Synapsida: A Guide to the Ancient Mammal-like Reptiles

Leave a comment / / Updated on: 24th September 2023

Taking a step back from our world as we know it means going back in time to prehistoric Earth, a period before the evolution of humans and eras that signified the rise and fall of magnificent creatures that once roamed and ruled our planet.

In more ways than one, the Earth eas very different from what we know it to be; supercontinents emerged and fragmented, shaping the very face of our planet.

Towering volcanoes painted the skies with fiery hues, while primordial seas teemed with an abundance of life, familiar and extraordinary.

During this remarkable era, life took its earliest steps, showcasing astonishing adaptations and embarking on a journey of evolution that would lead to the world as we know it today.

From colossal behemoths that shook the ground with each thunderous step to bizarre and fantastical beings that defied all expectations, this primeval realm holds the secrets of a bygone age waiting to unravel.

Synapsids | WillemSvdMerwe via Deviant Art

While these prehistoric times were more famous for giant creatures, they also housed other fascinating beings that captivated experts with their ingenuity and peculiar adaptations.

Yet, among these prehistoric inhabitants, one group would eventually claim the mantle of dominance and reshape life on Earth.

They were the ancestors of the incredible creatures we see today, the pioneers of the mammalian lineage.

These undersized, inconspicuous beings began their journey as humble foragers.

Their relentless pursuit of survival led to remarkable transformations, ultimately allowing them to conquer the land and give rise to the rich tapestry of mammalian diversity that surrounds us today.

One such group is the Synapsida, “mammal-like reptiles,” which arose during the late Carboniferous period, approximately 320 million years ago, and thrived until the end of the Triassic period, around 200 million years ago.

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This evolutionary lineage encompasses various fascinating creatures, ranging from small, elusive insectivores to enormous apex predators.

Through their evolutionary innovations and biological successes, the Synapsida group paved the way for the emergence of mammals, including our species.

The Synapsida group’s discovery was not in a single moment or at a specific location.

Instead, the understanding of the Synapsida group has developed over time through the study of fossil evidence found in various parts of the world.

The concept of the Synapsida group emerged as scientists explored and analyzed fossils from different periods and geographic regions.

The recognition and classification of the Synapsida group began in the 19th century when paleontologists such as Richard Owen and Thomas Henry Huxley examined fossil remains of mammal-like reptiles and recognized shared anatomical features that distinguished them from other reptiles.

Synapsid Skull | Gagea via Wikipedia (CC BY-SA 3.0)

The synapsid temporal fenestra, a specialized opening in the skull, played a pivotal role in identifying and defining this group.

Before the turn of the 20th century, some experts classified synapsids as one of the four major groups of reptiles.

Yet a closer look at the skeletal remains showed that this was not the case, as synapsids can be separated from reptiles by their distinctive temporal apertures.

Keep reading this article to discover more about this unique group.

Characteristics that Define the Synapsida Group

1. Temporal openings 

Eothyris, an early synapsid with multiple canines | ArthurWeasley via Wikipedia (CC BY 2.5)

One of the primary features of this group is its temporal openings found behind each eye orbit.

Synapsids exhibited three primary variations of temporal fenestrae: anapsid, synapsid, and diapsid.

Anapsids lacked temporal fenestrae altogether, while synapsids possessed a single pair.

Diapsids, on the other hand, had two sets of temporal openings.

This classification is crucial as it establishes synapsids as the evolutionary link between anapsid and diapsid reptiles, including modern-day reptiles and birds.

The temporal fenestrae underwent significant changes throughout the evolution of synapsids.

Pelycosaur (Dimetrodon) | MR1805 via Getty Images

Early synapsids, such as pelycosaurs, had relatively small fenestrae.

However, in later species, like therapsids, the temporal fenestrae increased in size, becoming more elongated and vertically oriented.

This trend continued in advanced therapsids, eventually forming the synapsid skull structure seen in modern mammals.

These temporal openings played several roles, such as developing jaw muscles of synapsids, thermoregulation, braincase expansion, etc.

2. Teeth

Gorgonopsian Teeth | Eurwentala via Deviant Art

The origins of teeth in the Synapsida can be traced back to their reptilian ancestors.

