Dinosaurs Were Reptiles! Here’s Why


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Dinosaurs are a diverse group of animals that dominated the earth about two hundred million years ago. Unfortunately, the terrestrial beasts were all wiped out after a meteor hit the earth 65 million years ago. Early researchers concluded that the animals were sluggish, cold-blooded and that they were either herbivores or carnivores. If you take a look at modern reptiles, you can’t help to notice the similarities, but there are still questions as to why dinosaurs were reptiles.

So why were dinosaurs reptiles? If dinosaurs were still in existence, they would be classified as reptiles under the sub-tribe Archosaurs that also includes crocodiles. The pelvic structures in dinosaur skeletons resembled those found in crocodiles and lizards, and they moved similarly. Additionally, dinosaurs laid eggs and built nests just like modern-day reptiles.  

Modern classification considers all dinosaurs to be reptiles and a lot to do with more than 80% similar characteristics to that of reptiles. They hatched from eggs, had scaly skin, and the word dinosaur, as earlier explained, means great terrible lizard. 

There is, however, more scientific detail that attempts to answer this seemingly straightforward question. The ideal answer depends wholly on taxonomy: the branch of biology concerned with classifying organisms. 

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Background Facts about Reptiles That Relate to Dinosaurs

Reptiles, in modern times, are not as diverse and populous as they were 200 million years ago. Maybe it has something to do with the fact that two-thirds of their ancestry went extinct simultaneous and suddenly. However, their physical attributes and behavior still cover a broad spectrum. There are numerous misunderstandings and myths about reptiles but not as many as in the past. Today, we have more insight into reptiles, and this information has brought to light numerous facts about these creatures. 

Reptiles Evolved from Amphibians

It might sound like it is a gross simplification; however, it is fair to think that fish evolved into tetrapods, the tetrapods into amphibians, and amphibians into reptiles. These events took place about four hundred million years ago. Moreover, about two hundred million years ago, the reptiles we now know as archosaurs evolved into dinosaurs. At the same time, the reptiles we now know as therapsids evolved into mammals.  

About fifty million years after these occurrences, the dinosaurs evolved into birds. This kind of overlapping might explain why reptiles are not as diverse and populous as they were since their descendants out-compete them in numerous ecological niches. 

There Are Four Reptile Groups

You can easily count the different types of reptiles alive: turtles, squamates, crocodilians, and tuataras. Their protective shells and slow metabolisms characterize turtles. Squamates, which include lizards and snakes, have wide-opening jaws and they shed their skins. Crocodilians are the closest relatives of both the now-extinct dinosaurs and modern birds. The strange creatures called tuataras are, today, only found in a few remote islands in New Zealand. 

So, just how far down from the top of the food chain did reptiles tumble? Well, the marine reptiles that ruled the oceans and pterosaurs that ruled the skies went extinct about 65 million years ago along with the dinosaurs. 

Reptiles Are Cold-Blooded

One of the primary characteristics that make a distinction between mammals and reptiles is that reptiles are cold-blooded or ectothermic. They rely on prevailing weather conditions to energize their physiological processes. Crocodiles and snakes, in particular, power their internal physiology by lying under the sun during the day. At night, when there is no available source of energy, reptiles are quite sluggish. The advantage enjoyed by cold-blooded organisms is that they need to eat less than equally sized mammals and birds. The disadvantage is that they are cannot sustain a high level of activity since it gets dark with time. 

Reptiles Have Scaly Skin

The rough and alien quality of reptile skin makes most people uneasy, yet the scales represent a significant evolutionary achievement. Thanks to the added layer of protection, animals could move from water bodies without the risk of drying out. Some reptiles shed their skin in one piece, while others might do it a few flakes at a time. 

As tough as it might seem, reptilian skin is quite thin, which is why reptile leather is only decorative when used to make cowboy boots. It is less useful than cowhide.

Reptiles Were the First Amniotes 

Amniotes, the first vertebrate animals that could incubate their fetuses inside the female’s body or lay eggs on land, represented a crucial transition in the evolution of life. Amphibians, which preceded reptiles, had to lay their eggs in water. Therefore, they could not move inland to colonize earthly continents. In this respect, it is natural to consider reptiles to be the intermediate phase between amphibians and fish.  

Why Reptiles Have Closer Links to Dinosaurs than Other Classifications

A Modern Approach

The modern way of classifying living organisms tends to look back at an animal’s ancestry. Animals that share the same ancestors are classified similarly. This system is known as phylogenetics. For instance, scientists could go back many years ago and find out which animal had the very first reptilian features. This animal would then be the group’s common ancestor. Scientists will, after that, group all of its descendants as reptiles. 

With this system of classification in mind, we should have a look at how dinosaurs evolved. 

How Dinosaurs Evolved

About three hundred million years ago, some amphibians gained the ability to lay eggs. They also had tough skin that reduced the risk of drying out as they moved on land. This evolutionary leap meant that the amphibians could survive on a broader array of habitats. These amphibians are considered to be the earliest reptile-like animals. 

