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Avian sternum and dinosaur gastralia: functional significance for respiratory apparatus

Ben Creisler

A new online paper:

Markus Lambertz & Steven F. Perry (2015)
Remarks on the evolution of the avian sternum, dinosaur gastralia, and
their functional significance for the respiratory apparatus.
Zoologischer Anzeiger - A Journal of Comparative Zoology (advance
online publication)

The sternum is a central part of the avian skeleton and, among other
functions, it serves as a key element of their locomotor apparatus by
providing the origin site for the primary flight musculature.
Understanding the evolutionary history of the sternum is critical for
understanding the origin of active flight: a fundamental
characteristic of one of the most diverse extant tetrapod lineages. It
recently has been proposed that the sternal elements in extinct basal
bird radiations may not be homologous and that the stem species of
Aves lacked a sternum. Here we show that sternal elements were indeed
present in the stem line throughout the evolutionary transformation
from non-avian theropod dinosaurs to modern birds. We further
demonstrate a trade-off between sternal elements and gastralia, dermal
bones of the belly wall that at least partially overlapped with the
respiratory functions of the sternum. As long as gastralia were
present and functioned in ventilation, the sternum could become
completely reduced such as possibly in Troodontidae, Archaeopteryx and
Sapeornis. However, once gastralia became lost in Neornithes, even the
flightless species needed to retain their sternum: no animal needs to
fly, but all need to breathe.