Archosaurs and Synapsids: two branches, not direct opposites

Archosaurs and synapsids are two important branches in the history of amniotes. Archosauria includes dinosaurs, birds, pterosaurs, crocodilians and extinct relatives. Synapsida includes Dimetrodon, therapsids, cynodonts and every living mammal, including humans.

It is not quite accurate to call Archosauria and Synapsida two immediate sister groups. The earliest major split among amniotes separated the synapsid line from the broader sauropsid line. Archosaurs appeared much later inside sauropsids. The comparison is useful for understanding the histories that eventually include dinosaurs and mammals, but it skips many sauropsids: lizards, snakes, turtles and extinct relatives among them.

What are amniotes?

Amniotes are vertebrates whose embryos develop within a set of membranes that includes the amnion. This reproductive system made development less dependent on open water than it is for many amphibians. The group includes mammals, birds, crocodilians, turtles, lizards, snakes, dinosaurs, pterosaurs and early synapsids.

The first amniotes appeared in the Carboniferous, around 320 million years ago. Early examples were small four-limbed animals that may look lizard-like in outline, but they were not members of modern lizard groups. The early amniote tree soon divided into the synapsid line and a reptile or sauropsid line. Archosaurs arose much later within the latter.

Why Archosauria is not the sister group of Synapsida

Synapsids split from the lineage leading to reptiles and birds in the Late Carboniferous. The sauropsid side then diversified into early reptiles, diapsids, lepidosauromorphs, archosauromorphs, marine reptile branches and many other groups. Archosaurs are only one later branch within that larger history.

A simplified nested tree is: Amniota; one branch is Synapsida; the other is the sauropsid line, within which are Diapsida, Archosauromorpha, Archosauriformes and finally Archosauria. Thus every archosaur is a sauropsid, but not every sauropsid is an archosaur. Every mammal is a synapsid, and synapsids are not a subgroup of modern reptiles in a phylogenetic classification.

The approximate timing is also different. Early amniotes appear around 320 million years ago; the synapsid–sauropsid split lies in the Late Carboniferous; therapsids arose by the Middle Permian; archosauromorphs are known from the Late Permian; archosaurs diversified in the Early Triassic; and the earliest dinosaurs occur later, in the Late Triassic. Early mammaliaforms also appear in the Late Triassic.

The exact divergence date cannot be read from one fossil. The fossil record documents organisms already present in particular layers; the evolutionary branch point must be at least as old as the first known members and may be older. The archosaur overview and the guide to Permian synapsids follow the two later histories in more detail.

What temporal openings tell us

Temporal openings are gaps in the skull behind the eye socket. They alter the side wall of the skull and provide space for jaw muscles. Historically, early amniotes were grouped by the number and position of these openings: anapsid forms lacked them, synapsids had one lower opening and diapsids had two. A so-called euryapsid condition, one upper opening, evolved secondarily in several marine reptile lineages.

This scheme remains useful for anatomy, but it cannot replace modern phylogenetics. Openings can change shape, merge, become obscured, or be lost with parts of the skull. A broken edge may create a false gap, and compression may distort the bone. Classification therefore uses the full character set rather than counting holes alone.

Early synapsids had one temporal opening low on each side of the skull. It increased space for jaw muscles. In therapsids and cynodonts, the temporal region expanded and the jaw apparatus became more specialised. By mammals, the original area had been extensively reorganised; a human skull does not contain a neat small synapsid opening like that of Dimetrodon.

Archosaurs inherited a diapsid skull pattern with upper and lower temporal openings. Many early archosauriforms also had an antorbital opening between the eye socket and nostril, while members of the crocodile line often had a mandibular opening. Birds remain archosaurs even though their bones fused, their openings changed and the teeth were lost. A modern bird does not look like a diagram of an early diapsid, but it remains nested within that branch.

Early synapsids were not mammals

Synapsida includes mammals, but its early members looked and lived very differently. Archaeothyris, Edaphosaurus, Dimetrodon, Ophiacodon and Sphenacodon are among early forms. Many had sprawling limbs, long tails, comparatively simple teeth and low bodies. Their soft coverings are poorly known.

“Mammal-like reptiles” is an outdated phrase. It arose from their appearance and mixture of features, but synapsids were not a branch inside modern reptiles. Most early synapsids were not ancestors of humans; they occupied side branches. The path to mammals ran through one therapsid line, the cynodonts, and the fossil record shows a series of anatomical changes rather than one abrupt transformation.

Therapsids and the route toward mammals

Therapsids arose within synapsids by the Middle Permian. Their branches included biarmosuchians, dinocephalians, anomodonts, dicynodonts, gorgonopsians, therocephalians and cynodonts. Across those groups, temporal regions enlarged, teeth became more specialised, jaw joints changed, limbs shifted in posture and new forms of chewing evolved. A secondary palate developed in some lines. Physiology also changed, although its pace and pattern remain debated.

Most therapsids were not direct mammal ancestors. They were part of a branching radiation. Only one cynodont lineage led to mammaliaforms and true mammals. Therocephalians were a neighbouring branch, not a stage every mammal ancestor passed through. More detail appears in the synapsids and therapsids guide.

Cynodonts and mammals

Cynodonts appear in the Late Permian and survived the end-Permian mass extinction. Their evolution involved enlargement of the dentary bone in the lower jaw, reduction of the rear jaw bones, more complex cheek teeth, a secondary palate, more upright limbs and changes to hearing anatomy. Hair may have appeared in advanced forms before true mammals, but the exact timing is uncertain.