The first reptiles, after that, split into a few other branches, such as the swimming reptiles, snakes and lizards, turtles, and archosaurs. The archosaurs then split into crocodiles, pterosaurs (flying reptiles), and dinosaurs. 

According to the phylogenetic system of classification, dinosaurs are the direct descendants of reptile-like amphibians that had moved from the water bodies. 

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A Traditional Approach

Traditionally, animals were classified together in light of their shared characteristics. Reptilian characteristics include:

  • Reptiles lay eggs
  • Reptiles are ectotherms
  • Reptiles are tetrapods

Arguably, dinosaurs did not display all features found in modern reptiles. However, they were still considered to have enough reptile-like features to be grouped as reptiles.  

Some Interesting Similarities

  • If dinosaurs were still in existence, they would be classified as reptiles under the sub-tribe Archosaurs that also includes crocodiles. The pelvic structures in dinosaur skeletons resembled those found in crocodiles and lizards, and they moved similarly. Additionally, dinosaurs laid eggs and built nests just like modern-day reptiles.  
  • Both reptiles and dinosaurs are vertebrate animals that have a spine with a few appendages. 
  • Both dinosaurs and reptiles possess dermal bone structures like the scales and plates on the skin and the endoskeletal present inside the muscles. 
  • Reproduction: the majority of reptiles are oviparous. It is a scientific term used to describe living things that reproduce by laying eggs. Only a few reptile species are ovoviviparous. It means that the young ones hatch from eggs incubated inside the body. It is similar to dinosaurs, which were all oviparous. Secondly, fertilization, in all reptiles, occurs internally. At birth, the offspring of dinosaurs and other reptiles were mini versions of adults. Therefore, the offspring are miniatures of the parents, and they do not change in appearance as they grow. 
  • The term ‘Squamata’ is the Latin variant for scaled. All reptiles, including dinosaurs, have scaled dry skin. The scales are smooth in some species, while they are keeled in others. Keeled scales give the animal a rough texture and appearance. 

Some Big Differences

Arguably, dinosaurs were merely lizards that had lived much longer in very favorable conditions of the early earth. This simplistic idea claims that a lizard would look like a dinosaur if it lived for decades. However, the notion doesn’t seem right. The differences between dinosaurs, lizards, and other reptiles are much more significant than they might appear at first sight. Numerous skeletal and fundamental differences can be noticed between all lepidosaurs (the collective term for all tuatara, living lizards, and all archosaurs.) 

What Characteristics Distinguish Dinosaurs from Other Reptiles? 

  • Most reptiles, such as lizards and crocodiles, possess legs that sprawl to the side. Their thigh bones are parallel to the ground in most cases. Most modern-day reptiles move in a side to side motion. On the other hand, dinosaurs stand upright with their legs positioned under their bodies. There is a hole in a dinosaur’s hip socket to permit this stance. Given this standing posture, dinosaurs could run with greater endurance and faster than most reptiles of a similar size. During the Mesozoic Age, there existed other huge reptiles that lived in water and on the land. While these organisms lived alongside dinosaurs, they lacked a hole on their hip socket. Therefore, scientists did not classify them as dinosaurs. Modern birds are closer to dinosaurs in this regard. Apart from the hole on their hip sockets, they have other features in common such as s-shaped necks and three-toed feet. 
  • Dinosaurs had five vertebrae inside their hips. Most modern-day reptiles only have two. 
  • Just like mammals, dinosaurs had their limbs under their bodies. Reptiles, on the other hand, have their limbs stretched sideways. 
  • Dinosaurs were upright and bipedal animals. Modern-day reptiles are neither upright nor bipedal. 
  • Dinosaurs were either herbivores or carnivores. Present-day reptiles, in contrast, are mostly carnivorous. 
  • Feathers: There is no doubt that a few dinosaur species had feathers. Some paleontologists have interpreted this as necessary for insulation. It provided more evidence that dinosaurs were endothermic. Present-day reptiles do not have feathers of any kind.  

Why Do Some Dinosaurs Look More like Birds?

Consider these facts: dinosaurs might have been warm-blooded, they might have had feathers, and they might have hatched on land from shelled eggs. With such characteristics in mind, couldn’t paleontologists conclude that dinosaurs were birds? 

There is, however, a significant complication to this thinking. If you think as most scientists do, then you must know that birds might have evolved from dinosaurs. It means that before dinosaurs, there might have been no birds. So how can we classify dinosaurs as birds, yet birds were not in existence? There is a consideration that dinosaurs were the first birds. However, given that not all dinosaurs were warm-blooded or feathered, it is quite likely that there was an overlap between birds and reptiles.  

So, why do some dinosaurs look more like birds? Dinosaurs are a very diverse group of animals, which emerged about four hundred million years ago. They represented the dominant life form for over two hundred million years until an asteroid hit the earth and sent them into extinction. Dinosaurs can be split into two groups: Ornithischia and Saurischia. 

