The common ancestors of birds and mammals were very early (stem)reptiles, which almost certainly possessed three-chambered hearts (two atria, one ventricle). Birds and mammals, however, are alike in having four-chambered hearts (two atria, two ventricles). The four-chambered hearts of birds and mammals are best described as

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The four-chambered hearts of birds and mammals are best described as homoplasies, which means they are similar structures that evolved independently in the two lineages after their divergence from a common ancestor with a three-chambered heart. This is not due to direct inheritance from a common ancestor with a four-chambered heart (which would be shared ancestry), but rather convergent evolution where similar environmental pressures led to the development of similar structures.

The Common Ancestors of Birds and Mammals Were Very Early