The earliest synapsids, such as the pelycosaurs, possessed simple, conical teeth for capturing and holding prey.

As the Synapsida evolved, their teeth became increasingly specialized to perform specific functions.

One significant development was the differentiation of tooth types within the jaw.

In later synapsids, such as the therapsids, different regions of the jaw exhibited distinct types of teeth, including incisors, canines, premolars, and molars.

Fossil of Chiniquodon, an extinct therapsid | Ghedoghedo via Wikipedia (CC BY-SA 3.0)

This dental differentiation allowed for a more efficient breakdown of food and enhanced feeding capabilities.

The evolution of heterodont dentition, characterized by different tooth types, is one of the pivotal dental innovations in the Synapsida group.

Heterodont dentition enabled synapsids to engage in more varied feeding behaviors, including processing diverse food types.

Incisors, located at the front of the mouth, were adapted for biting and cutting food.

Canines, often more pronounced in carnivorous species, were used for puncturing and tearing.

Premolars and molars, located toward the back of the jaw, were developed for grinding and shearing plant material.

3. Skin and Fur

Regisaurus jacobi | WillemSvdMerwe via Deviant Art

The Synapsida group is one of the most diverse animal groups ever, and this diversity is evident in several aspects, including their general appearance.

The group emerged in the Late Carboniferous period, giving rise to mammals.

Over the years, these creatures underwent an evolutionary process of various skin moderations – from osteoderms to scutes, hair or fur, scales, etc.

Unlike reptiles, mammals have a thick dermal layer.

The outermost layer, the epidermis, protects against environmental factors and pathogens.

Moschops Skin | CoreyFord via Getty Images

Beneath the epidermis lies the dermis, which contains blood vessels, nerves, and specialized structures such as hair follicles and sweat glands.

Contrarily, fur serves multiple purposes, including insulation, camouflage, communication, and protection against physical damage.

This skin structure has two main components: guard hairs and underfur.

Guard hairs are long, stiff hairs that provide structural support and help repel water, while the underfur consists of shorter, denser hairs that aid insulation.

4. Palate

Dental patterns in synapsids | Photo via Robert Reisz (Origin of dental occlusion in tetrapods: Signal for terrestrial vertebrate evolution?)

The Synapsida group arose from the reptilian lineage, branching off from sauropsids, which includes modern reptiles and birds.

Over time, Synapsida developed distinct adaptations, including changes in their palate, marking a significant step towards mammalian characteristics.

These adaptations ultimately allowed for improved feeding, respiration, and vocalization.

These creatures also developed a secondary palate, a roof-like structure formed by the fusion of several bones, separating the oral and nasal cavities.

Cleft palate in the human mouth. | Photo via Jamie A. Davies, in Mechanisms of Morphogenesis (Second Edition), 2013

Unlike reptiles, which possess a complete secondary palate formed by the maxilla and palatine bones, Synapsida developed a more advanced, partially or fully ossified secondary palate.

Separating the nasal and oral cavities facilitated the development of more efficient respiration.

The palate allowed air passage exclusively through the nasal cavity, improving airflow regulation.

This adaptation also played a pivotal role in transitioning to more sophisticated feeding strategies.

With a fully ossified palate, therapsids could chew and manipulate food more effectively, leading to better digestion and nutrient extraction.

Major Organism Groups of the Synapsida Order

1. Pelycosauria

Dimetrodon standing in a pond in the desert by day | Elenarts108 via Getty Images

The Pelycosauria is an early group of Synapsids that thrived during the Paleozoic Era, approximately 310 to 265 million years ago.

Pelycosaurs represent a transitional phase in reptilian evolution, exhibiting features that foreshadowed the emergence of mammalian traits.

These fascinating creatures displayed distinct features, such as their prominent dorsal sail composed of elongated neural spines, which are still subject to debate regarding their exact function.

Edaphosaurus | Elenarts108 via Getty Images

Their elongated skulls, large temporal fenestrae, and varied dentition further set them apart from other reptilian groups.

Ecologically, pelycosaurs displayed remarkable diversity.

