Animals like us have changed over time.
Long ago, our first kin lived on land. They grew into the animals we see now. Some even grew hair to stay warm.
This helped them live in new ways. Do you see any animals with hair?
Long ago, the first ancestors of mammals lived.
These animals were not reptiles. They were a different group.
Their skulls had special holes. These holes made their heads light.
Some early kin lived at night. They used a good sense of smell. This helped them stay safe. Living at night may have helped them grow hair.
As time passed, they changed more. Some animals lived in the water.
Others could glide through the air. Many new types of mammals appeared later. They filled many different roles in nature. It is amazing how they changed.
The story of mammals began a very long time ago.
Long ago, a group called synapsids appeared. They were not reptiles. Instead, they were the ancestors of all mammals. These animals had special holes in their skulls. These holes made their heads light and strong.
As time passed, these animals changed. Some lived during the day. Others lived at night. Living at night helped them stay safe from predators. This way of life may have helped them grow hair. It may also have helped them stay warm.
Mammals also changed how they heard sounds. Other animals use two bones in their jaw to hear. Mammals moved those bones into the ear. Now, mammals use only one bone for the lower jaw. This bone is called the dentary.
Many different kinds of mammals appeared later. Some lived in the water. Others could glide through the air.
After the dinosaurs died out, mammals grew into many new forms. They filled many different roles in the world.
The story of mammals is a very long journey. It began with a group of animals called synapsids. These ancestors first appeared during the Carboniferous period.
They were not reptiles, even though people used to call them "mammal-like reptiles." Scientists now call them stem mammals. These animals had special holes in their skulls. These holes made their heads lighter and stronger.
These holes also helped their jaw muscles work better.
As time passed, these animals changed in many ways. Some lived during the day, but others lived at night. This nocturnal lifestyle helped them hide from predators called archosaurs.
Living at night might have helped mammals develop hair. It may have also helped them develop endothermy, which is the ability to keep a warm body temperature. Some ancestors even grew a bony secondary palate. This is a structure in the mouth that helps animals breathe and eat at the same time.
One of the biggest changes was how mammals hear. Other animals use two bones in their jaw to hear. These bones are called the quadrate and the articular.
In mammals, these bones moved into the middle ear. They became the incus and the malleus. Now, mammals use a single bone for the lower jaw. This bone is called the dentary. This new jaw joint is a major way scientists identify mammal fossils.
Many different kinds of mammals appeared throughout history. During the Mesozoic era, some mammals lived in the water.
Others were gliders, like the Volaticotherium.
Some even ate small dinosaurs.
For a long time, these fossils were very hard to find. However, many important discoveries have been made in China since the mid-1990s. These fossils help us see how mammals grew and changed.
After the Cretaceous–Paleogene extinction event, the world changed again. This event wiped out the non-avian dinosaurs.
With the dinosaurs gone, mammals had more room to grow. They moved into many new roles in nature. This is called filling ecological niches. By the end of the Neogene period, all the modern groups of mammals had appeared. Today, we can see the results of this long history in every animal from a tiny mouse to a huge whale.
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.
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