We are independent & ad-supported. We may earn a commission for purchases made through our links.

Advertiser Disclosure

Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.

How We Make Money

We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently from our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

What are the Differences Between Synapsids and Sauropsids?

Michael Anissimov
Updated Mar 01, 2024
Our promise to you
InfoBloom is dedicated to creating trustworthy, high-quality content that always prioritizes transparency, integrity, and inclusivity above all else. Our ensure that our content creation and review process includes rigorous fact-checking, evidence-based, and continual updates to ensure accuracy and reliability.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

Editorial Standards

At InfoBloom, we are committed to creating content that you can trust. Our editorial process is designed to ensure that every piece of content we publish is accurate, reliable, and informative.

Our team of experienced writers and editors follows a strict set of guidelines to ensure the highest quality content. We conduct thorough research, fact-check all information, and rely on credible sources to back up our claims. Our content is reviewed by subject matter experts to ensure accuracy and clarity.

We believe in transparency and maintain editorial independence from our advertisers. Our team does not receive direct compensation from advertisers, allowing us to create unbiased content that prioritizes your interests.

Synapsids include mammals and our distant ancestors, including pelycosaurs and therapsids, while sauropsid is another word for reptiles. Synapsid means "fused arch," a reference to skull structure. Another name for a synapsid is theropsid, which means "beast face," in contrast to sauropsid, which means "lizard face." Synapsids are sauropsids are the two evolutionary lineages of amniotes, which includes all non-amphibians tetrapods and their descendants (such as whales, which descended from tetrapods but lost their legs when they became exclusively marine). Early synapsids used to be called "mammal-like reptiles," but this is a misnomer, as they were not reptiles at all.

Synapsids and sauropsids split off from each other approximately 320 million years ago, during the late Carboniferous period. Both looked like small lizards. At the time, tetrapods had existed in the water for about 45 million years, and on land for at least 20 million years. Both are amniotes, that is animals with complex eggs that can be laid on land, in contrast to amphibians, which must lay their eggs in water. Before synapsids and sauropsids split, there were some stem-group amniotes that didn't fit into either group. Amniotes were destined to inherit the Earth because they are the only land vertebrates that can venture significant distances from water and still survive.

The difference between sauropsids and synapsids is defined in terms of the openings in their skull. Synapsids have an extra hole, used to reduce skull weight and provide an attachment point for jaw muscles. Sauropsids began with no holes in their skull, then developed one pair, with each hole behind the eyes. Initially, both groups were "cold-blooded" (ectothermic).

Since the late Carboniferous, the colonization of the land by large creatures has been an evolutionary arms race between synapsids and sauropsids. Synapsids got off to a great start, diversifying more rapidly than sauropsids and giving rise to most of the large animals of the Permian, including the successful pelycosaurs, some of which were as big as trucks, and had the only apex predators of the time.

At the end of the Permian, the largest synapsids went extinct, leaving many niches open for exploitation. The sauropsids took advantage, eventually giving rise to dinosaurs, which dominated the Earth throughout the Mesozoic. About 65 million years ago, the tables turned again, when an asteroid wiped out all non-avian dinosaurs. Synapsids ruled the world again, in the form of mammals. Eventually, synapsids gave rise to humans, arguably the most evolutionarily successful terrestrial vertebrate in the history of life on Earth.

InfoBloom is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Michael Anissimov
By Michael Anissimov

Michael is a longtime InfoBloom contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Discussion Comments

By anon258380 — On Apr 01, 2012

To say that "Early synapsids used to be called "mammal-like reptiles," is a misnomer, as they were not reptiles at all." It is a truism, and it is only true because certain phylogeneticists decided that, in order to fit "the reptiles" into a tidy box that fitted their self-imposed rules of cladistics, they would have to redefine what they meant by "reptile". Before this, everyone knew what a reptile was -- scientists and laymen alike.

To force them to fit into a bonafide Clade, they had to decide what to do about the birds and the mammals. For the birds, they had a problem since they are more closely related to crocodiles than the crocs are to other reptiles. The only option open to them was to classify all birds as reptiles (which to me seems obvious nonsense).

Since mammals are so clearly not mammals, they had to exclude all of the ancestors of the mammals, hence the pelycosaurs and therapsids were reclassified and no longer considered reptiles.

I'll bet if you met Dimetrodon today there would be little doubt in your mind that you had just seen a reptile! I have no problem at all with phylogenetics and cladistics; I just wish they had confined themselves to using the perfectly reasonable term 'sauropsida' and left the already well understood term 'reptile' alone.

By pastanaga — On Oct 16, 2011

@Mor - You are forgetting that birds are also sauropsids. Considering the vast range of birds, I would hesitate to even say that synapsids are the dominant group at the moment. Sure, humans seem powerful, but for sheer numbers, chickens or sparrows or pigeons or seagulls would seem to be doing all right.

Put all those groups together and you've got something. I'm not sure how we are measuring success, but I wouldn't discount birds if it came to taking over the world.

By KoiwiGal — On Oct 15, 2011

@Mor - Well, if the sauropsids did manage to take over the earth again, it would be with species we hadn't seen before, most likely.

I mean, the dinosaurs weren't waiting in the wings to take over from the synapsids that were there before them.

After the extinction event, they gradually evolved to take their places, and they evolved to fill particular niches as well.

That's one of the reasons I don't think it's going to be all that strange to meet alien life forms that look familiar to us, if they evolved on a similar planet. There are always going to be small flying things, big flying things to eat the small flying things, swimming things, crawling things, running things, and so forth.

It's the same reason a shark and a dolphin look so much alike, even though they both evolved independently.

And the current day sauropsids would do it again, if they managed to get a foothold in time to beat out other species.

By Mor — On Oct 14, 2011

It's interesting to see that pattern, with the rises and falls of synapsids and sauropsids.

Since synapsids are the dominant group at the moment, it makes you wonder whether the sauropsids would have a chance if there was another extinction event?

Because at the moment, they don't really have any contenders. I might believe that crocodiles or maybe Komodo dragons might "conquer" the earth, but aside from that, I'm not sure who I would put my money on.

Too bad, I suppose, that all the dinosaurs died out.

Michael Anissimov

Michael Anissimov

Michael is a longtime InfoBloom contributor who specializes in topics relating to paleontology, physics, biology,...

Read more
InfoBloom, in your inbox

Our latest articles, guides, and more, delivered daily.

InfoBloom, in your inbox

Our latest articles, guides, and more, delivered daily.