The line separating dinosaurs from birds and reptiles is more blurry than you might imagine.
When you look at big, scary, carnivorous dinosaurs like the Tyrannosaurus rex, it’s easy to see their resemblance to crocodiles and other big lizards.
But not all dinosaurs were like this.
Some of them were tiny and feathery, much more similar to flying birds today.
It’s one of the central questions in the world of paleontology.
Most scientists now agree that birds are the closest living relatives to dinosaurs.
In fact, birds are technically dinosaurs, and they’re commonly referred to as avian dinosaurs to differentiate them from the non-flying dinosaurs (non-avian dinosaurs).
Yet, dinosaurs are generally classified as reptiles.
So, where do they really belong?
Are dinosaurs birds or reptiles?
In this article, we’ll explore this question in detail, defining dinosaurs, birds, and reptiles and highlighting their unique features.
We’ll also explore this question from both evolutionary and biological perspectives.
This will help us understand how these groups of animals have changed over the course of their evolutionary history and the unique attributes that differentiate them.
Dinosaurs are a group of terrestrial animals that dominated the Earth during the Mesozoic Era between 245 and 65 million years ago.
Dinosaurs are classified as reptiles—a group that includes lizards, crocodiles, and snakes.
However, they differ from other members of this group by their upright posture and the fact that they had limbs positioned under their body (perpendicular to the body) instead of the side of their body, like that of lizards and crocodiles.
All non-avian dinosaurs are extinct.
They died off at the end of the Cretaceous Period about 65 million years ago.
However, they did evolve into various body forms, shapes, and sizes before they eventually went extinct.
The term “Dinosauria” was coined In 1824 by British Scientist William Buckland to classify the remains of these extinct reptiles that were being discovered in various parts of England and other parts of the world at the time.
The term, which means “terrible lizard,” refers to their colossal size, fearsome appearance, and their similarities to lizards.
It would take years of research for scientists to discover that dinosaurs were not really lizards-just closely related to them.
As the most dominant animal group throughout the Mesozoic Era, our understanding of that period of Earth’s history is irrevocably tied to the discovery and study of dinosaurs.
This unique group of reptiles occupied virtually every niche imaginable, meaning the discovery of any new dinosaur fossil unlocks some information about Earth’s ecosystems and history spanning several million years.
As we gain a deeper understanding of Earth’s history, the study of dinosaurs remains a critical channel to further understand the concept of evolution, adaptation, and extinction, especially when they were alive.
Characteristics of Reptiles
The name reptile refers to a broad group of vertebrates that share unique attributes such as scaly skin, ectothermic (cold-blooded metabolism), and their tendency to lay amniotic eggs (eggs with embryo surrounded by a fluid-filled membrane called amnion).
The class Reptilia is the second-largest class of vertebrates after fishes and includes any animal that exhibits all of these characteristics.
Modern members of this group include turtles, tortoises, snakes, lizards, and crocodiles.
Dinosaurs are typically classified as reptiles.
Although they did not exactly check all the boxes, they exhibited most of the defining characteristics of the reptilian class.
They also shared a common ancestry with other members of this group.
For instance, the armor-like covering of scales, which is one of the defining features of the reptilian, is seen in many dinosaurs, too.
Although some dinosaurs did have a covering of feathers on some parts of their body, the presence of scales on several well-preserved dinosaur fossils suggests an affiliation with reptiles.
Dinosaurs also laid shelled eggs, which protected the growing embryo from drying out and provided essential nutrients to the developing embryo.
This is evidenced by the discovery of numerous dinosaur eggs, some of them with embryos preserved in them.
Of course, this is an attribute they also share with birds, which further deepens the debate about their true identity.
The question of whether dinosaurs were cold-blooded or warm-blooded is even more controversial and is a matter of ongoing debate in the scientific world.
In the past, dinosaurs were considered cold-blooded reptiles, which would mean they were slow and sluggish, considering their size.
Some scientists think the fact that these dinosaurs were so large yet active meant they were likely warm-blooded like birds.
But dinosaurs are not the only warm-blooded reptiles.
Some big turtles today, such as leatherback sea turtles, exhibit endothermic metabolism, meaning they are warm-blooded.
Experts also think that large reptiles can still be cold-blooded and active.
A few large land reptiles, such as Komodo dragons, have managed to reach massive sizes and are very active despite being cold-blooded.
Characteristics of Birds
Birds are a group of warm-blooded vertebrates characterized by their possession of feathers, wings, and beaks.
