In many of the books about dinosaurs I read as a child, the evolution of horned dinosaurs (ceratopsians) looked pretty straightforward. Early, lanky forms such as Psittacosaurus were succeeded by a miniature precursor of later types—
The new species, named Zhuchengceratops inexpectus, was discovered in a Cretaceous-age bone bed rich in the remains of the large hadrosaur Shantungosaurus and located in China's Shandong Province. Ribs, vertebrae, parts of the lower jaw and portions of the skull represent what we presently know about this animal, and there is enough of it to discern that it belonged to a peculiar group of horned dinosaurs called leptoceratopsids. These dinosaurs were relatively small—many were about six feet in total length—and, while they had short frills, they lacked the impressive horns of the ceratopsid dinosaurs. At one time they were thought to represent a rare and relatively short-lived part of horned dinosaur diversity, but recent discoveries and reanalysis of old data has shown that the leptoceratopsids were a relatively prolific and widespread group.
The evolutionary and environmental context of Zhuchengceratops underscores our changing view of the leptoceratopsids. Though it may have looked primitive compared to the ceratopsid dinosaurs, Zhuchengceratops and its close relatives overlapped in time and space with many of their larger, better-ornamented cousins. (Specifically, the newly described ceratopsid Sinoceratops was found just three miles away from Zhuchengceratops in rock of the same age, meaning that these dinosaurs were probably contemporaries.) And, while it is true that the relationships among the leptoceratopsids are still a little shaky, scientists have found so many forms in both North America and Asia that these animals now appear to have been members of a diverse, long-lived group which coexisted with the major radiation of ceratopsid species. Horned dinosaur evolution was not a matter of one stage giving way to the next, but is better understood as a wildly branching tree in which small, archaic types flourished alongside their well-decorated cousins.
Xu, X., Wang, K., Zhao, X., Sullivan, C., & Chen, S. (2010). A New Leptoceratopsid (Ornithischia: Ceratopsia) from the Upper Cretaceous of Shandong, China and Its Implications for Neoceratopsian Evolution PLoS ONE, 5 (11) DOI: 10.1371/journal.pone.0013835