Since reptiles are cold blooded and lay their eggs in unsafe zone hidden in the soil or sand so they have a had calcerious shell on their eggs and same we see in the case of birds as it also had common ansister archeoptrex

Reptilian Ancestry in Birds

Birds are descended from reptiles, specifically from a group of reptiles known as archosaurs. This reptilian ancestry is indicated by several characteristics that birds possess. Among the given options, the one that clearly indicates their reptilian ancestry is the fact that birds lay eggs with a calcareous shell.

Eggs with a Calcareous Shell

One of the most distinguishing features of birds is their ability to lay eggs. Unlike most reptiles, birds lay hard-shelled eggs. The eggshell is composed primarily of calcium carbonate, which gives it its calcareous nature. This calcareous shell provides protection for the developing embryo and helps prevent dehydration.

The presence of a calcareous shell in bird eggs is an important trait that birds inherited from their reptilian ancestors. This is because reptiles, including dinosaurs and other archosaurs, also lay eggs with hard shells. The calcareous shell is a characteristic feature of reptilian eggs and is not found in the eggs of other groups of animals, such as mammals.

The fact that birds lay eggs with a calcareous shell is a clear indication of their reptilian ancestry. This trait has been passed down through millions of years of evolution, from their reptilian ancestors to modern-day birds.

Other Options

While the other options mentioned in the question may also be present in birds, they do not necessarily indicate their reptilian ancestry:

- Scales on their hindlimbs: Scales are indeed found on the legs and feet of birds, but they have evolved independently from the reptilian scales. Bird scales are different in structure and function compared to reptilian scales.

- Four-chambered heart: Birds, like mammals, have a four-chambered heart. This is a characteristic shared by both birds and mammals, but it does not specifically indicate their reptilian ancestry.

- Two special chambers (crop and gizzard) in their digestive tract: The presence of a crop and gizzard in the digestive tract of birds is a unique adaptation for their specialized diet and digestion. While reptiles may have different digestive adaptations, the presence of a crop and gizzard in birds is not a direct indication of their reptilian ancestry.

Conclusion

Out of the given options, the characteristic that clearly indicates the reptilian ancestry of birds is the fact that they lay eggs with a calcareous shell. This trait is shared by reptiles, including archosaurs, from which birds evolved. The calcareous shell of bird eggs is a result of millions of years of evolutionary history and is a significant link between birds and their reptilian ancestors.