Abstract
Ontogenetic sequences are relatively rare among dinosaurs, with Ceratopsia being one of the better represented clades, and especially among geologically earlier forms, such as Psittacosaurus. Psittacosaurus is a small, bipedal basal ceratopsian abundant in the Lower Cretaceous deposits of Asia, whose cranial and endocranial morphology has been well studied, but only cursory details have been published on the bones surrounding the brain. Using reconstructions created from micro-computed tomography scans of well-preserved skulls from the Barremian–Aptian Yixian Formation, China, we document morphological changes in the braincase of Psittacosaurus lujiatunensis through three growth stages, hatchling, juvenile, and adult, thus providing the first detailed study of ceratopsian braincase morphology through ontogeny. Notable ontogenetic changes in the braincase of P. lujiatunensis include a dramatic relative reduction in size of the supraoccipital, an increase in the lateral expansion of the paroccipital processes and a decrease in the angle between the lateral semicircular canal and the palatal plane. These ontogenetic morphological changes in the braincase relate to expansion of the cranium and brain through growth, as well as reflecting the switch from quadrupedal juveniles to bipedal adults as documented in the changing orientation of the horizontal semicircular canal through ontogeny. Recognition of these patterns in a basal ceratopsian has implications for understanding key events in later ceratopsian evolution, such as the development of the parieto-squamosal frill in derived neoceratopsians.
Highlights
Psittacosaurus is an Early Cretaceous genus of Ceratopsia, a diverse and geographically widespread suborder of ornithischian dinosaurs (Dodson, Forster & Sampson, 2004; Chinnery-Allgeier & Kirkland, 2010; Dodson, 2013)
Chaoyangsauridae is a family of basal ceratopsian dinosaurs, that is either sister to Neoceratopsia (Xu et al, 2002; Han et al, 2017) or its most basal member (Sereno, 2000; You & Dodson, 2003)
The basioccipital contacts the basisphenoid anteriorly and the exoccipitals laterodorsally (Fig. 3D). It measures 5.4 mm anteroposteriorly by 5.8 mm transversely across its widest point. It forms the posterior floor of the braincase and contributes to the majority of the basal tubera (Fig. 2A: bo) with the basisphenoid contributing to the anteroventral-most regions
Summary
Psittacosaurus is an Early Cretaceous genus of Ceratopsia, a diverse and geographically widespread suborder of ornithischian dinosaurs (Dodson, Forster & Sampson, 2004; Chinnery-Allgeier & Kirkland, 2010; Dodson, 2013). Their position within ceratopsian phylogeny has been debated, from being basalmost Han et al, 2017), to more derived than Yinlong and chaoyangsaurids (Xu et al, 2006; He et al, 2015; Zheng, Jin & Xu, 2015). Chaoyangsauridae is a family of basal ceratopsian dinosaurs, that is either sister to Neoceratopsia (Xu et al, 2002; Han et al, 2017) or its most basal member (Sereno, 2000; You & Dodson, 2003)
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