Some species, like Dimetrodon, were apex predators with sharp teeth, while others, like Edaphosaurus, were herbivorous and had specialized grinding teeth for plant matter.

Also, because of their adaptable diet, omnivorous pelycosaurs, such as Ophiacodon, thrived in various environments.

2. Therapsida

Keratocephalus therapsids | Elenarts108 via Getty Images

The Therapsida lineage arose around 265 million years ago during the Permian period.

They descended from a group of reptiles called pelycosaurs, which were early members of the Synapsida order.

The transition from pelycosaurs to Therapsida involved significant anatomical and physiological changes, paving the way for more advanced mammalian traits.

Therapsids displayed fundamental features that set them apart from their reptilian ancestors and foreshadowed the rise of mammals.

Holotype skull of Raranimus dashankouensis, the most basal known therapsid | Jun Liu, Bruce Rubidge, and Jinling Li via Acta Paleontologica Polonica

They had differentiated teeth specialized for specific diets, a secondary palate separating the respiratory and digestive passages, an advanced jaw joint for efficient biting and chewing, and an enlarged braincase indicating increased neurological complexity.

Within the Therapsida group, two main subgroups emerged: the Dinocephalia and the Cynodontia.

Dinocephalians were primarily herbivorous and distinguished by their large heads and sturdy bodies.

Being herbivores, insectivores, and carnivores, cynodonts, on the other hand, inhabited different ecological niches and were quite diversified.

3. Mammaliaformes

Adelobasileus cromptoni, a mammaliamorph from the Late Triassic of Texas. Digital. | Nobu Tamura via Spinops

The Mammaliaformes represent a group of Synapsids that encompass the transitional forms between cynodonts and true mammals.

These organisms lived from the Late Triassic to the Early Cretaceous periods.

Mammaliaformes displayed a fascinating blend of reptilian and mammalian characteristics, offering valuable insights into the evolutionary history of mammals.

Dental specializations in Mammaliaformes demonstrate a progression towards mammalian dental characteristics, with specialized teeth adapted to different diets and ecological niches.

Life restoration of M. oehleri | FunkMonk via Wikipedia (CC BY-SA 3.0)

The secondary palate allowed simultaneous breathing and feeding, improving respiration and feeding strategies.

Changes in the jaw structure, including a more mammal-like jaw joint, facilitated efficient chewing and increased bite force.

Moreover, Mammaliaformes transitioned from the reptilian jaw joint to the mammalian middle ear bones, enhancing auditory capabilities.

4. Mammals

Rhino, Springboks, Zebra, Elephant, and Lion in Serengeti National Park, Tanzania. | ugurhan via Getty Images

Mammals evolved from synapsid ancestors, an order of mammal-like reptiles, around 200 million years ago during the Mesozoic Era.

The earliest mammals were small, nocturnal insectivores but diversified and adapted to various ecological niches.

Fossils have provided valuable insights into the transition from synapsids to mammals, with significant milestones including the emergence of specialized teeth, the development of a secondary palate, and the evolution of the mammalian ear.

This group possesses distinct traits that define their class.

They are warm-blooded, maintain a constant body temperature, and specialized mammary glands to produce milk, nourishing their young.

Hair or fur covers their bodies for insulation, camouflage, and communication.

Mammals also boast highly evolved brains, enabling complex behaviors and problem-solving abilities.

Notable Examples of Organisms within the Synopsida Order

1. Dimetrodon

Gage Beasley Prehistoric’s Dimetrodon Concept

Dimetrodon, an ancient reptile from the Permian period, holds a unique place in paleontology.

Despite its reptilian appearance, Dimetrodon is not a dinosaur but a member of the Synapsida order, closely related to mammals.

With its distinctive sail and specialized teeth, Dimetrodon stood as an apex predator of its time.

Dimetrodon’s importance extends beyond its ecological significance.

As a member of the Synapsida order, it offers valuable insights into the evolutionary transition from reptiles to mammals.

Dimetrodon displayed more mammal-like characteristics, such as erect limbs and specialized teeth.

Studying Dimetrodon helps scientists understand the steps and adaptations during this transition, providing a glimpse into the evolutionary history leading to the diverse mammalian fauna we see today.

