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Forearm bone mobility in dromaeosaurs, alligators + alligator middle-ear air-sinus system

Ben Creisler

Two recent papers in open access:

Joel D. HUTSON & Kelda N. HUTSON (2015)
An examination of forearm bone mobility in Alligator mississippiensis
(Daudin, 1802) and Struthio camelus Linnaeus, 1758 reveals that
Archaeopteryx and dromaeosaurs shared an adaptation for gliding and/or
Geodiversitas 37 (3): 325-344
doi:  http://dx.doi.org/10.5252/g2015n3a3

Free pdf:


In response to increased limb bone loads many tetrapod clades have
converged upon similar adaptations to reinforce the elbow joint by
reducing independent movements of the forearm bones. However prior
studies have not examined how these changes occurred phylogenetically
or functionally, such as during the transition from prehensile
forelimbs in dinosaurs to gliding/flapping flight in bird wings. Here,
a functional analysis of forearm bone mobility in extant archosaurs
shows that crossing and uncrossing of the radius and ulna can be
forced in alligators via a passive gliding mechanism recently
described in lacertilians, while birds are adapted to inhibit this
motion. A comparison of these findings with a sample of extinct
quadrupedal archosaur forearms strongly suggests that, due to the
highly conserved morphology of tetrapod forearms in general, the
lacertilian mechanism broadly describes the plesiomorphic mechanism
via which tetrapod forearm bones passively cross in response to
locomotor-induced torsion. Bipedal dinosaurs retained adaptations for
this passive mechanism, which indicates that they were unable to
perform active long-axis rotations to align their semi-pronated,
misaligned forearm joints. By contrast, analogous to birds and
pterosaurs, quadrupedal dinosaurs evolved immobilizing adaptations to
reduce or prohibit independent movements of the radius and ulna.
Notably, the elbow joints of Archaeopteryx von Meyer, 1861 and
dromaeosaurs are bird-like. This information, coupled with a lack of
non-aerial adaptations for increased limb bone loads, strongly
suggests that the forearms of deinonychosaurs were adapted to resist
the bending and torsional stresses incurred by leading edge air
streams during gliding and/or flapping.


David L. Dufeau &  Lawrence M. Witmer (2015)
Ontogeny of the Middle-Ear Air-Sinus System in Alligator
mississippiensis (Archosauria: Crocodylia).
PLoS ONE 10(9): e0137060

Modern crocodylians, including Alligator mississippiensis, have a
greatly elaborated system of pneumatic sinuses invading the cranium.
These sinuses invade nearly all the bones of the chondrocranium and
several bony elements of the splanchnocranium, but patterns of
postnatal paratympanic sinus development are poorly understood and
documented. Much of crocodylomorph—indeed archosaurian—evolution is
characterized by the evolution of various paratympanic air sinuses,
the homologies of which are poorly understood due in large part to the
fact that individual sinuses tend to become confluent in adults,
obscuring underlying patterns. This study seeks to explore the
ontogeny of these sinuses primarily to clarify the anatomical
relations of the individual sinuses before they become confluent and
thus to provide the foundation for later studies testing hypotheses of
homology across extant and extinct Archosauria. Ontogeny was assessed
using computed tomography in a sample of 13 specimens covering an
almost 19-fold increase in head size. The paratympanic sinus system
comprises two major inflations of evaginated pharyngeal epithelium:
the pharyngotympanic sinus, which communicates with the pharynx via
the lateral (true) Eustachian tubes and forms the cavum tympanicum
proprium, and the median pharyngeal sinus, which communicates with the
pharynx via the median pharyngeal tube. Each of these primary
inflations gives rise to a number of secondary inflations that further
invade the bones of the skull. The primary sinuses and secondary
diverticula are well developed in perinatal individuals of Alligator,
but during ontogeny the number and relative volumes of the secondary
diverticula are reduced. In addition to describing the morphological
ontogeny of this sinus system, we provide some preliminary exploratory
analyses of sinus function and allometry, rejecting the hypothesis
that changes in the volume of the paratympanic sinuses are simply an
allometric function of braincase volume, but instead support the
hypothesis that these changes may be a function of the acoustic
properties of the middle ear.