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 .

The four- chambered hearts of birds and mammals are best described as homoplasies.

Explanation:

In evolution, homoplasies are the trait shared in different species despite that their ancestors didn't have it at all.  In other words, these are traits that can be gained or lost during evolution in different lineages independently. For example, wings of birds and wings of bats do the same function (help in flying) but they were not there in the ancestors so both animals developed these structures independently.

In the given example, birds and mammals have four-chambered heart but their ancestors have 3 chamber hearts. They evolved independently and developed 4 chamber heart. This might be due to several reasons. However, their ancestors didn't have the feature so it is an example of homoplasy.

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