2. Edaphosaurus

Edaphosaurus | Elenarts108 via Getty Images

Belonging to the order Edaphosauridae within the Synapsida class, Edaphosaurus is a genus of herbivorous reptiles.

Its name, derived from Greek words meaning “ground lizard,” reflects its presumed terrestrial lifestyle.

This herbivorous animal, known for having a distinctive sail-like structure on its back, sheds important light on the development of synapsids and the origin of herbivory.

As one of the earliest-known herbivorous synapsids, Edaphosaurus had specialized teeth for plant consumption.

Its incisors were sharp and leaf-shaped, while its molars were flat for grinding vegetation.

Fossil evidence suggests that Edaphosaurus inhabited various environments, specifically swampy regions with abundant plant resources.

Its herbivorous lifestyle and feeding habits affected plant-animal interactions during the Permian period.

3. Thrinaxodon

Restoration of the Thrinaxodon | Nobu Tamura via Spinops

Thrinaxodon is an intriguing and pivotal genus of ancient synapsids that played a crucial role in the transition from reptiles to mammals.

Flourishing during the Early Triassic period, Thrinaxodon provides valuable insights into the evolutionary journey that led to the emergence of mammalian characteristics.

It had a body size ranging from that of a small dog to a large cat, with a streamlined shape optimized for terrestrial locomotion.

Notable mammalian traits included differentiated teeth, an erect limb posture, and specialized jaw joints that allowed more efficient chewing.

Its dentition showcases heterodont teeth, with sharp canines for capturing prey and molar-like teeth for efficient chewing.

This adaptation suggests a shift towards a more omnivorous or herbivorous diet, unlike its carnivorous ancestors.

Additionally, Thrinaxodon exhibited an erect limb posture, allowing more efficient movement, resembling the upright stance of modern mammals.

Feeding Strategies and Behavior of the Synapsida Order

Feeding Strategies

Sphenacodon ferocior | PaleoGuy via Deviant Art

The Synapsida group encompasses an extensive range of species that spanned various habitats and evolutionary periods.

This diverse group includes basal synapsids like pelycosaurs, which existed during the Permian period, and more derived synapsids like therapsids and cynodonts that thrived during the Permian and Triassic periods.

Within this vast taxonomic range, numerous feeding strategies evolved, reflecting the ecological niches occupied by these ancient creatures.

Several synapsids displayed herbivorous tendencies, marking an important shift from the carnivorous reptilian ancestors.

Herbivory evolved in different lineages within the Synapsida group, such as the case of pelycosaurs like Dimetrodon and Edaphosaurus, which possessed specialized teeth for grinding plant material.

Close-up shot of fern leaves. | borchee via Getty Images

These early herbivores likely fed on various plants, such as ferns, horsetails, and other primitive vegetation prevalent during their time.

While herbivory played a significant role in the Synapsida group, many species within this lineage retained carnivorous feeding habits.

The therapsids and cynodonts, for example, exhibited adaptations for capturing and consuming prey.

Some cynodonts, like Cynognathus, possessed well-developed canine teeth and robust jaws, suggesting they were capable of tackling larger prey.

These adaptations paved the way for the eventual development of more efficient carnivorous strategies among early mammals.

Emydops arctatus | WillemSvdMerwe via Deviant Art

Some synapsids adopted an opportunistic feeding strategy, using available food sources in their environment.

This approach allowed them to adapt to fluctuating conditions and take advantage of plant and animal resources.

For example, some cynodonts had dentition capable of handling different food items, indicating omnivorous dietary flexibility.

This opportunistic feeding strategy likely contributed to synapsids’ survival and adaptive radiation during periods of environmental change.


Dimetrodon primitive predators stick together looking for prey | CoreyFord via Getty Images

Synapsids exhibited various social structures, ranging from solitary individuals to complex social networks.

Fossil evidence suggests that some synapsids, such as the famous Dimetrodon, displayed territorial behavior.

These territorial boundaries likely played a crucial role in resource acquisition, mating opportunities, and reducing competition within a specific area.

Communication among synapsids likely involved a combination of visual displays, vocalizations, and chemical signals.

