Before coming to the question of common ancestry of birds, mammals and reptiles, you need to understand what distinct characteristics the animals belonging to these classification groups have. Scientifically known as Aves, birds form one of the major classes of phylum Chordata in kingdom Animalia. The members of this clade are possessed with wings, feathers, two legs and a beak. They lay eggs and are capable of maintaining thermal homeostasis with the help of internal metabolic processes. Mammals form another major class of vertebrate animals and are characterized by the distinguishing features of hair on the body, mammary glands, three middle ear bones and a region of the brain, called neo-cortex. Turning to reptiles, these tetrapod vertebrates constitute an evolutionary grade of animals. Their members, without four limbs, like snakes, have descended from the four-limbed ancestors.
Similarities among Birds, Reptiles and Mammals:
Listed below are some of the prominent similarities among the classes of reptiles, birds and mammals.
- Birds, mammals and reptiles are all vertebrate animals and belong to the phylum Chordata.
- All the animals, from these classes, are collectively termed as tetrapods, which means they have four limbs attached to the body.
- They are all multi-cellular and have well developed brain and sensory organs.
- They are all amniotes as their embryos develop inside special membranes, called amnions. These membranes keep the developing embryos moist.
- There is a special type of kidney in all these animals, called metanephros, which is capable of producing concentrated urine.
- In the animals, from all these classes, there is internal fertilization and, in most of the cases, there can be seen a male copulatory organ, which may be a penis or a semi-penis.
- The second cervical vertebra in the neck of all these animals has a specialized shape and is known and axis. Both atlas, the first cervical vertebra, and axis contribute to the production of a wide variety of movements for the head in these animals.
- In the animals of all these classes, you can see that the tips of fingers are protected by claws, which may be modified in the form of nails or hooves.
- Mammals have hair on their body; birds have feathers and wings covering their skin; and reptiles do not have such outgrowths.
- There are scales on the bodies of reptiles which are absent in case of mammals and birds.
- Birds and mammals usually have four limbs, while in some reptiles, like snakes, these structures have gone extinct.
- Reptiles are coldblooded animals, i.e. they cannot maintain thermal homeostasis with the help of internal metabolic processes, whereas mammals and birds are warm-blooded. Warm-blooded animals can keep their body temperatures at a nearly constant level and are capable of resisting fluctuations in the internal and external environments.
- Mammals have mammary glands which are absent in birds and reptiles.
- Three middle ear bones distinguish mammals from members of the classes of birds and reptiles.
- A region of the brain, called neocortex, is found only in mammals and cannot be seen in members of other two classes.
- Blue whale, measuring up to 33 meter in size, is the largest animal on the planet earth. It belongs to the class of mammals.
What Do These Similarities Suggest? A Common Ancestor?
All these similarities of features between the members of birds, reptiles and mammals suggest that they evolved from a common ancestor during the course of evolutionary history. However, with the passage of time, there occurred divergence on their path. As a result of this divergence, the members of one class developed features which were different from those of other classes. Now, as you can see, the descendants of a common ancestor have become so dissimilar that they have been grouped into three distinct classes, called mammals, birds and reptiles. Here, it is also important to visualize some of the prominent dissimilarities existing among them.
Differences among Birds, Reptiles and Mammals:
Here are some of the prominent differences among the members of classes of reptiles, mammals and birds which serve as the distinguishing features for each class.