EVOLUTIONARY CHANGES IN PUBIC ORIENTATION IN DINOSAURS ARE MORE STRONGLY CORRELATED WITH THE VENTILATION SYSTEM THAN WITH HERBIVORY

Among dinosaurs, the pubis has convergently retroverted four times in Maniraptora (Theropoda) and once in Ornithischia. Although a clear correlation has not been demonstrated, it has been previously proposed that two traits were related to pubic retroversion: the reduced importance of cuirassal ventilation, and a herbivorous diet. Here, we analyse the possible influence of these traits on pubis orientation. Cuirassal ventilation was plesiomorphically present as an accessory ventilation mechanism in Dinosauria and was powered by the M. ischiotruncus, which was probably connected to a propubic pelvis. Cuirassal ventilation was reduced in both Ornithischia and Maniraptora, some of which also evolved herbivory. According to our results, cuirassal ventilation is more strongly correlated with pubic orientation than herbivory. The retroversion of the pubis during the evolution of birds resulted in major changes in the musculature of the tail. These changes increased the efficiency of the pubocaudalis muscles, which enhanced the birds' capability for take-off from the ground. The release of the evolutionary constraint on pubic orientation through changes in the ventilatory system can therefore be considered to be an important step in bird evolution.