Diapsid vs. Synapsid

What's the Difference?

Diapsids and synapsids are two major groups of reptiles that diverged from a common ancestor during the early evolution of reptiles. Diapsids are characterized by having two openings on each side of their skull, known as temporal fenestrae, which provide attachment points for jaw muscles. This group includes reptiles such as lizards, snakes, crocodiles, and birds. On the other hand, synapsids have a single opening on each side of their skull, known as the temporal fenestra, which is larger and more elongated than that of diapsids. Synapsids include mammals and their extinct relatives, such as the famous dinosaurs. While both groups have evolved to be successful and diverse, synapsids, particularly mammals, have achieved greater ecological dominance and have evolved complex features such as hair, mammary glands, and a highly specialized dentition.


Number of temporal fenestraeTwoOne
Skull structureDiapsid skull with two pairs of temporal fenestraeSynapsid skull with one pair of temporal fenestrae
Evolutionary historyEvolved during the late Carboniferous periodEvolved during the early Permian period
Extant representativesCrocodilians, birds, lizards, snakesMammals
Number of openings in the skullTwo temporal fenestrae and additional openingsOne temporal fenestra and additional openings
Types of teethVaried types including conical, incisors, canines, molarsVaried types including incisors, canines, premolars, molars
Respiration mechanismMostly lungsLungs
Reproductive strategyMostly oviparous (lay eggs)Mostly viviparous (give live birth)

Further Detail


Diapsids and synapsids are two major groups of reptiles that have played significant roles in the evolution of terrestrial vertebrates. While both groups share common ancestry, they have distinct characteristics that set them apart. In this article, we will explore the attributes of diapsids and synapsids, highlighting their anatomical, physiological, and ecological differences.

Anatomical Features

One of the key distinctions between diapsids and synapsids lies in their skull structure. Diapsids possess two pairs of temporal openings on each side of their skull, known as diapsid fenestrae. These fenestrae provide attachment points for jaw muscles, allowing for powerful biting and chewing. In contrast, synapsids have a single pair of temporal openings, known as synapsid fenestrae, which are located higher on the skull. This unique skull structure in synapsids allows for a more efficient attachment of jaw muscles, enabling a stronger bite force.

Furthermore, diapsids typically have a more elongated snout compared to synapsids. This elongation is often associated with adaptations for capturing prey, such as in crocodiles and lizards. Synapsids, on the other hand, exhibit a wider range of snout shapes, from short and robust in carnivorous mammals to long and slender in herbivorous species like horses.

Another notable anatomical difference is the presence of scales. Diapsids, including reptiles like snakes and turtles, possess scales that cover their bodies, providing protection and reducing water loss. In contrast, synapsids, which include mammals, have evolved various types of hair or fur, which serve multiple functions such as insulation, camouflage, and sensory perception.

Physiological Characteristics

When it comes to physiological attributes, diapsids and synapsids also exhibit distinct features. Diapsids are ectothermic, meaning their body temperature is primarily regulated by external factors such as sunlight and ambient temperature. This characteristic allows diapsids to conserve energy and survive in a wide range of environments. However, it also limits their activity levels, as they rely on external heat sources to maintain optimal body temperature.

In contrast, synapsids are endothermic, meaning they can generate and regulate their own body heat internally. This ability allows synapsids, including mammals, to maintain a relatively constant body temperature regardless of external conditions. Endothermy provides several advantages, such as increased metabolic rates, allowing for sustained activity levels, and the ability to inhabit diverse habitats, including colder environments.

Furthermore, synapsids possess a more advanced respiratory system compared to diapsids. Mammals, as synapsids, have a diaphragm, a muscular structure that aids in breathing by expanding and contracting the lungs. This specialized respiratory system allows for more efficient gas exchange, enabling mammals to sustain higher levels of aerobic activity compared to diapsids.

Ecological Roles

Diapsids and synapsids have occupied different ecological niches throughout their evolutionary history. Diapsids, with their diverse adaptations, have successfully colonized various habitats, including terrestrial, arboreal, and aquatic environments. For example, reptiles like lizards have evolved adhesive toe pads, enabling them to climb trees, while crocodiles have adapted to aquatic life with their streamlined bodies and powerful tails for swimming.

Synapsids, particularly mammals, have also diversified into numerous ecological roles. Mammals have evolved specialized teeth and jaws for different diets, ranging from carnivorous to herbivorous. Additionally, the development of mammary glands in mammals allowed for the production of milk, providing nourishment to their young. This unique adaptation has contributed to the success of mammals in nurturing their offspring and has played a crucial role in their evolutionary success.

Moreover, synapsids, as endothermic animals, have the ability to maintain a constant body temperature, which has allowed them to occupy a wide range of habitats, including polar regions. This adaptability has enabled mammals to thrive in diverse ecosystems, from deserts to rainforests, and has contributed to their global distribution and ecological dominance.


In conclusion, diapsids and synapsids represent two distinct groups of reptiles with contrasting attributes. Diapsids possess a unique skull structure with two pairs of temporal openings, elongated snouts, and scales, while synapsids have a single pair of temporal openings, varied snout shapes, and hair or fur. Physiologically, diapsids are ectothermic, relying on external heat sources, while synapsids are endothermic, generating their own body heat. Ecologically, diapsids have diversified into various habitats, while synapsids, particularly mammals, have occupied diverse ecological roles and habitats. Understanding these attributes helps us appreciate the remarkable diversity and evolutionary success of both diapsids and synapsids in shaping the natural world.

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