Feathers are perhaps the most distinctive characteristics of modern birds that set them apart from other animal groups.
Like fur in mammals, birds are the only living animals with their body (partially or completely) covered by feathers.
The feathers serve a wide range of functions, including insulation, display, flight, and even sound production.
While feathers are undeniably emblematic of birds, it is also one feature they share with certain dinosaurs.
Some dinosaur fossils (especially theropod dinosaurs) have been identified with feathers or features similar to feathers on them.
This was the first evidence that suggested a link between birds and some groups of dinosaurs.
In modern birds, the beak is an essential tool for feeding.
While some reptiles (such as turtles) have evolved beak-like mouths, the beaks of birds are far more distinctive and have evolved into different forms in modern bird species.
Some birds, like herons, have probing beaks, while others, like parrots, have beaks adapted for crushing food.
These adaptations reflect the diverse ecological niches that birds occupy today.
Beak-like structures have been identified in various dinosaur species as well.
Another unique attribute of birds that sets them apart from cold-blooded reptiles is their warm-blooded nature.
This refers to their ability to maintain a consistent internal body temperature.
Warm-blooded animals manage to remain active despite the ambient temperature in their surroundings, unlike cold-blooded animals that are docile or less active when the temperature drops.
This metabolic vigor allows birds to engage in various activities, such as hovering mid-air and traversing vast distances during migration.
Many modern studies suggest that most dinosaur groups were warm-blooded, like birds.
One research found that some dinosaurs (such as theropods) were probably warm-blooded, while others, like the stegosaurs and ceratopsians, were cold-blooded.
The Evolutionary Link Between Birds and Dinosaurs
These similarities between birds and certain dinosaurs aren’t out of place because they explain their evolutionary relationship.
Scientists think birds descended directly from dinosaurs.
Based on recent research, birds are considered a distinct group within the larger dinosaur group that developed feathers and the ability to fly about 150 million years ago.
In addition to the features earlier highlighted, birds share other similarities with their ancestors, such as hollow bones and a wishbone (furcula).
The earliest known bird fossils are from the Jurassic Period.
Around this time, some groups of theropod dinosaurs started transitioning gradually from non-avian forms to bird-like forms.
This transition involved a wide range of anatomical and behavioral adaptations that culminated in the emergence of dinosaurs with the ability to fly.
These early changes also laid the groundwork for the extraordinary diversity that we see in modern birds.
The Evolutionary Perspective
The key to answering the question of whether dinosaurs are reptiles or birds is to examine their evolutionary journey starting from the point where they diverged from the same common ancestors.
The Linnaean system of classifying organisms, which groups animals based on their physical characteristics, places birds and reptiles into distinct groups, with dinosaurs lumped into the reptilian class.
The alternative system of classifying animals is the phylogenetics system.
This system is based on their ancestry or evolutionary relationship.
Based on this system, birds, dinosaurs, and some reptiles (the crocodilians) all belong to the same group known as the archosaurs.
Reptiles emerged from amphibian ancestors during the Carboniferous Period about 320 million years ago.
Several million years down the line, a unique branch of reptiles known as the archosaurs emerged (a contrast to the lepidosaurs, which gave rise to modern tuataras, lizards, and snakes).
During the Triassic Period, about 250 million years ago, some of these archosaurs began to develop adaptations that allowed them to explore new ecological niches.
The Development of Unique Traits Within the Dinosaur Lineage
As the dinosaur lineage continued to evolve, several groups of dinosaurs, all with unique traits and adaptations, began to emerge.
This diversification also allowed them to develop specialized traits that would set the different groups of dinosaurs apart.
The carnivorous theropods were large but agile predators with massive jaws and other adaptations for taking down prey.
At the height of this diversification, some theropod dinosaurs also began to develop avian traits.
The transition from agile ground-dwelling bipedal theropods into winged flying birds began about 160 million years ago.
First, the theropod group that would eventually evolve into birds began to shrink in size.
As they grew smaller, several other body parts were modified.
For instance, their already small forelimbs became modified into wings.
The bones of their first and second digits consolidated to form a semi-circular structure that could rotate sideways to create thrust for flight.
Similarly, their short, hair-like feathers, which only grew on parts of their body and served the primary purpose of insulation, became modified into more complex structures capable of powering flight.
These are just a few of the features developed by these theropod dinosaurs as they gradually transitioned into the early birds of the Jurassic and Cretaceous periods.
