Are Alligators Related to Dinosaurs? (Classification, Similarities)


When we look at modern-day animals, especially fast and aggressive reptiles, it easily makes one wonder – are alligators related to dinosaurs? The answer is not so straightforward as there are no living dinosaurs of the fossils we find. Luckily, we have tools and scientists to help us understand the relationships, like classification, dinosaur fossils, and of course, the living alligators themselves. Not only that, by using tools like classification, we can also see what the similarities are even if dinosaurs do not exist anymore.

So, are alligators related to dinosaurs? Yes, both alligators and dinosaurs are archosaurs that have common ancestors from the mid-Triassic period, roughly 230 million years ago. They share common characteristics such as laying eggs, long hind legs, and short forelimbs and teeth that are set in sockets.

It’s also important to point out some of the details of modern classification that will enable you to see the bigger picture and then also to cover some of the differences as well as the similarities that dinosaurs and alligators had in common. Because both came from a common ancestor, we’ll take a look at the groups of those common ancestors and try to match it to a rough timeline. Let’s jump in.

I’ve researched information to write this article from many sources – multiple internet sites and magazines. However, some of the best reference knowledge is from books. If you are interested in checking out the best dinosaur books for adults on Amazon, you can find them by clicking here: Best Dinosaur Books for Adults

What Dinosaur Did Alligators Evolve From? Scientific Classification Can Help Clarify

To answer what dinosaur did alligators evolve from, we need to understand a bit of scientific classification to illustrate the thinking that scientists have.

are-alligators-related-to-dinosaurs-adventuredinosaurs
Alligators are related to dinosaurs and classifications systems help to show the similarities – AdventureDinosaurs

The Two Basic Classification Systems

Taxonomy and classification is something that continues to change and evolve as more extinct species are found and also because our knowledge of living species is growing. There are two common classification methods.

The Linnaean- In this system, which was created and developed by Carolus Linnaeus during the 1730s, it groups organisms by physical characteristics and doesn’t always consider or factor in what ancestors they came from. The classification that is often taught in schools uses this system.

The Phylogenetic – Developed in the early 20th century, in this system, the ancestors are given priority and grouped by ancestry, using characteristics to help discover who the ancestors are. It uses clades, which was coined by Lucien Cuénot in 1940.

Many paleontologists like using this system to classify dinosaurs. Some paleontologists or paleobiologists have further evolved the thinking of the Phylogenetic classification to include new fossil discoveries. I find it useful because they use clades or groups.

The cool thing about using these two classification systems is what it gives you when using both of them together. It helps to give a broader view of both dinosaurs and modern-day reptiles, using their strengths of each system as needed to make things more understandable, and in a visual way.

The classification system can help us understand how alligators are related to dinosaurs – AdventureDinosaurs

Modern classification uses a mix of Linnaean Classification and the Phylogenetic System. Although there are many additions and adjustments as biologists work with the system and publish new versions, one of the most easily visible is the inclusion of clades in the listing of taxonomy.

I think it is not only interesting but useful as well, to cover each system separately so that we get a deeper understanding.

Classifications of Ancestral Alligators Related to Dinosaurs

When taking a look at what dinosaur did alligators evolve from, it is probably more accurate to ask what group of dinosaur ancestors did alligators evolve from. In this way, we can take a look at classifications of extinct dinosaurs and find the closest group.

YouTube Video – Spinosaurus Compared to an Alligator

Watch YouTube Video by Nova PBS that compares a Spinosaurus fossil to a modern-day alligator. Also, further in this article is a table comparing the two – AdventureDinosaurs

All reptiles had common ancestors based on the characteristics of the Archosaurs. Using the Linnaean classification system, here is a simplified breakdown of Archosaurs, which highlights dinosaurs and reptiles.

figure-01-linnaean-chordata-to-archosauromorpha
Figure 1. The traditional Linnaean classification for alligators. Reptiles and alligators are in the Archosauromorphia sub-class – AdventureDinosaurs

The Linnaean classification system tells us that, in general, all Archosaurs were Amniotes. They laid eggs that didn’t need water to survive, the eggs had a substantial amount of yolk and had a harder shell so it could endure on land.

I’ll detail the common features for dinosaurs and alligators later in the article.

Here is a shortlist of the physical features of Archosaurs that evolved and that are present in ancestors of dinosaurs and alligators (Source):

A lightened skull which enable more organ tissue developments
The presence of serrated teeth set in sockets on the top and bottom of the jaw
The fifth toe on the claw reduced in size
Double row of bony plates which ran along the backbone to its tail

Now let’s take a look at the Phylogenetic classification.

As mentioned, sorts animals by ancestry and gets help from the animal’s characteristics to better determine its ancestors as well as using clades (or groupings) to visualize the classification better. In structure, it uses Linnaean classification as a base.

