shovel-beaked hadrosaurs, spike-tailed stegosaurs, heavily-armored ankylosaurs, dome-headed pachycephalosaurs, and multi-horned ceratopsids. But, contrary to their timid image, such dinosaurs frequently fought members of their own species. Raymond M. Alf Museum of Paleontology scientist Andrew Farke reviews the evidence in a new Journal of Zoology paper.

Farke lays out three lines of inference that clue paleontologists in to dinosaur combat. First, and most flimsy, is analogy to modern animals. Since antelope and other bovids sometimes lock horns in tussles, Farke points out, paleontologists thought that ceratopsid dinosaurs such as Triceratops did the same. That’s a fair hypothesis in broad strokes, but it quickly breaks down in the finer fossil points.

Bovids have sinuses in their skulls. Researchers thought that the spaces acted as shock absorbers for cranial clashes, and therefore similar structures in dinosaur skulls were believed to perform the same function. As Farke found out through some of his own research, however, the sinuses of bovids have little to do with combat, and so dinosaur sinuses probably were not shock-absorbers, either. Not to mention that dinosaur horns and bovid horns are different in shape, size, orientation, and number. A Styracosaurus is not a sable antelope.

Biomechanical studies get a little closer to what was actually possible for dinosaur fights. By simulating head-butting, spike-swinging, and other combat behaviors, paleontologists can estimate whether the animals were actually capable of inflicting, and surviving, the trauma seen in movies and richly-illustrated books. As Farke is quick to note, though, such analyses “are not proof that the behavior actually happened.” There may be not better case of this distinction than the thick-skulled pachycephalosaurs.

The bipdeal pachycephalosaurs are immediately recognizable for their tonsure-style skulls – bald, often-rounded tops with a ring of spikes of varying sizes around the back. And despite the fact that the anatomy doesn’t match up that well, the late paleontologist Edwin Colbert thought that such skulls had evolved for ramming contests of the sort bighorn sheep engage in. Researchers have since tried to model how pachycephalosaurs such as Stegoceras and Pachycephalosaurus would have coped from head-on collisions, but without much agreement. Where some researchers have concluded that the reinforced skulls of these dinosaurs were well-suited to butting, other scientists have proposed that such cracks on the noggin would have been extremely traumatic, if not fatal.

For decades, there was no independent line of evidence for dome-smacking dinosaurs. The closest paleontologists could get was modeling the fights. Then, in 2012, paleontologists Joseph Peterson and Christopher Vittore published a study focused on lesions pocking a Pachycephalosaurus skull. Along with further instances Peterson published with other authors the following year, such injuries could have been caused by head-butting. And this is Farke’s third category of evidence – paleopathology.

Was head-butting the cause of the skull damage? That’s still a matter of debate among experts. It’s hard enough to diagnose the cause of a modern injury or disease, much less what created a pathology in an animal that lived over 66 million years ago. And what was once perceived as an injury could turn out to be caused by something else. The odd holes in the frill of a horned dinosaur named Nedoceratops – possibly a Triceratopspossibly a Triceratops – were once thought to be damage caused by a rival, but are now interpreted as possible signs of an as-yet-unknown disease, a case of individual variation, or a sign of transformation through growth. No one knows for sure. Nevertheless, pathologies are signs of real events in dinosaurs lives. When successfully decoded, injuries are signs of behaviors from feeding to fights.

While the holes in the frill of Nedoceratops probably weren’t punctures created by the horns of a rival, for example, Farke’s research with Ewan Wolff and Darren Tanke has made a strong case that lesions along the frills of multiple Triceratops skulls are indicators that these dinosaurs locked horns in predictable patterns. Similar cases have been harder to find, but some densely-armored ankylosaurs are a good place to look. Through extensive research on ankylosaurid tail clubs, paleontologist Victoria Arbour has suggested that these mace-like weapons were better suited to fights between dinosaurs of the same species than defense against carnivores. Injuries to the hips and ribs of ankylosaurids could be signs of tail-whacking contests, although clear evidence of battering battles hasn’t been found just yet.

Cases for dinosaur combat are strongest when all three lines of evidence come together, Farke notes. And beyond continuing to scour the fossil record, Farke urges fellow researchers to gain a better understanding of modern animals. If researchers can identify adaptations for combat in modern animals, as well as patterns of pathology that result from fights, then paleontologists will gain a more focused frame of reference for what to look for among dinosaurs. And even though there’s on ongoing debate about the evolution and function of “bizarre structures” in dinosaurs, paleontologists can still approach how dinosaurs tussled and the range of possible behaviors for flashy pieces of anatomy.

We may never really know whether or not Iguanodon used those thumb spikes for defense or why about ten percent of Stegosaurus spikes were broken during their owner’s lifetime, but Farke’s paper boils down to a simple conclusion – dinosaurs fought, and we’re only just beginning to understand how they pummeled each other during prehistory.

Reference:

Farke, A. 2014. Evaluating combat in ornithischian dinosaurs. Journal of Zoology. doi: 10.1111/jzo.12111