Evidence From the Fossil Record
The transition of dinosaurs into birds took place over several million years.
This left several pieces of evidence in the fossil record in the form of transitional fossils.
These are fossils that exhibit traits common to both birds and dinosaurs.
Arguably, the most well-known of these transitional fossils is the Archaeopteryx.
Commonly referred to as the first bird, Archaeopteryx shows several reptilian features despite having notably feathered wings.
Several other transitional fossils similar to the Archaeopteryx provide a glimpse into how dinosaurs gradually acquired bird-like traits.
These dinosaurs show a mosaic of traits that demonstrates their journey from reptiles to birds and underscores the gradual nature of these transformations over the course of several million years.
Warm-Blooded or Cold-Blooded?
Since dinosaurs are traditionally classified as reptiles, it raises the question of whether they are warm-blooded or cold-blooded.
Since the early days of dinosaur research, they have always been considered cold-blooded reptiles.
This means they’re similar to modern reptiles in terms of their low metabolic rate, which is strongly influenced by the ambient temperature.
This would mean the activity levels of the dinosaurs were highly dependent on the external thermal conditions in their environment—similar to what reptiles do today.
Scientists who support this hypothesis point to bone histology studies as evidence.
These studies suggest that dinosaurs exhibited growth patterns similar to that of modern reptiles.
These growth patterns are characterized by distinct lines in bone tissue, similar to growth rings in trees.
The implication is that dinosaur growth slowed during unfavorable conditions, and they experienced quick growth during favorable periods.
In contrast, some scientists think certain dinosaurs exhibited metabolic rates similar to modern birds.
Being warm-blooded would allow them to maintain a constant internal body temperature regardless of the ambient temperature.
Animals with an endothermic strategy tend to generate heat internally.
This makes it possible for them to sustain various activities and live in a wide range of environments.
Advocates of the warm-blooded perspective highlight evidence of high vascularization and complex tissue in some dinosaur fossils, suggesting a potential for internal heat generation.
Also, the presence of feathers in certain dinosaur species confirms the link between birds and dinosaurs while also underscoring similarities in their mechanisms for temperature regulation.
The Discovery of Endothermic Dinosaurs and Its Impact on This Debate
The discovery of endothermic dinosaurs has added a new layer of complexity to the debate about how dinosaurs regulate their temperature.
These dinosaurs show clear evidence that they could maintain stable internal and external temperatures using different adaptations.
The fuzzy Yutyrannus is a clear example of this.
This theropod dinosaur lived in a region of Asia during the Cretaceous that was characterized by extremely cold temperatures during certain seasons.
To maintain its temperature, Yutyrannus had feathers covering parts of its body.
It was the largest-known theropod dinosaur that preserves direct evidence of feathers, which confirms that the dinosaur was most likely endothermic.
The feathered Sinosauropteryx is another notable example of a potentially endothermic dinosaur.
Unlike other dinosaurs that only had feathers on some parts of their body, evidence suggests that the entire body of the Sinosauropteryx was likely covered in primitive down feathers, which would have provided insulation for the dinosaur.
Sinosauropteryx was the first dinosaur to be discovered with this type of dinosaur.
However, many more like it have turned up in the fossil record, forcing scientists to reevaluate their stance on the metabolic strategies used by dinosaurs.
Feathers and Scales
The discovery that certain dinosaur species had feathers is another common point of controversy in classifying dinosaurs as either birds or reptiles.
The possession of scales is one of the main properties of reptiles.
Birds, on the other hand, are the only living animals with their body covered by feathers.
Until the 1990s, most scientists thought dinosaurs only had scales on their bodies.
This was confirmed by the discovery of several dinosaur remains with patches of skin showing extensive evidence of scales.
Like other reptiles, dinosaur scales kept their body from losing water excessively in the harsh environment of the Mesozoic.
However, fossils discovered in more recent years have revealed evidence of feathers or proto-feathers on various dinosaurs.
This has led some paleontologists to question the traditional classification of dinosaurs and their relationship with birds and reptiles.
But while evidence suggests that some theropods, such as the dromaeosaurids and tyrannosaurs, had extensive plumage, it doesn’t exactly affect the classification of dinosaurs.
Feathers were the exception in the world of dinosaurs, not the rule.
Most dinosaurs were scaly reptiles, and only a few (especially those most closely related to birds) had feathers.
