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From theropod hallux to perching digit in birds (free pdf)

Ben Creisler

A recent paper not yet mentioned:

João Francisco Botelho, Daniel Smith-Paredes, Sergio Soto-Acuña, Jorge
Mpodozis, Verónica Palma & Alexander O. Vargas (2015)
Skeletal plasticity in response to embryonic muscular activity
underlies the development and evolution of the perching digit of
Scientific Reports 5, Article number: 9840


Most birds have an opposable digit 1 (hallux) allowing the foot to
grasp, which evolved from the non-opposable hallux of early theropod
dinosaurs. An important morphological difference with early theropods
is the twisting of the long axis of its metatarsal. Here, we show how
embryonic musculature and the onset of its activity are required for
twisting of metatarsal 1 (Mt1) and retroversion of the hallux.
Pharmacologically paralyzed embryos do not fully retrovert the hallux
and have a straight Mt1 shaft, phenocopying the morphology of early
tetanuran dinosaurs. Molecular markers of cartilage maturation and
ossification show that differentiation of Mt1 is significantly delayed
compared to Mt2-4. We hypothesize on how delayed maturation may have
increased plasticity, facilitating muscular twisting. Our experimental
results emphasize the importance of embryonic muscular activity in the
evolutionary origin of a crucial adaptation.