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Ceratopsid dinosaurs squamosal shape evolution



From: Ben Creisler
bcreisler@gmail.com

In the new JVP:


Leonardo Maiorino, Andrew A. Farke, Paolo Piras, Michael J. Ryan,
Kevin M. Terris & Tassos Kotsakis (2013)
The evolution of squamosal shape in ceratopsid dinosaurs (Dinosauria,
Ornithischia).
Journal of Vertebrate Paleontology 33 (6):  1385-1393
DOI: 10.1080/02724634.2013.779922
http://www.tandfonline.com/doi/full/10.1080/02724634.2013.779922#.UoJN6fk_sbk

Ceratopsidae represents one of the last and best-known radiations of
non-avian dinosaurs. Interspecific variation is well documented
qualitatively with linear measurements, but little has been done to
quantify shape differences in the frill that may indicate functional
or evolutionary signals. In order to investigate shape change in the
squamosal across Chasmosaurinae and Centrosaurinae, we applied
geometric morphometrics to the outline of the squamosal for 155
specimens representing 27 ceratopsid species and Protoceratops spp. A
Mantel test suggests that the shape of ceratopsid squamosals is highly
constrained by phylogeny. MANOVA, ANOVA, and their phylogenetic
versions suggest that an evolutionarily significant allometric signal
exists between the two clades, but not within clades. Principal
component analysis indicates that centrosaurines have a uniform
squamosal shape, with the exceptions of Spinops and Diabloceratops.
Even when accounting for phylogeny, the relationship between squamosal
shape and size is significant. Mapping shape onto the phylogeny, we
estimated ancestral shapes at nodes. The transition from the
non-ceratopsid to ceratopsid condition is characterized by a squamosal
with a wider angle between the infratemporal process and the
caudoventral margin, and a more dorsoventrally elongated and caudally
expanded blade. From root to tips, centrosaurine squamosals were found
to be conservative, but exhibit a slight dorsoventral expansion and a
narrow angle between the infratemporal process and the caudoventral
margin in more derived taxa. Chasmosaurines, compared with
centrosaurines, show a derived morphology, with a trend towards a
blade that is strongly expanded dorsoventrally and with a narrower
angle between the infratemporal process and the caudoventral margin.