I’ll simplify the structure to stay focused on showing how alligators are related to dinosaurs, but keep in mind that there are volumes of research on each classification topic, and this is just a snapshot.

figure-03-cladogram-Sauropoda-to-Archosauria-cladogram
Figure 2. Using Phylogenic cladograms, the common clade of Archosauria has clades of Pseudosuchia (common ancestry of alligators) and Orinthosuchia (common ancestry of dinosaurs and birds) – AdventureDinosaurs

The ancestral amniotes were divided into two main categories, Reptiles, and Synapsids.

Reptiles were divided into three categories, which led two of them to modern reptiles. The third category was Diapsids. Diapsids were evolving between 250 to 201 million years ago at the end of the Triassic period.

physical-characteristics-of-alligators-compared-to-dinosaurs-adventuredinosaurs
The modern-day alligator looks similar to its Archosaur ancestors – AdventureDinosaurs

Starting in the Jurassic period, some animals started to evolve with new characteristics. From this point, there is a stronger relationship of alligators to dinosaurs with a common group of ancestors.

The category of Diapsids had four sub-categories. The Archosaurs were the ancestors of both dinosaurs and alligators.

figure-02-linnaean-amniota-to-diapsid
Figure 3. The Linnaean classification has all reptiles as Amniotes and has four sub-classes. Most reptiles belong to the sub-class Diapsida – AdventureDinosaurs

One way to answer the question about what dinosaur group did alligators evolve from simply is, based on what we have covered, to say that:

Alligators are Archosaurs that evolved from dinosaur ancestors that were Amniotes (egg-laying) and Diapsids (having two arches on the skull).

Moving further on to the sub-groups of Archosaurs, about 141 million years ago during the Jurassic period, the branch of Archosaurs divided into three main groups.

● The first sub-category was Pterosaurs. These were the reptiles that could fly.

● The second was Dinosaurs. From this point forward, the era of dinosaurs had begun, and they ruled the world for millions of years until the C-T Extinction.  This grouping of dinosaurs evolved, resulting in hundreds of new species, including carnivores like T-Rex and the plant-eating, long-necked Sauropods.

● The third and last group was Crocodilians from which modern-day alligators and crocodiles evolved from. 

Let’s look at the classification of crocodiles and alligators for further clarity on how alligators are related to dinosaurs.

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Classification of Reptiles – Pseudosuchia

Let’s take a look at the Phylogenetic system’s classification of Reptiles, which are Pseudosuchia. In the Phylogenetic system, it can be more detailed with multiple branches (or Clades) as it links extinct animals as well as current living animals.

Here are the simplified versions of Phylogenetic clades for Vertebrata, which lead to dinosaurs and reptiles.

Finally, here is the combined modern version from 2011. I use this one because it details the clades specifically to Crocodilia and Dinosaurs, which is the focus of the article. See Figure 4 below. It will help with the discussion.

figure-05-phylogenetic-pseudosuchia-and-avemetatarsalia
Figure 4. Phylogenic classification for Archosauria which include the clades of Pseudosuchia from which alligators belong and Avemetatarsalia, from which dinosaurs belong – AdventureDinosaurs

Modern classification for Archeosaurs definition is structured as:

Kingdom -Animalia
Phylum -Chordata
Class -Reptilia
Clade -Archosauria

Classification of Dinosaurs – Avemetatarsalia- (From Crown Group Archosauria)

The Phylogenic cladogram of Avemetatarsalia (Source) has branches grouped, starting with Avemetatarsalia. It has two branches, Orinthodira and Dinosauromorphia. See Figure 4 above.

It is here where you can see the connection between birds and dinosaurs. Up until here, dinosaurs and birds have common features. After this, these two branches then develop separately.

From Dinosauromorphia, it further groups into Dinosauroformes and then Dinosauria, where the clades include the species of dinosaurs. It breaks down further into Saurischia and Theropoda.

prehistoric-alligators-may-have-hunted-t-rex-adventuredinosaurs
Ancient alligators lived together with dinosaurs and it is even thought that alligators hunted them – AdventureDinosaurs

Did Alligators Live with Dinosaurs? What Time Period Are Alligators and Dinosaurs From?

Dinosaurs lived throughout the Mesozoic era, the earliest estimated to have appeared during the Triassic period. They evolved from reptiles, and the distinguishing characteristics as per modern classification is that they were Amniotes, had Diapsid skull and the Orinthian hip.

It’s exciting to realize the fact that during the Mesozoic era, other important species were living alongside the dinosaurs that were evolving and also had an impact on the prehistoric eras. It’s often overlooked since dinosaurs were so predominant.

One of these groups was the Crocodilia, from which alligators came from.

The geological eras in which the dinosaurs lived was the Mesozoic, ending in the C-T Extinction that happened in the Cretaceous period at the end of the Mesozoic. The ancestors of alligators survived the C-T Extinction and survived in the modern-day Cenozoic geological era.

