Exploring Whether Alligators and Crocodiles Are True Dinosaurs

Leave a comment / / Updated on: 24th September 2023

The name “dinosaur” means “terrible lizard,” and when you think about the biggest and baddest lizards today, alligators and crocodiles might come to mind.

Ever since the first dinosaur fossils were discovered, people have been captivated by these incredible creatures and wondered if they or any of their close relatives are still around today. 

This has prompted comparison between the crocodilians and the dinosaurs. 

But are alligators and crocodiles really dinosaurs hiding in plain sight?

are crocodiles and alligators dinosaurs
A group of crocodiles | Simon Dux via Getty Images

It’s an ongoing debate that we can now answer with some degree of certainty. 

The short answer is no!

Alligators and crocodiles are not true dinosaurs. 

But to fully understand why this is so, we must take a few steps back and explore the evolutionary relationship between crocodilians and dinosaurs. 

are crocodiles and alligators dinosaurs
Alligator | BirdImages via Getty Images

A quick search and you’ll find that crocodiles and alligators are just as old as the dinosaurs; both animals lived alongside each other at some point in geologic history. 

They also share a common ancestry because they descended from the same ancestors. 

That explains many of the similarities between them, but it isn’t enough to say they’re the same. 

In this article, we’ll explore the question of whether alligators and crocodiles are true dinosaurs in greater detail to help you understand the relationship and distinctions between them. 

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

Shared Ancestry and Evolution

are crocodiles and alligators dinosaurs
Crocodile close-up | Veni via Getty Images

To understand prehistoric life, you’ll have to untangle an intricate web of evolutionary relationships between various animal groups. 

One of the key concepts to understand in order to unravel these relationships is the idea of common ancestry. 

When two animals are alike but not the same, it could be because they share a common ancestor. 

This is an ancestral organism that gave rise to these two distinct lineages at some point in the prehistoric past. 

How similar the two lineages will be depends on how long ago they split off from this common root. 

are crocodiles and alligators dinosaurs
You said you liked crocodiles? | KongSan via Getty Images

This is the case with dinosaurs and the crocodilians. 

Sharing a common ancestor explains some of their physical similarities, such as their rubbery armored skin, fierce teeth, sharp claws, and even similarities in behavior, such as their reproductive habits. 

The archosaur lineage is the common ancestor of both dinosaurs and crocodiles. 

Pterosaurs and avian dinosaurs also descend from this same lineage. 

Evolutionary History of Archosaurs 

Gage Beasley Prehistoric's Dakosaurus Concept
Gage Beasley Prehistoric’s Dakosaurus Concept

Archosaurs are a subgroup of the reptilian group known as the archosauriforms

The archosauriforms evolved during the Late Permian Period, about 260 million years ago. 

But the true archosaurs evolved during the Early Triassic, about 250 million years ago, after the Permian–Triassic extinction event.

The subsequent diversification of this group of reptiles laid the foundation for the emergence of crocodiles, dinosaurs, and a variety of other related species.

During the early stages of archosaur evolution, these reptiles exhibited a suite of adaptations that set them apart from their reptilian relatives. 

are crocodiles and alligators dinosaurs
Life restoration of S. galilei | Nobu Tamura via Spinops

These include a more erect posture, improved locomotion, an efficient respiratory system, and other adaptations that favored their survival in the predominantly arid Triassic environment.  

As archosaur evolution continued during the Early Triassic, they branched into two main lineages: the pseudosuchians and the avemetatarsalians. 

The pseudosuchians were a diverse group of reptiles, including the ancestors of modern-day crocodiles, alligators, and their relatives. 

On the other hand, the avemetatarsalians, like the Alamosaurus, gave rise to the dinosaurs and their descendants, which ultimately included birds.

Alamosaurus 3D illustration
Alamosaurus 3D illustration – Warpaintcobra via Istock

The pseudosuchians diversified into various forms throughout the Triassic. 

Some of them developed more elongated snouts and other specialized adaptations that allowed them to occupy both terrestrial and aquatic environments. 

By the end of the Triassic Period, an extinction event wiped out all the other pseudosuchians leaving only two groups: Sphenosuchia and Crocodyliformes. 

The latter group—Crocodyliformes—is the ancestor of modern crocodiles, alligators, and caimans. 

Meanwhile, the evolution of the avemetatarsalians took a different trajectory. 

The first true dinosaurs emerged from the ashes of the end-Triassic extinction event and soon evolved into a wide range of body sizes and shapes. 

Gage Beasley Prehistoric's Pentaceratops Concept
Gage Beasley Prehistoric’s Pentaceratops Concept

They also adapted to various ecological roles, and the numerous species in this group soon became the most dominant land animals for the rest of the Mesozoic Era

So while crocodiles and dinosaurs share a common ancestor within the archosaur lineage, their subsequent evolutionary paths led to distinctions in their morphology, physical adaptations, and ecological roles. 

