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Arrhinoceratops and other horned dinosaur skull ontogeny

Ben Creisler

A new paper:

Jordan C. Mallon, Michael J. Ryan and James A. Campbell (2015)
Skull ontogeny in Arrhinoceratops brachyops (Ornithischia:
Ceratopsidae) and other horned dinosaurs.
DOI: 10.1111/zoj.12294

Disentangling ontogenetic from interspecific variation is key to
understanding biodiversity in the fossil record, yet information on
growth in the ceratopsid subfamily Chasmosaurinae is sparse. Here, we
describe the partial skull of a juvenile chasmosaurine, attributed to
Arrhinoceratops brachyops, within the context of more mature specimens
of this species, to better understand the ontogenetic transformations
therein. We show that as A. brachyops matured, the postorbital
horncores became longer and shifted from a posterior to an anterior
inclination, the delta-shaped frill epiossifications became lower and
fused to the underlying frill, and the face became more elongate. In
these respects, A. brachyops closely resembled Triceratops, suggesting
that these ontogenetic changes may have been common to all long-horned
chasmosaurines. However, an event-paired cladistic analysis of
Chasmosaurinae using a standardized matrix of 24 developmental
characters reveals that the relative timing of ontogenetic events in
Arrhinoceratops was more like that of Chasmosaurus, particularly in
the relatively late reduction in scalloping around the frill margins.
Thus, the ontogenetic similarities between Arrhinoceratops and
Triceratops appear to be plesiomorphic, partly related to the
retention of the elongate postorbital horncores, which are primitive
for Ceratopsidae. This study elucidates the otherwise contentious
evolutionary relationships of Arrhinoceratops, and highlights the
importance of ontogenetic data for resolving phylogenies when
morphological data from adults alone are inadequate.