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Alioramus (tyrannosaurid) and evolution of cranial pneumaticity in theropods

From: Ben Creisler

A new paper available on the BioOne site. It is not yet posted in open
access on the AMNH site.

Maria Eugenia Leone Gold, Stephen L. Brusatte and Mark A. Norell (2013)
The Cranial Pneumatic Sinuses of the Tyrannosaurid Alioramus
(Dinosauria: Theropoda) and the Evolution of Cranial Pneumaticity in
Theropod Dinosaurs.
American Museum Novitates Number 3790 : 1-46
doi: http://dx.doi.org/10.1206/3790.1

[AMNH open access link:

Archosaurs and mammals exhibit skeletal pneumaticity, where bone is
infilled by air-filled soft tissues. Some theropod dinosaurs possess
extensively pneumatic skulls in which many of the individual bones are
hollowed out by diverticula of three main cranial sinus systems: the
paranasal, suborbital, and tympanic sinuses. Computed tomography (CT
scanning) permits detailed study of the internal morphology of cranial
sinuses. But only a few theropod specimens have yet been subjected to
this type of analysis. We present CT scans of the remarkably preserved
and disarticulated skull bones of the long-snouted tyrannosaurid
theropod Alioramus. These scans indicate that Alioramus has extensive
cranial pneumaticity, with pneumatic sinuses invading the maxilla,
lacrimal, jugal, squamosal, quadrate, palatine, ectopterygoid, and
surangular. Pneumaticity is not present, however, in the nasal,
postorbital, quadratojugal, pterygoid, or angular. Comparisons between
Alioramus and other theropods (most importantly the closely related
Tyrannosaurus) show that the cranial sinuses of Alioramus are modified
to fill the long-snouted skull of this taxon, and that Alioramus has
an extreme degree of cranial pneumaticity compared to other theropods,
which may be the result of the juvenile status of the specimen, a
difference in feeding style between Alioramus and other theropods, or
passive processes. Based on these comparisons, we provide a revised
terminology of cranial pneumatic structures and review the
distribution, variation, and evolution of cranial pneumaticity within
theropod dinosaurs. This review illustrates that most theropods
possess a common “groundplan” in which the maxilla and lacrimal are
pneumatized, and that various theropods modify this groundplan by
pneumatizing numerous other bones of the skull. Tyrannosaurids are
very pneumatic compared to other theropods, particularly in the
development of extensive ectopterygoid, quadrate, and palatine
sinuses, as well as a pneumatic invasion into the surangular.
Tyrannosauroids seem to retain many cranial sinuses, such as the jugal
and nasal recesses, which are primitive for coelurosaurs but lost or
apomorphically modified in taxa more closely related to birds.