For instance, the elaborate sail-like structure found in Dimetrodon might have served as a visual signal for species recognition, mate attraction, and threat displays.

Dimetrodon sail | Elenarts108 via Getty Images

Vocalizations and chemical signals, such as pheromones, could have facilitated communication between individuals, especially during mating seasons or territorial disputes.

Some synapsids displayed social behaviors akin to herding or flocking.

Fossils of herbivorous synapsids like Lystrosaurus suggest that they formed large groups, possibly for protection against predators, enhanced foraging efficiency, or other mutual benefits.

Group living may have also facilitated the sharing of information, such as food sources or predator warnings.

In addition to group living, synapsids may have exhibited cooperative behaviors within their social structures.

Cooperation could have manifested in collective defense against predators, coordinated hunting strategies, or shared parenting responsibilities.

Lisowicia bojani | tuomaskoivurinne via Deviant Art

However, the extent and complexity of cooperative behaviors among synapsids remain speculative due to the lack of direct fossil evidence.

Understanding the social behaviors of synapsids provides valuable insights into their ecological impact and the functioning of prehistoric ecosystems.

Their behaviors, such as territoriality and group living, influenced resource utilization, population dynamics, and predator-prey relationships.

The social interactions of synapsids also shaped the selective pressures that contributed to the evolution of more complex social behaviors in their descendants, the mammals.

Paleobiogeography of the Synapsida Order

Keratocephalus | WillemSvdMerwe via Deviant Art

The Synapsida group, often referred to as “stem mammals,” is a diverse and significant group of reptiles that flourished during the Paleozoic and Mesozoic eras.

Synapsids are crucial in understanding the evolutionary history of mammals, as they are considered the earliest amniotes to possess mammal-like characteristics.

This group of reptiles emerged during the Paleozoic and Mesozoic eras and possessed unique skull structures that set them apart from other reptiles.

These reptiles originated in Laurasia during the early Paleozoic, specifically in regions that are now North America and Europe.

As time progressed, they expanded their range to Gondwana, encompassing present-day South America and Africa.

The Late Paleozoic era marked a period of diversification and widespread distribution for synapsids.

Life in the early Paleozoic | H. G. Wells via The Outline of History

During this time, they occupied various ecological niches and experienced remarkable radiation.

Synapsids experienced remarkable radiation and occupied diverse ecological niches across the globe.

Pelycosaurs, a group of early synapsids, were particularly diverse and widespread during the Permian.

With the arrival of the Mesozoic era, the synapsids faced a decline in diversity due to the emergence and dominance of diapsids, including the dinosaurs.

However, a branch of synapsids, known as the therapsids, continued to thrive.

These therapsids displayed more mammal-like features and occupied various habitats across Pangaea.

Several factors influenced the paleobiogeography of synapsids.

Pangea Earth Supercontinent | Ianm35 via Getty Images

The movement of continents played a crucial role in shaping the distribution of synapsids.

The formation and breakup of supercontinents facilitated or restricted the dispersal of these reptiles.

As continents shifted, barriers such as oceans and mountain ranges emerged, affecting species distribution and leading to the divergence of distinct lineages.

Fluctuations in climate during the Paleozoic and Mesozoic eras impacted the distribution and evolution of synapsids.

Changes in temperature, precipitation patterns, and sea levels influenced habitat availability and species migration.

Some synapsids adapted to arid environments, while others thrived in humid and forested regions. Interactions with other organisms, such as competition and predation, influenced the distribution of synapsids. 

Hovasaurus boulei, a tangasaurid diapsid reptile from the Late Permian of Madagascar | Nobu Tamura via Spinops

As diapsids diversified and became dominant in certain regions, synapsids faced increased competition and predation pressure.

These ecological factors likely played a role in the decline of some synapsid lineages.

The most significant extinction event in Earth’s history, the Permian-Triassic extinction event, occurred approximately 252 million years ago.

This event resulted in the loss of an estimated 90% of marine species and 70% of terrestrial species, including many early synapsids.

The causes of this mass extinction are still debated, but they likely involved a combination of factors such as volcanic activity, climate change, oceanic anoxia, and possibly asteroid impacts.


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