  • Saurischia comprised of all herbivorous dinosaurs (sauropods) and the giant carnivorous dinosaurs (theropods). 
  • Ornithischia is a group of beaked, herbivorous dinosaurs. A few notable members include the Stegosaurus and Iguanodon. 

Interestingly, birds might have evolved from Saurischian dinosaurs. According to modern paleontology, birds started emerging during the Jurassic age about 150 million years ago. 

Birds form part of the Maniraptora classification. It might seem strange, but most scientists believe that birds emerged from dinosaurs. It is because some dinosaurs were covered in feathers, had very light bones, and laid eggs. The dinosaurs had numerous characteristics in common with the modern bird. 

Indeed the transition might have taken place over millions of years with a few evolutionary changes. The most famous fossil is Archaeopteryx. 

The Archaeopteryx might have lived about 150 million years ago. At this time, Europe was still an archipelago of small islands in a warm tropical sea. The species represents a clear example of transitional dinosaurs; they showed characteristics from both birds and dinosaurs. Most of the fossils discovered include some impressions of feathers. Moreover, since the feathers could have been used for flight (an advanced form), the fossils were proof that the evolution of feathers began even before the late Jurassic period. 

It, however, took a long while before birds started diversifying. For millions of years, birds had teeth and clawed wings. Modern birds could have appeared about 100 million years ago before the massive asteroid hit the earth. While the birds were not top of the food chain at that time, they were the only group of dinosaurs to survive the Cretaceous. 

A Few Mysteries and Related Questions

1.    Were Dinosaurs Warm or Cold-Blooded? – Even though this might seem like a simple question, it is quite a tough dilemma that gives headaches to paleontologists all over the world. What we know about dinosaurs and how we perceive them can change radically depending on the answer. Did the ‘terrible lizards’ depend on their environs to determine their metabolisms like amphibians and reptiles? Or did they have an endothermic system like birds and mammals?

Paleontologists have come up with a wide array of arrangements: from dinosaurs warmed by the fermenting vegetation in their guts to mesothermal dinosaurs. Paleontologists will continue debating and investigating the matter, especially since dinosaurs were quite varied in size and form. 

2. How Did Dinosaurs Mate? – Undoubtedly, female dinosaurs laid eggs. While this point remains clear, how parent dinosaurs coupled to conceive is not clear. There is no fossilized evidence of dinosaur mating. Paleontologists can only fill the gap through imagination. 

Additionally, even the most basic sexual anatomy in dinosaurs remains a mystery. Some scientists believe that they had a cloaca, one orifice for the excretory, reproductive, and urinary tracts as in crocodiles and birds. Moreover, male dinosaurs could have had an intromittent organ like the one found in ostriches and ducks.  

3. How Did Dinosaurs Learn to Fly? – Paleontologists speculate several ways through which dinosaurs learned how to fly. The trees down theory, for example, envisioned arboreal dinosaurs that glided first before they could flap. The ground-up theories, however, suggested that dinosaurs learned how to flap while still on the ground. Ongoing research on dinosaurs is still yet to provide sufficient detail as to how and when dinosaurs learned how to fly. 

4. Did Dinosaurs Hunt in Packs? – Paleontologists still do not know whether dinosaurs coordinated in packs to hunt for prey. While very rare trackways show that some dinosaurs could have walked together, the strolls do not tell us why they walked side by side. Scientists will have to find something rarer: a group of dinosaur tracks intercepting a prey’s path. Preferably, the discovery should also show signs of a scuffle or a skeleton at the end. Most bone beds give information about the burials and deaths of dinosaurs. However, they remain unclear on whether the animals formed hunting packs to bring down prey. 

5. Which Species Was the First Dinosaur? – For scientists, the earliest evidence of a species in any lineage is always a goldmine. The problem is that fossil records are made up of small historical snippets. Therefore, finding out the dawn of dinosaurs relies solely on luck. So far, the findings suggest that dinosaurs evolved about three hundred million years ago. 

Final Thoughts

The term “terrible lizard” automatically gets us thinking that dinosaurs were reptiles. Interestingly, some of the big Hollywood movies give ideas that dinosaurs were more bird-like than reptile, and this tends to confuse us. I hope that this article provides a bit more clarity, at least from the scientific point-of-view, of why modern classifications rank dinosaurs as reptiles. 

Lastly, if you or someone you know loves dinosaur toys, then check out my favorite dinosaur toy gifts on Amazon: Best Dinosaur Toys – Remote Control

Michael Haralson

I'm the owner of Adventure Dinosaurs website. Although I have an extensive business background, I am fascinated with dinosaurs and have been since childhood. I'm fortunate enough to have visited fossil museums in Europe (UK, Germany, and Spain), the US (California, Texas) and in Asia (China). Currently, I'm location independent with a home base in Kirkkonummi, Finland.

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