The common ancestors of birds and mammals were very early (stem) amniotes, 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

homoplasies

Explanation:

According to my research on studies conducted by various biologists, I can say that based on the information provided within the question the four-chambered hearts of birds and mammals are best described as homoplasies. In the context of biology, homoplasy is a trait or character that is found in a set of species but not present in their common ancestor.

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