Synapsida

Long ago, a special group of animals lived.

They were the first ancestors of mammals. These animals had one hole in their skulls. This hole was behind each eye. The hole helped their jaw muscles work better. This gave them a very strong bite.

They also had different kinds of teeth. Some teeth were long and sharp. Others were flat for grinding food.

These old animals led to all mammals today.

Synapsids are a group of animals that includes all mammals. They are also the relatives of mammals that lived long ago.

You can tell a synapsid apart by its skull. It has one hole behind each eye. We call this a temporal fenestra. This hole helps jaw muscles attach to the skull. This makes the bite much stronger.

These animals also had special teeth. They had different kinds like canines and molars. Over time, their jaws began to change. In reptiles, the lower jaw has many small bones. In mammals, the lower jaw has only one main bone. Some of those old jaw bones even moved into the ear!

Synapsids lived a long time ago. Some were large predators like Dimetrodon. Others, like the Lystrosaurus, lived in huge numbers.

Many synapsids died out during mass extinctions. However, one group called cynodonts survived. These small animals eventually led to the mammals we see today.

Synapsids are a very important group of animals. They include all mammals alive today and their extinct relatives. Scientists call them a clade, which is a group that shares a common ancestor.

You might hear people call them "mammal-like reptiles." However, that name is actually outdated. Modern science shows they are a separate branch from true reptiles. Today, we often call these ancient relatives stem mammals or proto-mammals.

One way to spot a synapsid is by looking at its skull. They have a single temporal fenestra, which is an opening behind each eye socket. This hole creates a bony arch under the skull roof.

This opening was very useful for the animal. It allowed larger jaw muscles to attach to the skull. This made their bite much more efficient and strong. Over time, these openings changed as mammals evolved.

Synapsids have a very long and interesting history. They first appeared about 318 million years ago during the Late Carboniferous period.

Early members were often lizard-like with a sprawling walk. Later, more advanced groups called therapsids appeared. These animals had a more upright pose and maybe even hair. Many of these groups lived during the Permian period. This was a time when synapsids were the main animals on land.

There are many amazing facts about these creatures. During the Permian, they were the biggest animals on land. Some predators, like Dimetrodon, were at the top of the food chain.

Other animals, like the Lystrosaurus, were very successful. At one time, Lystrosaurus made up 95% of all land species! Even though many groups died out during mass extinctions, some survived. The cynodonts were a special group that lived past the Triassic period.

We can see how synapsids are related to us by looking at their bodies. Their teeth changed from simple shapes to different types like canines and molars.

Their jaws changed too. In reptiles, the lower jaw has many small bones. In mammals, the lower jaw is just one large bone called the dentary. Some of those old jaw bones even moved to become middle ear bones! This connection shows how a small change can lead to the animals we see today.

Synapsida is a major clade of tetrapod vertebrates. This group includes all living mammals and their extinct relatives. Synapsids are one of two primary branches of the Amniota. The other branch is called Sauropsida, which contains all modern reptiles and birds.

For a long time, scientists called synapsids "mammal-like reptiles." This term is now considered incorrect in modern biology. We now know that reptiles are more closely related to each other than to synapsids. Because of this, scientists use terms like stem mammals or pan-mammals. These names show that synapsids are an independent branch on the tree of life.

The most famous feature of a synapsid is the temporal fenestra. This is a single opening in the skull roof located behind each eye socket. This opening leaves a bony arch beneath the skull roof.

This structure was a major evolutionary advantage. It provided new attachment sites for larger jaw muscles. These larger muscles allowed for a much more efficient and powerful bite. In early mammals, this opening eventually merged with the orbit, or eye socket. This change helped shape the modern mammalian skull.

Synapsids are often divided into two main evolutionary grades. The first group consists of the more primitive stem mammals. These animals were often lizard-like with a sprawling gait. They may have had protective scutes or scales on their skin.

The second group is the more advanced therapsids. Therapsids evolved from sphenacodontoid pelycosaurs. They tended to have a more upright body pose. Some forms of therapsids may have even possessed hair. This progression from stem mammals to therapsids eventually led to the evolution of true mammals.

The history of synapsids is a story of great success and sudden change. They first appeared about 318 million years ago during the Late Carboniferous period. During the Permian period, from 299 to 251 million years ago, they were the largest land vertebrates. They were the dominant land predators of the late Paleozoic and early Mesozoic. For example, eupelycosaurs like Dimetrodon were apex predators.

However, several mass extinction events severely reduced their numbers. The Capitanian and the Permian–Triassic extinctions were especially devastating. Only two therapsid groups, the dicynodonts and eutheriodonts, survived into the Triassic.

Even after major extinctions, some synapsids were incredibly successful. The dicynodont known as Lystrosaurus acted as a disaster taxon. At one point, it made up as much as 95% of all land species. Eventually, archosaurian sauropsids like dinosaurs began to take over their niches. The cynodont group, which includes Mammaliaformes, were the only synapsids to survive past the Triassic. During the Jurassic, these cynodonts mostly lived nocturnal lives to avoid competing with dinosaurs.

After the Cretaceous–Paleogene extinction event, mammals finally rose to dominance in the Cenozoic.

We can see the evolution of mammals by looking at specific physical changes. One major change happened in the teeth. Early synapsids had simple teeth, but they eventually developed differentiated teeth. These include incisors, canines, and molars.

Another major change occurred in the jaw. In reptiles, the lower jaw is made of several small bones. In mammals, the lower jaw is a single bone called the dentary. As synapsids evolved, the smaller jaw bones, like the articular, shrank. These bones eventually moved into the ear to become middle ear bones like the malleus.

Finally, the internal structure of the mouth also changed over time. Synapsids developed a secondary palate to separate the mouth from the nasal cavity. In early ancestors, this began as a partial structure on the sides of the maxilla. Eventually, the sides curved together to form a complete U-shaped barrier. This allowed animals to breathe and eat at the same time. This development, along with changes in skin and fur, helped set the stage for the diverse mammals we see today.