Name: Fruitadens haagarorum
Etymology: From the Fruita locality in Colorado), and Latin "dens" (tooth); and from the surname Haaga
Distribution: Late Jurassic (early Tithonian) of North America
Type Specimen: Jaw, vertebrae and limb bones of a near-adult individual
Estimated size: 65 - 75 cm body length
It's really not surprising that small (relative to humans) dinosaurs existed: they were a dominant group, spread across the whole globe, persisting for ~200 million years, who filled most ecological niches (habitats with specific resources such as different food types). However, up until 2010, the only small dinosaurs described were theropods. Then along came the smallest known ornithischian, Fruitadens haagarorum.
Remember how I mentioned niches before? Basically, in any given environment, there are different food types to consume. If an animal can eat a variety of food types, it can occupy a variety of niches. The teeth of F. haagarorum, along with other heterodontosaurids, suggest they were omnivorous (eating both plants and animals). It is possible that this omnivory, and resulting occupation of many niches, was the reason for the 100 million year success of the heterodontosaurids.
Butler, R., Galton, P., Porro, L., Chiappe, L., Henderson, D. and Erickson, G., 2010. Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America. Proc. Royal Soc. B 277: 375 - 381.
Butler, R., Porro, L., Galton, P., Chiappe, L., 2012. Anatomy and Cranial Functional Morphology of the Small-Bodied Dinosaur Fruitadens haagarorum from the Upper Jurassic of the USA. PLoS ONE 7(4): e31556. doi:10.1371/journal.pone.0031556.