Are birds considered mammals or reptiles?

So, a long time ago, back when fungus could grow as tall as a modern building, reptiles started to roam the earth. The earlier reptiles were the dinosaurs, even before crocodiles and lizards emerged.

Several years have passed and records says birds might have been reptiles too. Before we discard that information, let’s find out why evolutionists are hell bent on this idea.

Are birds considered mammals or reptiles?

Birds are reptiles when the term is being used as a synonym for sauropsids. Sauropsids is a clade that diverged from the first amniotes. The other clade is synasipda which include mammals.

The sauropsids line split into diapsids and anapsids. The modern diapsids now have three primary clades, which are the lepidosauria, testudines and archosauria which include the crocodilians and dinosaurs. This is where we might find birds, because avilae is a clade of theropod dinosaurs.

So birds together form the group aves, which is a subset of the therapods, who are a subset of the dinosaurs, who are the subset of the sauropsids.

At the end of the day, we see how birds are significantly more in common with crocodile than mammals, we can even go ahead to say they have more in common with lizards than crocodiles.

If one still wishes to stick closely as possible to scientific cladistic classification, then “Reptile” can map to either amniota or sauropsida.

The problem with the definition of reptiles

Let’s give a proper definition to the term “reptiles” to have a better understanding of why birds are closely related to them. There are many ways to define reptile as a group.

To make a long story short, whether birds are reptile or not, depends on the definition of reptile you are using. It seems that treating Reptilia has a monophyletic group including birds and warm blooded Sauropsids is becoming more and more popular. People would prefer to call it Sauropsids and abandon Reptilia.

Other reptiles have scales and so do birds, right along with the feathers. Other birds lay eggs, at least if not all, and so do birds.

The problem comes with the old definition of reptiles. Reptiles were considered vertebrates a long time ago, this is because they developed the amniotic eggs but are not warm blooded. Because of this definition, birds and mammals both are descended from reptiles and the common ancestor of birds and mammals was a reptile, but they themselves are not.

Many years have passed and a new definition for reptiles have emerged, in which reptiles are defined by a hole in the bone in the skull for air passages and muscle attachments, as well as other attributes that do not fossilize, such as four-coloured vision. Because of this definition, birds are considered as reptiles.

Birds as reptiles because of similar structural features

But one can also define “Reptile” by distinct features. In this scenario, depending on which features you picked, you could define birds as not reptiles. But in doing so you would also end up defining most dinosuars as not reptiles, either.

Earlier classification systems differentiated animals by how they looked, so scaly cold-blooded animals got put into one category, while feathery, warm blooded animals put into another. Years later and now we tend to classify animals by how they are related to each other and not just by how they look alone.

Let’s compare birds with other reptiles. Birds walk on two legs and are warm blooded – also true for theropod dinosaurs). Birds have feathers – so did many but not all theropod dinosaurs. Birds have beaks and so do turtles.

Another consideration is in the skeletal structure of birds. Birds have a very similar skeleton to some dinosaurs, which are reptiles. From fossils, we were able to discover that dinosaurs have long arms like birds, also in their beaks and feathers.

Evidence from fossil records.

This is an evidence for an occurrence about some million years ago, where bird like theropods and theropod like birds appeared in the fossil records, and somewhat prior to that going back some 200 years ago, bird like traits appeared in reptiles in the fossil succession.

So ultimately what we see in the fossil record is the initial appearance of reptiles some 300 million years ago, but somewhere around the middle of the mesozoic, reptiles become more and more bird like, untill it became almost impossible to differentiate between birds and dinosaurs.

The closer we look at the fossil records with more and more precision, the more blurred the lines become between birds and reptiles.

Birds as reptiles vs mammals

Zoologist may attempt to critique the theory that birds evolved from reptiles, but suggesting that birds do not evolve from dinosaurs do not also mean birds do not evolve from mesozoic reptiles, they only that birds descended from other dinosaur like reptiles, but not dinosaurs themselves.

Mammals are closely related to achosuars (crocodiles, reptiles and snakes) than suarians are. The reason is because egg laying mammals have sex chromosomes that resemble sex chromosome of bird than that of placental mammals.

The males in placental and marsupial mammals are the heterogamous sex. The females in the birds are the heterogamous sex. This proves that the birds and dinosaurs are closely related to mammals than the lizards and snake.

Related information.

Years ago, people thought dinosaurs were like giant lizards, who needed to warm up in the morning and sat for days motionless and people saw dinosaurs as reptiles and birds as birds. They envisioned mammals as the superior class to the slow lumbering reptiles that would eventually inherit the earth.

Dinosaurs having birds as their closest relatives is because they are genetically closer to birds than they are to other living reptiles. The closest living reptile to dinosaurs are crocodiles. The Dinosauria class include the first warm blooded animals that became dinosaurs, pterosaurs, and some other warm-blooded triassic reptiles very similar to dinosaurs.


Cladistically, birds descended from reptiles and this evolution unfolded sometime between the triassic to jurassic periods of the mesozoic.

So when people say that dinosaurs are extinct, we know that is untrue. We see living mini dinosaurs flying all over our gardens every day.