EVOLUTIONARY TRENDS IN HORNED DINOSAURS: NEW EVIDENCE FROM ASIAN BASAL NEOCERATOPSIANS

Ceratopsians first appeared in the Late Jurassic of Asia as small bipeds. Retaining this conservative body plan basal neoceratopsians underwent a modest diversification in Asia during the Early Cretaceous, when they also dispersed to Europe and North America. Large bodied and frilled ceratopsids evolved in North America and underwent an explosive diversification during the last 20 million years of the Late Cretaceous.

Autapomorphies for Ceratopsia include the rostral which forms the upper half of a pointed beak. The initial development of the frill as a modest parietosquamosal shelve in basalmost neoceratopsians appears to be linked to the modification of jaw musculature related, in part, to the acquisition of the rostral. Subsequent development of the large, ornamented frill in ceratopsids was probably related to sexual selection which may also have played a factor in the evolution of large body size.

The recent description of new basal neoceratopsians, including Koreaceratops, have allowed for a comprehensive phylogenetic analysis of the Ceratopsia to be conducted to investigate evolutionary trends within integrated skeletal systems. Ceratopsoidea is united by 13 synapomorphies that are closely related to a shift to larger body sizes in ceratopsids. Three unambiguous postcranial synapomorphies of the neural spines and chevrons diagnose Neoceratopsia, with the tall caudal neural spines of Koreaceratops, Montanocertops, Udanoceratops, Protoceratops, and Bagaceratops being secondarily lost in Ceratopsidae. This suggests that elongation of caudal neural spines may have evolved as a functionally integrated module in non-ceratopsid neoceratopsians more derived than Archaeoceratops.

The ratio of the skull to hind limb length appears to be related to the mode of locomotion, and implies that the development of large skulls may have required a shift to a quadrupedal lifestyle. Obligate quadrupedalism occurred gradually in neoceratopsians, developing from bipedal (probably including both Koreaceratops and Cerasinops), to facultative quadrupedal (Graciliceratops, Leptoceratopsidae), and finally to fully quadrupedal (Coronosauria) forms through a progressive increase of body and skull size with a relative elongation in trunk length, and the modification of claws into hoof-shaped unguals.