Synapsida

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Diagram comparing five types of reptile skulls-synapsid, parapsid, anapsid, euryapsid, diapsid-and a lizard skull, each labeled with bones such as postorbital, squamosal, and postfrontal, highlighting differences in skull openings and structure.

Collage showing a dog plus a saber-toothed cat equaling a prehistoric, quadrupedal creature with thick limbs and dark, hairless skin.

Synapsida

fossil tetrapod subclass

Also known as: synapsid

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July 14, 2026

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Diagram comparing five types of reptile skulls-synapsid, parapsid, anapsid, euryapsid, diapsid-and a lizard skull, each labeled with bones such as postorbital, squamosal, and postfrontal, highlighting differences in skull openings and structure. — synapsid and reptilian skull types Synapsid and reptilian skull types.

Synapsida is a subclass of extinct reptiles that lived during the Pennsylvanian, Permian, and Triassic periods. These reptiles are characterized by a single pair of lateral temporal openings in their skulls. Synapsids are significant because they are considered ancestral to mammals.1

They were present in the Carboniferous Period and became the dominant reptiles during the Permian Period. The therapsids, a group within the synapsids, are particularly important as they gave rise to mammals in the Triassic Period. This transition from reptiles to mammals occurred over millions of years, with therapsids showing increasingly mammal-like features in their dentition and bone structure. The Permian extinction event, the most severe mass extinction in Earth's history, significantly impacted many terrestrial groups, including pelycosaurs and therapsids.234

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vertebrate
Annotated classification
Subclass Synapsida †Extinct; mammal-like; lateral temporal opening. Class Aves Warm-blooded; skull has only 1 condyle; front limbs primarily modified for flight; hind limbs are legs with 4 or fewer toes; body covered with feathers; scales on feet; 4-chambered heart; no teeth; horny beak; lungs with extended…
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Subclass Synapsida †Extinct; mammal-like; lateral temporal opening. Class Aves Warm-blooded; skull has only 1 condyle; front limbs primarily modified for flight; hind limbs are legs with 4 or fewer toes; body covered with feathers; scales on feet; 4-chambered heart; no teeth; horny beak; lungs with extended…
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reptile
Historical development
…million years ago), fossils of synapsids (mammal-like reptiles) and early reptiles occur together in the same fossil beds. These earliest known synapsids and reptiles had already developed some traits that would persist in their descendants, modern mammals and reptiles. One example of a feature both groups held in common was…
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…million years ago), fossils of synapsids (mammal-like reptiles) and early reptiles occur together in the same fossil beds. These earliest known synapsids and reptiles had already developed some traits that would persist in their descendants, modern mammals and reptiles. One example of a feature both groups held in common was…
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Jurassic Period
Vertebrates
In Triassic terrestrial ecosystems, synapsids and therapsids—ancestors of modern mammals and their relatives, often called “mammal-like reptiles”—were dominant. They occupied several ecological niches and grew to large sizes. By the start of the Jurassic, these groups became rare, a minor component of fossil assemblages; individuals were very small—no larger…
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In Triassic terrestrial ecosystems, synapsids and therapsids—ancestors of modern mammals and their relatives, often called “mammal-like reptiles”—were dominant. They occupied several ecological niches and grew to large sizes. By the start of the Jurassic, these groups became rare, a minor component of fossil assemblages; individuals were very small—no larger…
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mammal
The evolution of the mammalian condition
…therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles), generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 359 million to 299 million years ago) and are one of the earliest known reptilian groups. They were the dominant…
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…therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles), generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 359 million to 299 million years ago) and are one of the earliest known reptilian groups. They were the dominant…
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Permian Period
Emergence of important reptiles
…reptiles of the Mesozoic; and synapsids, a common and varied group of mammal-like reptiles that eventually gave rise to mammals in the Mesozoic.
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…reptiles of the Mesozoic; and synapsids, a common and varied group of mammal-like reptiles that eventually gave rise to mammals in the Mesozoic.
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