Evolution of mammals

The history of mammals is a story of gradual change spanning hundreds of millions of years. It began with a group of animals called synapsids. These ancestors first appeared during the Pennsylvanian sub-period of the late Carboniferous period.

Synapsids are distinct from the sauropsid lineage, which includes reptiles, dinosaurs, and birds. While early scientists often called synapsids "mammal-like reptiles," this term is now considered outdated. They were not part of the reptile lineage. Instead, paleontologists use terms like "stem mammals," "protomammals," or "paramammals" to describe these ancestors.

One of the most important features of early synapsids was the structure of their skulls. They possessed a distinctive pattern of holes located behind each eye. These openings, known as temporal fenestrae, provided several biological advantages. They made the skull lighter without losing strength. They also allowed the animal to save energy by using less bone. Furthermore, these holes provided better attachment points for jaw muscles. This allowed muscles to be longer and exert a strong pull over a wide range of movement.

As the Permian period progressed, a group called the therapsids descended from earlier synapsids. These animals showed more advanced features, such as larger temporal fenestrae and more specialized teeth. Some therapsids, known as cynodonts, began to develop a bony secondary palate. This is a structure in the mouth that separates the airway from the oral cavity. Many scientists believe this allowed animals to breathe and eat at the same time. This ability may have supported a higher metabolic rate. A higher metabolism helps an animal maintain a constant body temperature, a process called endothermy.

A major turning point in mammalian evolution involves the structure of the jaw and the ear. In most other amniotes, the jaw joint is made of the articular bone and the quadrate bone.

However, in mammals, these bones underwent a significant transformation. During the Jurassic period, the quadrate and articular bones moved and became the incus and malleus bones in the middle ear. This left the lower jaw to be composed of a single bone called the dentary. This dentary-squamosal joint is a primary way paleontologists identify mammalian fossils.

During the Mesozoic era, mammals began to occupy many different ecological niches. While many large animals like dinosaurs dominated the landscape, mammals explored specialized lifestyles. For example, the *Castorocauda* became an aquatic animal.

The *Volaticotherium* evolved to be a glider.

Some mammals, such as *Repenomamus*, were even capable of feeding on juvenile dinosaurs.

These diverse lifestyles show that mammals were active and varied long before the dinosaurs disappeared.

The lineage of modern mammals eventually split into several distinct groups. In the Jurassic, the line leading to today's mammals diverged. Some ancestors, like *Ambondro*, were more closely related to monotremes. Others, like *Dryolestes*, were more closely related to placental and marsupial mammals. Later, the eutherian and metatherian lineages separated. Metatherians are the ancestors of marsupials, while eutherians are the ancestors of placental mammals. We know the eutherian lineage is very old because the earliest known eutherian, *Juramaia*, lived approximately 160 million years ago.

A massive turning point occurred during the Cretaceous–Paleogene extinction event. This event wiped out the non-avian dinosaurs and several other mammalian groups. This cleared the way for placental and marsupial mammals to diversify. They moved into many new ecological roles throughout the Paleogene and Neogene periods. By the end of the Neogene, all modern mammalian orders had appeared. This long process of evolution turned small, nocturnal stem mammals into the wide variety of animals we see today.