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Fibular reduction in birds and its evolution from dinosaurs



Ben Creisler
bcreisler@gmail.com

A new paper:

João Francisco Botelho, Daniel Smith-Paredes, Sergio Soto-Acuña,
Jingmai O'Connor, Verónica Palma and Alexander Vargas (2016)
Molecular development of fibular reduction in birds and its evolution
from dinosaurs.
Evolution (advance online publication)
DOI: 10.1111/evo.12882
http: // onlinelibrary.wiley.com/doi/10.1111/evo.12882/abstract

Birds have a distally reduced, splinter-like fibula that is shorter
than the tibia. In embryonic development, both skeletal elements start
out with similar lengths. We examined molecular markers of cartilage
differentiation in chicken embryos. We found that the distal end of
the fibula expresses Indian Hedgehog (IHH), undergoing terminal
cartilage differentiation, and almost no Parathyroid-related-protein
(PTHrP), which is required to develop a proliferative growth plate
(epiphysis). Reduction of the distal fibula may be influenced earlier
by its close contact with the nearby fibulare, which strongly
expresses PTHrP. The epiphysis-like fibulare however then separates
from the fibula, which fails to maintain a distal growth plate, and
fibular reduction ensues. Experimental downregulation of IHH signaling
at a post-morphogenetic stage led to a tibia and fibula of equal
length: The fibula is longer than in controls and fused to the
fibulare, while the tibia is shorter and bent. We propose that the
presence of a distal fibular epiphysis may constrain greater growth in
the tibia. Accordingly, many Mesozoic birds show a fibula that has
lost its distal epiphysis, but remains almost as long as the tibia,
suggesting that loss of the fibulare preceded and allowed subsequent
evolution of great fibulo-tibial disparity.