Plant-eating dinosaurs dominated the Jurassic and Cretaceous eras because they were able to make effective use of the new plant species that appeared at the time. But a new study of their secret weapon, powerful jaws, shows that their evolution was not driven by changes in the available food.
The carnivores might get the glory, but vegetarian dinosaurs outnumbered their predators hundreds to one, just as today it takes many zebras to support a single lion. It is only because the plant eaters, ornithopods in particular, were so successful at exploiting the conifers and newly emerged flowering plants that such enormous beasts could survive in large numbers. So the question of how they did this holds considerable interest.
Eddy Strickson, a Master's student at the University of Bristol said in a statement: "The plant-eating ornithopods showed four evolutionary bursts; one in the middle of the Jurassic, and the other three in a cluster around 80 million years ago in the Late Cretaceous. This was down to innovation in their jaws and improved efficiency."
Strickson is first author of a Scientific Reports paper on the changes in ornithopod jaws. Intriguingly, he found, “These evolutionary bursts do not correspond to times of plant diversification, including the radiation of the flowering plants.” It seems the ornithopods evolved their new teeth of their own accord, rather than responding to changes in the plant food on offer.
The first flowering plants (angiosperms) appeared 160 million years ago, but they didn't become widespread until well into the Cretaceous. Their spread was accompanied by increased diversity in many categories of terrestrial animals known as the Cretaceous Terrestrial Revolution (KTR). It would therefore not have been surprising if the ornithopods also benefited from so much potential new food, in turn providing more meals for predatory dinosaurs.
However, the paper notes, “The timings of these diversification shifts are telling.... The first evolutionary acceleration long predated the KTR, and the final three substantially post-dated the radiation of angiosperms.” Whatever sparked these bursts of evolution, it wasn't changes in the food supply.
Moreover, it seems angiosperms were not the preferred food of the highly successful hadrosaurids, the “duckbilled” dinosaurs who look like they were designed by George Lucas. Co-author Dr Albert Prieto-Marquez pointed out, “Their tooth wear patterns, and especially close study of their coprolites – that's fossil poops – shows they were conifer specialists, designed to crush and digest the oily, tough needles and cones."
The hadrosaur family tree. Debivort via Wikimedia Commons. CC-BY-3.0
The first ornithopods were relatively sparse in the tooth department. However, the development of smaller, but far more numerous, teeth was associated with the hadrosaurid expansion around 90 million years ago. This produced 40 percent of known ornithopod species in a short (by dinosaur standards) period of time.
All these extra teeth may have contributed to the fact, confirmed by Strickson and Prieto-Marquez, that the crested hadrosaurs were one of the few groups of dinosaurs not already in decline before the asteroid wiped them out, instead feasting on the vast conifer forests that dominated North America at the time.