For instance, while crocodiles stuck to a semi-aquatic lifestyle, dinosaurs occupied terrestrial and aerial niches. 

As a result, the crocodilians developed traits that favored their ability to hunt prey in aquatic environments, which are still present today. 

Distinctive Features of Crocodiles

are crocodiles and alligators dinosaurs
Saltwater crocodile leaps out of the water | chameleonseye via Getty Images

As they evolved, crocodiles developed a distinctive set of anatomical features that made them different from dinosaurs. 

Some of these features have evolved to suit their semi-aquatic lifestyle as against the largely terrestrial lifestyle of the dinosaurs. 

They include: 

  • Long, streamlined bodies complemented by powerful limbs, which allow them to move swiftly on land and in water. 
  • Formidable jaws, armed with rows of sharp teeth, well-suited for catching and gripping prey. 
  • Eyes and nostrils situated atop their heads, allowing them to remain mostly submerged while maintaining awareness of their surroundings.
  • Thick-plated skin made up of scales that are arranged in a regular pattern. The scales are made of Keratin, the same material that makes up human hair and fingernails. 

It is worth noting that modern crocodiles differ slightly from alligators in terms of their general anatomy, behavior, and habitat. 

The most notable anatomical difference is in the shape of their snout. 

Crocodiles have long snouts with a V-shaped tip, while alligators have a shorter and broader snout with a U-shaped tip.

are crocodiles and alligators dinosaurs
Nile Crocodile | BirdImages via Getty Images

Because of the shape of its snout, a crocodile’s lower teeth are visible when its mouth is closed. 

This is not the case with alligators, whose lower teeth are always hidden when their mouth is closed. 

In terms of their behavior, crocodiles are generally more aggressive than alligators. 

They’re also more adaptable because they have salt glands, allowing them to live in both saltwater and freshwater. 

Crocodiles are found in Africa, Asia, Australia, and the Americas, while alligators are only found in the Americas.

Crocodiles as Living Fossils

are crocodiles and alligators dinosaurs
Crocodile swimming through blue water. | Tammy616 via Getty Images

Crocodiles and some other animals today are commonly referred to as living fossils. 

The term refers to species that evolved several million years ago and have managed to endure till present times with minimal morphological changes. 

Crocodiles and alligators fit neatly into this category. 

The evolution of their ancestors happened more than 200 million years ago. 

Even present-day crocodiles came on the scene during the Cretaceous Period about 95 million years ago. 

are crocodiles and alligators dinosaurs
Top view of a wild crocodile in blue waters of Lake Everglades | Wirestock via Getty Images

They (and the birds) are the only archosaurs that managed to survive the end-Cretaceous extinction event that wiped out the dinosaurs. 

What’s probably more remarkable is the fact that present-day forms share striking similarities with their prehistoric ancestors. 

Although they evolved many of their physical and behavioral adaptations several million years ago, the crocodilians haven’t changed a lot since their early days. 

Some scientists think this is because crocodiles and alligators found the most suitable anatomical features that favored their survival in the habitat and ecological niche they occupied. 

This allowed them to remain in a state of evolutionary stability without changing significantly. 

Differentiating Dinosaurs

are crocodiles and alligators dinosaurs
Dinosaurs | ALLVISIONN via Getty Images

So if dinosaurs and crocodilians have similar prehistoric origins and descended from the same common ancestors, what makes them different? 

Aside from the fact that dinosaurs are now extinct, both groups of animals would have looked completely different even if dinosaurs were still alive today. 

That’s because dinosaurs had unique bone structures and other anatomical features that would have set them apart from crocodiles, alligators, and other reptiles. 

One of the main differences between dinosaurs and other reptiles is that the dinosaurs had upright limbs positioned under their body instead of legs sprawling out to the side like the crocodilians. 

Their thigh bones are almost parallel to the ground and are connected to a hole in the hip socket. 

Hip joints and hindlimb postures of: (left to right) typical reptiles (sprawling), dinosaurs and mammals (erect), and rauisuchians (pillar-erect) | Fred the Oyster via Wikipedia (CC BY-SA 4.0)

This permits an upright stance and makes dinosaurs more agile on the ground compared to crocodilians that have to walk or run with a side-to-side motion of their body. 

Other anatomical differences between dinosaurs and crocodiles or alligators include: 

  • Hollow bones: many dinosaurs had hollow bones, which reduced their overall body weight and made them more agile. 
  • S-shaped neck: many dinosaurs, especially the larger theropods, had an S-shaped neck, which allowed for a wider range of movement and facilitated better balance.
  • Fused pelvis: the pelvis of dinosaurs was often fused or more tightly connected than in other reptiles. 
  • Scales: in the past, scientists assumed that all dinosaurs had scales. This has been found to be inaccurate. Many species of dinosaurs had feathers on their body, similar to modern birds. 

