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Theropod quadrate terminology and function

From: Ben Creisler

Two new preprint papers in the open-access online journal PeerJ:

Christophe Hendrickx, Ricardo Araújo & Octávio Mateus (2014)
The nonavian theropod quadrate I: standardized terminology and
overview of the anatomy, function and ontogeny.
PeerJ PrePrints 2:e379v1
doi: http://dx.doi.org/10.7287/peerj.preprints.379v1

By allowing the articulation of the mandible with the cranium, the
quadrate of diapsids and most other tetrapods plays an important role
morphofunctionally. In Theropoda, its morphology is particularly
complex and varies importantly among different clades of nonavian
theropods so that the quadrate possesses a strong taxonomic potential.
Inconsistencies in the notation and terminology used in discussions of
the theropod quadrate anatomy have been noticed, a number of no less
than height different terms being sometimes given to a same structure.
A standardization list of terms and notation for each quadrate
anatomical entity is here proposed, with the goal of facilitating
future descriptions of this important cranial bone. An overview of the
quadrate function, pneumaticity and ontogeny in nonavian theropods is
also given. The quadrate of the large majority of nonavian theropod is
akinetic and the diagonally oriented sulcus of the mandibular
articulation allowed both rami of the mandible to move laterally when
opening the mouth in many of them. Pneumaticity of the quadrate is
also present in most of tetanuran clades and the pneumatic chamber,
invaded by the quadrate diverticulum of the mandibular arch pneumatic
system, was connected to one or several pneumatopores on the medial,
lateral, posterior, anterior or ventral sides of the quadrate. Absence
of a quadrate foramen in allosauroid embryos and a poor delimitation
of mandibular condyles in both embryonic and juveniles tetanurans
seems to be ontogenetic features of some theropods. Finally, the
numerous morphological differences existing in the quadrates of the
two specimens of Shuvuuia deserti, interpreted by some as juvenile and
adult individuals, are considered as ontogenetic, taphonomic, and
perhaps also taxonomic variations.


Christophe Hendrickx, Ricardo Araújo & Octávio Mateus (2014)
The nonavian theropod quadrate II: systematic usefulness, major trends
and cladistic and phylogenetic morphometrics analyses.
PeerJ PrePrints 2:e380v1
doi: http://dx.doi.org/10.7287/peerj.preprints.380v1

The skull-bone quadrate in nonavian theropods is very diverse
morphologically alongside the disparity of the group as a whole.
However this disparity has been underestimated for taxonomic purposes.
In order to evaluate the phylogenetic potential and investigate the
evolutionary transformations of the quadrate, we conducted a
Catalano-Goloboff phylogenetic morphometric analysis as well as a
cladistic analysis using 98 discrete quadrate related characters. The
cladistic analysis provides a fully resolved tree mirroring to some
degree the classification of nonavian theropods. The quadrate
morphology by its own provides a wealth of data with strong
phylogenetic signal and allows inference of major trends in the
evolution of this bone. Important synapomorphies include: for
Abelisauroidea, a lateral ramus extending to the ectocondyle; for
Tetanurae, the absence of the lateral process; for Spinosauridae, a
medial curvature of the ventral part of the pterygoid ramus occurring
just above the mandibular articulation; for Avetheropoda, an anterior
margin of the pterygoid flange formed by a roughly parabolic margin;
and for Tyrannosauroidea, a semi-oval pterygoid flange shape in medial
view. The Catalano-Goloboff phylogenetic morphometric analysis reveals
two main morphotypes of the mandibular articulation of the quadrate
linked to function. The first morphotype, characterized by an
anteroposteriorly broad mandibular articulation with two
ovoid/subcircular condyles roughly subequal in size, is found in
Ceratosauria, Tyrannosauroidea and Oviraptorosauria. This morphotype
allows a very weak displacement of the mandible laterally. The second
morphotype is characterized by an elongate and anteroposteriorly
narrow mandibular articulation and a long and parabolic/sigmoid
ectocondyle. Present in Megalosauroidea, Carcharodontosauridae and
Dromaeosauridae, this morphotype permits the lower jaw rami to be
displaced laterally when the mouth opened.