Did Alligators Live with Dinosaurs? Ancient Species of Alligator

Yes, ancient species of alligators (and multiple species of crocodiles) lived at the same time as dinosaurs.

Several species of the order Crocodilia that include Crocodiles and Alligators of the modern-day evolved during the Mesozoic era. In terms of looks, the amazing thing is that modern-day alligators and crocodiles didn’t look too different from those of today.

The earliest alligator ancestors evolved during the Triassic, then in the late Cretaceous, one fossil that was found to be an Alligatoroid was Brachychampsa.

Brachychamsa evolved during the late-Cretaceous, and fossil remains have been found in Montana, USA, and Kazakhstan. As mentioned above, it looked similar to modern-day alligators. One biological difference was an enlarged tooth in the upper jaw of the Brachychamsa (Source Wikipedia)  

Deinosuchus was also a closely related ancestor to alligators, classified as a match to modern-day crocodiles. Like Brachychamsa, it lived during the late-Cretaceous. Its fossils have been found in North America and Mexico. It was considerably larger than modern-day alligators at 12m (39 ft), and scientists believe it preyed on plant-eating dinosaurs. (Source)

prehistoric-alligators-lived-during-the-late-cretaceous-adventuredinosaurs
Ancient crocodiles like Deinosucus and alligators like Brachychamsa lived in the late-Cretaceous period – AdventureDinosaurs

What Time Period Did Alligators and Dinosaurs Live Together

If we talk about the time period at which dinosaurs and alligators were sharing the world, we have to go back a long time. As we know, the first dinosaurs appeared approximately 230 million years ago. 

Some of the first dinosaurs, such as the Plateosaurus was one of the first dinosaurs that appeared, and they lived until the Cretaceous-Tertiary mass extinction (C-T Extinction). Some species were thought to be the survivors of C-T Extinction.

The first of the ancestors of alligators emerged from Diapsids about 245 million years ago. These were the Archosaurs, and they were the common ancestor of alligators and dinosaurs. The first crocodilians approach about 90 million years ago during the Cretaceous period.

The first alligator species, now extinct, was Brachychampsa. They lived about 93.5 million years ago and went extinct 63 million years ago.

So, we can estimate that during the time of dinosaurs, there were an ancient species of alligators that existed, and they shared the earth about 30 million years, which is quite a long time frame.

Dino and Alligator Anatomy – What Are the Differences and Similarities?

We have already covered the ancestry of dinosaurs and alligators. To find the differences between them, the anatomical differences and similarities between dinosaurs and alligators can be looked at with the classification characteristics and clades.

Common Anatomy of Dinosaurs and Alligators

Dinosaurs are the reptiles of land. There were many kinds of dinosaurs, they are different in each species, but here I’ll point out their most common anatomy.

They have acetabula or hip socket which contains ilium, ischium, and pubis
They have acromion a bony ridge that binds the clavicle bone
They have angular is the bone of the lower jaw
They have three sets of vertebrae (cervical, dorsal and caudal) which connect the neck to the tail area
The back legs contain femur, fibula, and tibia bone
They have dorsal vertebras attached with rib sections
The front legs contain bones of humerus, radius, and ulna

An Alligator’s anatomy matches closely with birds. Scientific classification represents this with the classification structure, characteristic features, and having related clades that point out its ancestors. Here are some top-level points about alligator anatomy:

● The skull contains mandibles
● They contain cervical, thoracic, lumbar, and caudal vertebrae
● The thoracic vertebrae have an attachment to the rib section. And caudal vertebrae’s make the tail section
● Under the ribs they contain sternum
● They also contain the front pair of legs and back pain. The front legs contain humerus, radius, and ulna
● The back legs contain femur, fibula, and tibia. And foot made up of tarsal and metatarsals

A-spinosaurus-compared-to-an-alligator-adventuredinosaurs
The Spinosaurus was related to alligators but there were some characteristics that are different. They also had similar ancestors – AdventureDinosaurs

TABLE – Similarities of Spinosaurus aegyptus Dinosaur vs. Alligator

In this table, we look at the similarities between a dinosaur and an alligator. I chose the Spinosaurus as the dinosaur example as the recent fossil finds such as the Spinosaurus aegyptus look amazingly like an alligator ancestor. Now let’s try to spot the differences.

Final Thoughts

Over the years, scientists have been studying and discussing not only how dinosaurs are related to alligators by using classifications together with fossils to make the connections, but even adding clades to clarify the relationships further. It gives great insight into whether alligators are related to dinosaurs.

Although there are modern-day animals that are closer to dinosaurs than alligators, I can’t help but think when I see an alligator in the zoo or on video that I’m looking at something alive and well during the time of the dinosaurs or even had similar features. Just take a look at the Spinosaurus, whose skull and tail resemble alligators!

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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