Ontogenetic braincase development in Psittacosaurus lujiatunensis (Dinosauria: Ceratopsia) using micro-computed tomography

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.

The internal cranial anatomy of Champsosaurus (Choristodera: Champsosauridae): Implications for neurosensory function

“…Although the relationship between semicircular canal morphology and ontogeny has received some attention 102,103 , there is not enough information here to comment on whether the variation in canal morphology between CMN 8920 and CMN 8919 is ontogenetic, due to small sample size. Slow moving taxa, such as sloths, have greater variation in canal morphology than most animals due to their slow-moving lifestyle that has lessened selective pressures on the morphology of the semicircular canals 10 .…”

Endocranial anatomy of the ceratopsid dinosaur Triceratops and interpretations of sensory and motor function

“…The ASC is round in general morphology. In particular, the arc of the ASC is relatively low dorsoventrally, differing from tall arcs of both Psittacosaurus (Zhou et al, 2007;Bullar et al, 2019;Napoli et al, 2019) and Protoceratops (Hopson, 1979). The PSC is slightly lower dorsoventrally than the ASC when the LSC is oriented horizontally (Fig.…”

“…They observed various sensorineural anatomical features of Psittacosaurus on the basis of virtual endocasts. Bullar et al (2019) compared the inner ear morphology of three specimens of P. lujiatunensis and discussed that their head posture changes with growth. Napoli et al (2019) described the brain endocast and endosseous labyrinth of P. amitabha.…”

A test of the lateral semicircular canal correlation to head posture, diet and other biological traits in “ungulate” mammals

“…This is backed by the hypothesis that a horizontal orientation of the LSC would mechanically maximize the recording of rotational and linear head movements made in the horizontal plane by placing the sensory hair cells of the semicircular canal and its associated ampulla perpendicular to the horizontal plane [12][13][14][15][16] . The subsequent use of the orientation of the plane of the LSC as a proxy to infer head posture in fossil vertebrates has grown more popular among paleontologists, as it is being applied to dozens of extinct taxa such as archosaurs, including dinosaurs, and synapsids, including mammals 12,[17][18][19][20][21][22][23][24][25][26][27][28][29] . This has raised discussion on some crucial paleobiological questions, such as the evolution of bipedalism in ancient hominin 14,18 and paleodiets.…”

“…The bony labyrinth is one of the first organs to completely ossify in mammals as its adult size and shape are reached at mid-gestation 50,51 . However, the orientation of the LSC seems to show age-related variations in some tetrapod species, including humans, which may impact their head posture 26,52,53 . As such, juveniles were excluded from the dataset.…”