In addition to these physical differences, dinosaurs had other behavioral and ecological peculiarities that set them apart from crocodilians. 

For instance, with a few exceptions (such as the Spinosaurus), most dinosaurs were fully adapted to terrestrial life. 

Iguanodon dinosaurs herd | Elenarts108 via Getty Images

Also, while crocodiles and alligators are carnivorous predators, Dinosaurs diversified into a wide range of ecological roles. 

They included herbivores, carnivores, and omnivores with a wide range of feeding habits. 

Dinosaurs exhibited different group behavior as well, with some of them forming hunting packs, family groups, and herds. 

Even burrowing forms of dinosaurs like the Oryctodromeus have been discovered with habits similar to that of modern-day burrowing mammals. 

With only 28 living species, crocodilians are generally less diverse compared to dinosaurs.  

Fossil Evidence and Paleontology

Postosuchus (Rauisuchidae) and Desmatosuchus (Aetosauria), both pseudosuchians | Kim Alaniz via Flickr (CC BY 2.0)

Since crocodiles, alligators, and their relatives have been around for several million years, their fossils are unsurprisingly abundant in ancient rocks from the Jurassic Period till more recent times. 

The fossil record shows that the earliest pseudosuchians were small, two-legged terrestrial animals. 

Two of such crocodilian ancestors discovered so far are Erpetosuchus and Doswellia

Erpetosuchus lived during the Late Triassic and was first discovered in Northeastern Scotland in the Late 1800s. 

Doswellia had a more crocodilian form and lived in North America during the Late Triassic Period. 

In the Jurassic, these prehistoric crocodiles began to make a transition from terrestrial to aquatic habitats. 

Kaprosuchus | MR1805 via Getty Images

Consequently, they developed a more elongated body, narrow snouts, and powerful jaws. 

Their diet also continued to change throughout the Jurassic and Cretaceous period. 

Some of them, such as the Cretaceous Stomatosuchus, survived on a diet of plankton and krill similar to modern whales. 

As mentioned earlier, the crocodilians haven’t changed much since their early days. 

Many of the Late Cretaceous and Early Cenozoic forms of this reptile looked surprisingly similar to modern forms. 

They had long snouts, powerful tails, armored skins, and a sprawling gait. 

Life restoration of Aegisuchus witmeri, a giant, flat-headed, ornamented crocodyliform from the Late Cretaceous of northern Africa. | Henry P. Tsai, University of Missouri via Plos One

However, the physical similarities aside, several exotic and unfamiliar crocodilian relatives have been discovered in the fossil record. 

Some of them, like the Early Cretaceous Sarcosuchus and Late Cretaceous Deinosuchus. grew to massive sizes of over 30 feet. 

They also varied in their habitats as well. 

While some of them, like the Aegisuchus, lived in rivers, others, like the Dakosaurus, developed flippers and lived exclusively in the ocean. 

Even some fast-running terrestrial crocodilians like the Baurusuchus have been identified in the fossil record. 

This shows that the crocodiles were once more diverse than they currently are, as evidenced by the fossil record and their evolutionary history. 

Prehistoric Crocodile Species

As the fossil record shows, a diverse array of crocodilians or crocodile-like creatures once occupied the Earth’s terrestrial and aquatic landscapes. 

Because of how long ago they lived, many of them are wrongly characterized as dinosaurs even though they’re not. 

Some of the most notable prehistoric crocodile species include: 


Gage Beasley Prehistoric’s Sarcosuchus Concept

Often referred to as the “SuperCroc,” Sarcosuchus was a massive crocodilian that inhabited what is now Northern Africa during the Early Cretaceous Period, approximately 112 million years ago. 

Although it looked and behaved similar to modern crocodiles, Sarcosuchus was significantly bigger.

It was over 30 feet long and is often regarded as the largest crocodile to have ever lived. 


Gage Beasley Prehistoric’s Deinosuchus Concept

This crocodile’s name translates as “terrible crocodile,” which is fitting considering it was one of North America’s most formidable predators of the Late Cretaceous ecosystem. 

Deinosuchus had an impressive set of sharp teeth with a robust body. 

This massive reptile was more closely related to modern alligators than to crocodiles. 


Boverisuchus | Photo via Dinosaur Wiki

Although many prehistoric crocodilians were massive, some were smaller, about the same size as modern species. 

An example is the Boverisuchus, which measured about 3.6 meters on average. 

Modern Scientific Insights

Freshwater Crocodile | mollypix via Getty Images

The idea that crocodiles are related to dinosaurs dates back more than a century ago. 

However, the distinction between these two animal groups was established early in the history of their story. 