Also, in many of the feathered species, their feathers mainly served the purpose of insulation (and probably reproductive display) but were not useful for flight like that of modern birds.
The coexistence of feathers and scales in dinosaurs does not affect their classification as reptiles.
Instead, it paints a nuanced picture of their biology.
It also reflects the diverse range of adaptations that these dinosaurs exhibited, which made it possible for them to survive in a wide range of habitats.
Other Dinosaur Features
Although dinosaurs are reptiles, they do have some distinguishing features that set them apart from other members of the reptilian group.
We explored some of these differences earlier in this article, but here’s a more in-depth look at some of the most notable traits exhibited by dinosaurs.
Dinosaurs had an upright stance.
They were either bipedal (stood on two legs) or quadrupedal (walked on all fours).
In either case, their limbs were long and upright, held in a position parallel to their body.
This was a transformative departure from the sprawling posture of early reptiles and many of the modern members of this group.
Dinosaurs had holes in their pelvis that served as articulation points for their limb bones.
This shift in posture allowed for increased mobility for dinosaurs.
It also freed up the forelimbs of some groups of dinosaurs for more specialized functions, such as grasping prey or plucking food from trees.
The difference in posture also affected how dinosaurs navigated their ecosystem.
Although this varied from one dinosaur group to the other, most dinosaurs were swift and agile, while some of them, such as the massive sauropods, were much slower.
Evidence from fossilized trackways, communal nesting sites, and extensive bone beds suggests that some dinosaurs exhibited complex social behavior and interactions.
This is another deviation from typical reptilian behavior since most reptiles today are largely solitary.
Even in reptilian groups that form aggregations, such as the crocodilians, there are no notable social interactions between them.
For dinosaurs, scientists have discovered mass graveyards and other evidence that point to herding behavior and potential parental care.
Dinosaurs also evolved a wide range of unique anatomical features, such as elaborate crests, frills, or horns that likely served the purpose of visual signals and communication.
These features are less common in reptiles but more commonly observed in birds, the direct ancestors of the dinosaurs.
The Debate in Modern Paleontology
Most modern paleontologists agree with the Linnaean system that classifies dinosaurs as reptiles.
Scientists in this category emphasize the reptilian attributes of dinosaurs, such as their possession of scales, similarities in growth patterns, and various skeletal characteristics that connect them to their reptilian relatives.
However, more recent discoveries, starting from the 1990s, have been revealing a deeper connection between birds and dinosaurs.
This includes the discovery of dinosaurs with feathers and endothermic dinosaurs with the potential for warm-bloodedness.
All of this evidence points to a shared ancestry with modern birds.
Based on these findings, some scientists are reimagining dinosaurs as less of actual reptiles and more of a transitional lineage that bridges the gap between reptiles and birds.
Recent advancement in scientific research methods continues to revolutionize how we classify dinosaurs and their relationship with other animal groups.
Most notably, advancements in imaging, molecular analysis, and biomechanical modeling allow scientists to scrutinize fossils more than ever before.
These techniques allow us to uncover insights relating to the microscopic structures of bones or soft tissues like scales and feathers.
The potential for deciphering genetic clues from ancient DNA also paves the way to a better understanding of how dinosaurs are related to other reptiles and birds.
With more fossil discoveries and technological breakthroughs like this, we can begin to reexamine the evolution of dinosaurs and their closest relatives through a fresh lens.
So, are dinosaurs reptiles or birds?
This is one of the intriguing questions coming to the fore as we continue to explore the evolutionary history of dinosaurs and their relationship with other animal groups.
Although dinosaurs have been traditionally classified as reptiles, the discovery of their ancestral relationship with birds is causing scientists to reexamine this view.
Dinosaurs evolved from the same ancestors as other reptiles, then gradually evolved into birds.
This means birds are technically dinosaurs.
It would, therefore, be accurate to describe dinosaurs as a sort of transitional form between reptiles and birds.
They’re not quite birds, but not entirely reptiles either.
This view is similar to how amphibians are transitional between fish and reptiles, showing features of both groups but being a distinct clade of animals with their own unique adaptations.
The classification debate is still on, and as you research and learn more about this topic, you’ll find many more fascinating questions with equally interesting answers to satisfy your curiosity.
Jerry Young is a self-proclaimed prehistoric animal nerd. He has been fascinated with these ancient creatures for as long as he can remember, and his passion for them continues to this day. With his extensive knowledge and love for prehistoric animals, he is the perfect fit for Gage Beasley Prehistoric.