Aquilops americanus (Farke et al. 2018; Early Cretaceous) was originally considered a small, early neoceratopsian based on a partical skull. Here it nests more basally prior to Yinlong and Leptoceratops. The rostral bone is hooked and the antorbital fenestra is surrounded by a fossa. A small parietal shield appears for the first time.

Yinlong downsi (Xu et al. 2006) Late Jurassic ~150 mya, ~1.2 m in length was considered the earliest known member of the ceratopsian clade that ultimately produced Triceratops. Derived from a sister to Hexinlusaurus, Yinlong nested at the base of the Ceratopsia, including Triceratops.

Distinct from Hexlinusaurus, the skull of Yinlong retained three large fangs on the premaxilla, as in Daemonosaurus. A rostral bone, found only in ceratopsians, tipped the rostrum. The postorbital was elongated. The postcranial skull was elongated, increasing the size of the temporal fenestra. This represents an increase in jaw muscle size. The jugals and squamosals flared slightly.

Both Yinlong and Heterodontosaurus had an older common ancestor, probably in the Late Triassic. Yinlong did not have the extended premaxillary posterior process, reduced fangs, double posterior process of the jugal and taller quadratojugal of Heterodontosaurus. Heterodontosaurus did not have the round antorbital fossa shared by Daemonosaurus and Yinlong.

Farke AA, Maxwell WD, Cifelli RL and Wedel MJ 2014. A Ceratopsian Dinosaur from the Lower Cretaceous of Western North America, and the Biogeography of Neoceratopsia. PLoS ONE. 9 (12): e112055. doi:10.1371/journal.pone.0112055

Osborn HF 1923. Two Lower Cretaceous dinosaurs from Mongolia. American Museum Novitates 95: 1–10.

Schmidt H 1969. Stenopelix valdensis H. v. Meyer, der kleine Dinosaurier des norddeutschen Wealden. Palaeontologische Zeitschrift 43(3/4):194-198.

Sereno PC 2000. The fossil record, systematics and evolution of pachycephalosaurs and ceraptosians from Asia. In: M.J. Benton, M.A. Shiskin, D.M. Unwin, and E.N. Kurochkin (eds.), The Age of Dinosaurs in Russia and Mongolia, 480–516. Cambridge University Press, Cambridge.

Sues H-D and Galton PM 1982. The systematic position of Stenopelix valdensis Reptilia: Ornithischia) from the Wealden of north-western Germany. Palaeontographica Abteilung A 178(4-6): 183-190.

Xu X, Forster CA, Clark J M and Mo J 2006. A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences. First Cite Early Online Publishing. online pdf

You H−L, Tanou K and Dodson P 2008. New data on cranial anatomy of the ceratopsian dinosaur Psittacosaurus major. Acta Palaeontologica Polonica 53 (2): 183–196.

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wiki/Yinlong
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