Little variation in the morphology of the atria across 13 orders of birds.
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Hearts of the endothermic mammals generate much greater blood flows and pressures than the hearts of ectothermic reptiles that represent the mammalian ancestral setting. Among mammals, the variation in atrial morphology is greater than in ectothermic reptiles. Possibly, the transition from ectothermy to endothermy and high cardiac performance associated with greater variation in cardiac structure. To test this association, we investigated the variation in cardiac morphology among the endothermic birds, as birds evolved from ectothermic reptiles and their hearts generate greater blood flows and pressures than the hearts of mammals. Hearts were assessed by gross morphology and histology, and we focused on the atria as they have multiple features that lend themselves to quantification. We found the bird hearts to have multiple features in common with ectothermic reptiles (synapomorphies), for instance three sinus horns. Convergent features were shared with crocodylians and mammals, such as the cranial offset of the left atrioventricular junction. Other convergent features like the compact organization of the chamber myocardium were shared with mammals only. Some features were distinctively avian (apomorphies), including the left atrial antechamber, and a ventral merger of the left and right atrium was found in parrots and passerine birds only. Most features, however, exhibited little variation. For instance, there were always receives three systemic veins and two pulmonary veins, whereas among mammals the number of veins are 2-3 and 1-7, respectively. Our findings suggest that the transition to high cardiac performance does not necessarily lead to greater variation in cardiac structure.