Lead author Richard Butler describes this paper as "the result of a nearly 30 year long project," building on a substantial body of work by coauthor Peter Galton. Heterodontosaurids were bipedal ornithischians, notable for their varied dentition. Many have pronounced fangs, or caniniforms, in addition to leaf-shaped teeth which would have been good for munching vegetation. And one of them, Tianyulong, was preserved well enough to reveal that it bore long, hollow quill-like filaments along its back.
Fruitadens restored by Nobu Tamura, featuring quills like those of Tianyulong.
Understanding the evolutionary trends of these primitive ornithischians is important for what it can reveal about the common ancestor of all dinosaurs. Unfortunately, there are few windows into heterodontosaurid evolution, with the most productive strata dating to the early Jurassic of South Africa, when Heterodontosaurus, Abrictosaurus, and Lycorhinus lived. The Middle Jurassic of Europe has offered a plethora of teeth which hint at substantial populations of heterodontosaurids, but so far has not coughed up any significant remains to help fill in the details of the heterodontosaurid story. But what we have is enough to build a rough sketch of how the clade evolved over the 55 million year stretch that it existed.
Fruitadens was likely an omnivore, eating what plants, insects, and other invertebrates it could find. Like closely related Tianyulong, it was smaller and less specialized for herbivory than the earlier Heterodontosaurus, which was made to process tough plant material. Butler notes that "it appears that Late Jurassic-Early Cretaceous heterodontosaurids were smaller than most Early Jurassic members of the clade, and displayed less sophisticated skull and dental morphologies." They were becoming generalists. As the clade spread across Pangaea, which was fragmenting by the Late Jurassic, evolutionary pressures - perhaps competition from the sauropodomorphs - were pushing the heterodontosaurids into a more opportunistic lifestyle. Sort of a dinosaurian version of a raccoon, scratching out a living any way they could.
More:
Andy Farke's take on PLoS Blogs
A note from lead author Richard Butler
Brian Switek at Dinosaur Tracking
