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Evolution of Theropod Tail into Stiff Aerodynamic Surface

From: Ben Creisler

New in PLoS ONE:

Michael Pittman, Stephen M. Gatesy, Paul Upchurch, Anjali Goswami &
John R. Hutchinson (2013)
Shake a Tail Feather: The Evolution of the Theropod Tail into a Stiff
Aerodynamic Surface.
PLoS ONE 8(5): e63115.

Theropod dinosaurs show striking morphological and functional tail
variation; e.g., a long, robust, basal theropod tail used for
counterbalance, or a short, modern avian tail used as an aerodynamic
surface. We used a quantitative morphological and functional analysis
to reconstruct intervertebral joint stiffness in the tail along the
theropod lineage to extant birds. This provides new details of the
tail’s morphological transformation, and for the first time
quantitatively evaluates its biomechanical consequences. We observe
that both dorsoventral and lateral joint stiffness decreased along the
non-avian theropod lineage (between nodes Theropoda and Paraves). Our
results show how the tail structure of non-avian theropods was
mechanically appropriate for holding itself up against gravity and
maintaining passive balance. However, as dorsoventral and lateral
joint stiffness decreased, the tail may have become more effective for
dynamically maintaining balance. This supports our hypothesis of a
reduction of dorsoventral and lateral joint stiffness in shorter
tails. Along the avian theropod lineage (Avialae to crown group
birds), dorsoventral and lateral joint stiffness increased overall,
which appears to contradict our null expectation. We infer that this
departure in joint stiffness is specific to the tail’s aerodynamic
role and the functional constraints imposed by it. Increased
dorsoventral and lateral joint stiffness may have facilitated a
gradually improved capacity to lift, depress, and swing the tail. The
associated morphological changes should have resulted in a tail
capable of producing larger muscular forces to utilise larger lift
forces in flight. Improved joint mobility in neornithine birds
potentially permitted an increase in the range of lift force vector
orientations, which might have improved flight proficiency and
manoeuvrability. The tail morphology of modern birds with tail fanning
capabilities originated in early ornithuromorph birds. Hence, these
capabilities should have been present in the early Cretaceous, with
incipient tail-fanning capacity in the earliest pygostylian birds.