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The evolution of limb function from theropods to birds
Naturwissenschaften. 2008 Dec 24.
The evolutionary continuum of limb function from early theropods to
birds.Hutchinson JR, Allen V.
Structure and Motion Laboratory, Department of Veterinary Basic Sciences, The
Royal Veterinary College, University of London, Hatfield, Hertfordshire, AL9
7TA, UK, email@example.com.
The bipedal stance and gait of theropod dinosaurs evolved gradually along the
lineage leading to birds and at some point(s), flight evolved. How and when did
these changes occur? We review the evidence from neontology and palaeontology,
including pectoral and pelvic limb functional morphology, fossil
footprints/trackways and biomechanical models and simulations. We emphasise
that many false dichotomies or categories have been applied to theropod form
and function, and sometimes, these impede research progress. For example,
dichotomisation of locomotor function into 'non-avian' and 'avian' modes is
only a conceptual crutch; the evidence supports a continuous transition.
Simplification of pelvic limb function into cursorial/non-cursorial
morphologies or flexed/columnar poses has outlived its utility. For the
pectoral limbs, even the classic predatory strike vs. flight wing-stroke
distinction and separation of theropods into non-flying and flying-or
terrestrial and arboreal-categories
may be missing important subtleties. Distinguishing locomotor function between
taxa, even with quantitative approaches, will always be fraught with ambiguity,
making it difficult to find real differences if that ambiguity is properly
acknowledged. There must be an 'interpretive asymptote' for reconstructing
dinosaur limb function that available methods and evidence cannot overcome. We
may be close to that limit, but how far can it be stretched with improved
methods and evidence, if at all? The way forward is a combination of techniques
that emphasises integration of neontological and palaeontological evidence and
quantitative assessment of limb function cautiously applied with validated
techniques and sensitivity analysis of unknown variables.