Synapsida

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.