The first scientific paper that recognized the dinosaurs as a distinct group of Mesozoic Era reptiles distinguished between them and other reptiles, including crocodiles. 

So early researchers treated them as completely distinct and unrelated groups. 

The establishment of the Archosauria group in 1869 by Edward Drinker Cope in 1869 helped to establish an ancestral relationship between dinosaurs, crocodilians, birds, and other reptilian groups. 

Subsequent research on the archosaur group focused on explaining the differences between the different members of the group and determining when they split into different evolutionary lines. 

Most of the early studies involved analyzing fossil evidence and comparing the skeletal similarities and distinctions between dinosaurs and crocodiles. 

Gavial Skeleton, prepared and articulated by Skulls Unlimited International. | Sklmsta via Wikipedia (CC0)

More recently, scientists have been investing efforts not just to establish the differences between these groups of archosaurs but also to determine how they’re related to each other. 

Advancements in genetic studies and molecular biology have helped to shed new light on the intricate web of genetic relationships that connect crocodiles, dinosaurs, and other archosaurs. 

One pivotal discovery that has made this easier for scientists is the recognition that birds are actually the closest living relatives of dinosaurs. 

With this knowledge in mind, scientists can now carry out in-depth genetic sequencing of crocodilian genomes and compare them to that of birds, the closest dinosaur relatives around. 

One study, published in the Nature Journal in 2014, sequenced the genomes of three crocodilian species: the American alligator, the Indian gharial, and the saltwater crocodile. 

The researchers then compared the crocodilian genomes to the genomes of birds and other reptiles. 

A crocodile from the Louisiana swamp grins for the camera | beckyjenson via Getty Images

They found that the crocodilian genomes were more similar to that of birds than any other group of reptiles. 

This suggests that crocodiles and birds are more closely related to each other than either group is to other reptiles.

Since no intact DNA has so far been found for fossil dinosaurs, modern analytics to compare dinosaurs and crocodiles have to rely on chromosomes and genomes derived from different species of birds. 

Using these tools and molecular clocks, based on the rate of genetic mutations, scientists have been able to estimate likely divergence times between different archosaur lineages. 

This approach, combined with the available fossil evidence, has made it possible to identify valuable reference points within the evolutionary timelines of these different groups to determine their branching points within the larger archosaur lineage. 

With future advancements in molecular biology, such as next-generation sequencing techniques and comparative genomics, our understanding of the evolutionary history of crocodiles, dinosaurs, and archosaurs will continue to improve. 

Theories and Debates

Saltwater crocodile in the Adelaide River, Northern Territory, Australia. Closeup of head. | benedek via Getty Images

But while the answer to the question of dinosaur and crocodilian relationship might seem straightforward, there are still some gray areas and controversies that are yet to be resolved. 

Different scientific theories and debates still exist regarding the evolutionary relationships between these groups. 

One of the fundamental debates centers on the precise closeness of the relationships among archosaurs. 

In the past, scientists believed crocodiles and birds (avian dinosaurs) were more closely related than they were to dinosaurs. 

However, fossil discoveries that show birds as the direct descendant of theropod dinosaurs and recent genetic studies suggest that the crocodiles were closer to the base of the archosaur tree than initially assumed. 

A cast of the pterosaur Dorygnathus banthensis at Göteborgs Naturhistoriska Museum, Gothenburg, Sweden. | Gunnar Creutz via Wikipedia (CC BY-SA 4.0)

Based on these findings, dinosaurs and birds form a separate branch known as the Avemetatarsalia, distinct from the pseudosuchians that gave rise to the crocodilians. 

Another intriguing aspect of crocodilian evolution is the diversity of the crocodylomorphs themselves (the broader group that includes crocodiles and their extinct relatives). 

Fossil evidence reveals a wide array of crocodile-like forms with different physical attributes. 

This diversity prompts questions about the ecological niches these creatures occupied and why the current crocodilians seem less diverse than their ancestors.

Living crocodilians show even less diversity compared to dinosaurs and birds (28 species compared to more than 10,000 species), which is quite puzzling considering both groups evolved from the same root and survived the end-Cretaceous extinction together. 


So are alligators and crocodiles true dinosaurs? 

We have been able to establish that the answer to that question is no! 

Although they both share a common ancestry, they diverged along separate branches of the archosaur family tree long ago in the Triassic. 

This split early on in their evolutionary history explains why both groups exhibit completely different physical and behavioral attributes. 

Yet, crocodiles and birds are the closest relatives of dinosaurs that we have around. 

Crocodiles are even more morphologically similar to some dinosaurs because they have managed to retain their reputation as dangerous, menacing predators that dinosaurs are known for. 

So even though alligators and crocodiles might not be true dinosaurs, we can still look at them and appreciate a time long ago when beasts like them roamed Earth’s terrestrial landscape as the top predators of the terrestrial ecosystem.


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