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What do you imagine when you hear the word "dinosaur"? Sure, first of all, a prehistoric reptile or a giant lizard. But did it ever occur to you that lizards might have included not only cold-blooded predators and herbivores but also mammals?

However, we won't be the first to think about it. The Society of Paleontologists has already put forward the theory that the first primitive mammals - our distant ancestors - also appeared a long time ago, along with the first dinosaurs. Paleontologists believe that the large animal reptiles were shredded and turned into mammals precisely because only in this way they could escape from the new terrible bipedal predators, who were the first of the large animals learned to run quickly - from the dinosaurs and their closest relatives thecodonts. But were the first mammals so different from lizards and their feathered congeners?

 are dinosaurs mammals or reptiles?

What’s the difference between mammals and reptiles?


Modern mammals and birds are warm-blooded animals, while fish, reptiles, and amphibians are cold-blooded. And dinosaurs, as it turns out, were an intermediate variant. Comparing mammals and reptiles, one can immediately identify significant differences in structure.



  • Feed their calves with milk thanks to the mammary glands of the females.
  • They are "live-bearing", a characteristic feature is internal fertilization.
  • They have a hair cover, sweat, and sebaceous glands.
  • The highly organized nervous system, all the senses are present.
  • The distinctive feature of animals are the teeth, which "sit" in special cells and have different functions - incisors, primary.
  • The respiratory system is more developed, there is a diaphragm.
  • The four-chamber heart provides the body with arterial blood.
  • They're believed to have appeared at the end of Mesozoic.



  • Interpreters reproduce by laying eggs or egg-laying.
  • The skin of reptiles is covered with scales, no glands.
  • The nervous system is worse developed, the visual analyzer is weak.
  • Primitive respiratory system.
  • The three-chamber heart provides the body with mixed blood, which makes reptiles cool.
  • Appeared at the end of the Paleozoic period.


Are all dinosaur reptiles?

are all dinosaurs reptiles?

Dinosaurs are reptiles, they belong to the group of archosaurs (Archosauria), which also includes crocodiles and birds. The first archosaurs appeared at the turn of the Permian and Triassic periods, while dinosaurs appeared a little later - in the middle of the Triassic, about 230 million years ago.

The word "dinosaur" itself, derived from the Greek language, can be translated as "Terrible lizard". At first they were thought of as a mixture of a lizard and hippopotamus, and for a long time they were depicted as four-legged massive clumsy creatures. As research progressed, the idea of prehistoric creatures changed. At first, it became clear that there were many bipedal dinosaurs. Then scientists dispelled the myth that the tail of bipedal dinosaurs served as a support and dragged on the ground while walking. Then paleontologists were faced with the question of whether or not the dinosaurs were cold-blooded. Further study of the structure of their organisms made most scientists adhere to the opinion about their warm blood. But, since these lizards are not cold-blooded like lizards or crocodiles, can they be considered reptiles? After all, many, especially small dinosaurs, had a feather cover that served as thermal insulation and was not gentle to cold-blooded creatures. They, like modern birds, had a very perfect respiratory system with airbags - air reservoirs, traces of which (cavities, which included these bags) can be seen even on fossilized bones. So now dinosaurs are presented as fast and energetic animals similar to modern birds.

The "first birds" came from a group of Theropods. It was in this squad that dinosaurs covered with feathers met. A vivid example is Archaeopteryx, a typical theropod dinosaur in terms of skeleton structure.

Many paleontologists consider birds to be direct descendants of dinosaurs, and many even call birds modern dinosaurs who survived extinction. So it is difficult to say it or not yet, but the relationship between dinosaurs and birds is obvious - birds, if not descendants, then certainly "brothers" of dinosaurs.

Here is an interesting article if you want to know more about what is the closest thing to a modern-day dinosaur.

Were there mammals when dinosaurs lived?

little prehistoric mammals 

In the past, scientists believed that most of the mammals that lived in dinosaur times were small insect eaters because there were not yet enough remains to refute such a version. However, over time, more and more such fossils were found, and, having compared them to each other, scientists noticed that the different biological species living in the dinosaur times, were very different. This, in turn, indicates that they had significantly different diets, which means that they occupied very different ecological niches


What were the first mammals like?


The very first mammals, according to scientists, appeared 216 million years ago, and they looked like little shrew or mice. They fed on insects and plants and spent nights in trees. The wool and other distinctive features of mammals have evolved over millions of years. 

  • A striking example of the first mammals is the Tritylodontidae, which lived in Triassic. It was just over 1 meter long and had, as you can see, very unusual teeth. Tritylodontidae was a long time alternating between mammals and thermomorphic reptiles. The structure of the teeth, with a lumpy crown and bifurcated roots, tritilodont undoubtedly belongs to mammals, and the structure of the rest of the skull is close to the thermomorphs.


Illustration of a Tritylodontidae
  • Another ancestor of modern mammals is the Triconodonta. The triconodonta lived in the late Jurassic period and was the size of today's average domestic cat. However, it was obviously a predator and hunted a wide variety of small animals.


Illustration of a Triconodonta

Illustration of a Triconodonta by P.Riha

  • And here's another representative, Megazostrodon. He lived in Triassic, like the tritilodont. It was a small animal, similar to a shrew, which only grew to 13 cm. However, it fed mainly on insects and was only released at night when the risk of being eaten by predators was reduced.

 Megazostrodon reenactment

Megazostrodon reenactment

However, the researchers note that many ancient mammals suffered the same fate as the dinosaurs, and in this matter, their diet probably played a key role. Those animals whose diets were monotonous died without being able to feed themselves under changed conditions, while those whose diet was more flexible survived and became the ancestors of mammals living today.

Scientists cannot yet say with certainty what allowed ancient mammals to occupy a variety of ecological niches, but it is believed that the spread of flowering plants played a role in this.

Researchers believe that a successful diet allowed the ancestors of modern birds to survive mass extinction - they are believed to have eaten seeds that remained in large quantities even after the fall of a meteorite on Earth. It reduced the number of vegetation, and then, along the chain - herbivorous and predatory animals.