In mammals, bones that once formed part of the jaw joint became elements of the middle ear. This transformation did not happen in one step; fossils preserve intermediate configurations. Increasingly specialised teeth and jaw motion made more varied food processing possible. These are features in the ancestry of mammals, not a description of every synapsid.

Early archosaurs were not all dinosaurs

Archosauria is much broader than Dinosauria. After archosaurs appeared in the Triassic, their history split into crocodile-line and bird-line branches. The crocodile line included aetosaurs, ornithosuchids, poposauroids, large terrestrial predators, early crocodylomorphs and modern crocodilians. It contained armoured herbivores, bipedal forms, fast small runners and marine animals.

The bird line included pterosaurs, dinosauromorphs, dinosaurs and birds. Many Triassic archosaurs looked dinosaur-like but were on the crocodile branch. Other forms were close relatives of dinosaurs without belonging to Dinosauria. The dinosaur ancestors guide describes how these branches fit together.

Within the bird-line archosaurs, dinosauromorphs preceded true dinosaurs. Changes to the hindlimbs, ankle, body posture and locomotion accumulated across several lineages. The first dinosaurs appear in the Late Triassic, but they did not immediately dominate all ecosystems. They lived alongside large pseudosuchians, therapsids and amphibians.

How dominance shifted before and during the age of dinosaurs

Early synapsids were important terrestrial animals in the Late Carboniferous and Early Permian. Therapsids occupied many large-bodied roles in the Middle and Late Permian, including herbivores and predators. The end-Permian extinction about 252 million years ago removed most lineages and disrupted ecosystems on land and sea.

Some dicynodonts, therocephalians, cynodonts, archosauromorphs and other groups survived into the Triassic. During the Middle and Late Triassic, archosaurs became especially diverse. It is too simple to say that they “replaced” synapsids in a single competitive event. Mass extinction, climate, geography, diets, competition and chance all shaped which branches survived and diversified.

Archosaurs and synapsids did not stop coexisting. Birds and crocodilians are archosaurs; humans, dogs and mice are synapsids. In the Mesozoic, dinosaurs lived alongside cynodonts, early mammals and several surviving therapsid groups. Mammals were already present before non-avian dinosaurs disappeared.

Are dinosaurs closer to mammals or crocodiles?

Dinosaurs are much closer to crocodilians. Both are archosaurs, and their shared archosaur ancestor lived long after the earliest synapsid–sauropsid split. Dinosaurs and mammals are both amniotes, but their common ancestor is far older.

In nested terms, a bird and a crocodile are archosaurs; a bird and a tyrannosaur are both dinosaurs; a person and Dimetrodon are both synapsids; a person and a bird are amniotes from different major branches. A crocodile is therefore a closer living relative of a non-avian dinosaur than any mammal is.

Dimetrodon is more closely related to humans than to dinosaurs, in the sense of shared ancestry. That does not make it a direct human ancestor. It belonged to a side branch of early synapsids and vanished long before the first dinosaurs. But the most recent common ancestor of Dimetrodon and humans lived later than their common ancestor with any dinosaur.

Teeth, jaws and limbs followed different paths

Early members of both branches could have simple cone-shaped teeth, after which their anatomy diversified. In therapsids and cynodonts, incisors, canines and postcanine teeth became more distinct. Among archosaurs, teeth could be set in separate sockets and evolved into slicing predator teeth, leaf-shaped herbivore teeth, beaks without teeth and complex batteries.

Both early synapsids and sauropsids had lower jaws made of several bones. The ancestral jaw joint involved the articular and quadrate bones. In the cynodont line, the dentary expanded and the rear bones shrank; in mammals those older joint bones became auditory ossicles. Archosaurs retained a different jaw-joint arrangement, with modifications of their own. Birds later fused and lightened many skull bones, but they did not follow the mammalian conversion of jaw-joint bones into the three middle-ear ossicles.

Early forms in both branches often had limbs set out to the sides. Therapsids gradually evolved more upright posture, especially in the hindlimbs. Archosaurs also developed erect limbs, but through a different arrangement of the hip and ankle. Dinosaurs held their legs beneath the body. Similar posture does not mean each anatomical detail came from the same immediate ancestor.

Warm-blooded versus cold-blooded is not the dividing line

It is wrong to divide all archosaurs into “warm-blooded” and all synapsids into “cold-blooded,” or the reverse. Physiology evolved gradually and could vary within each branch. Early synapsids may have had lower metabolic rates than later forms; therapsids and cynodonts show a series of traits approaching mammalian physiology.

Among archosaurs, birds are endothermic, crocodilians have a different metabolic pattern, and extinct dinosaurs and pterosaurs may have had high or intermediate physiological regimes. Modern crocodiles are not a perfect model for every archosaur, and a mouse does not represent the metabolism of Dimetrodon.

What were their body coverings?

The skin or fur of early synapsids is poorly documented. A thick coat on Dimetrodon or a gorgonopsian should not be presented as a fossil fact. Hair likely appeared within the cynodont line before true mammals emerged, but its exact timing remains uncertain.

On the crocodile line, scales and bony osteoderms were widespread. The bird line evolved pterosaur pycnofibres, dinosaur feathers and the specialised plumage of birds. Feathers did not appear only after the origin of birds. These differences are patterns across branches, not a claim that every species had the same covering.

Both branches still live today

Birds, crocodiles, alligators, caimans and gharials represent living archosaurs. Mammals, including monotremes, marsupials and placentals, represent living synapsids. That includes humans, whales, bats, elephants, mice, cats and platypuses. These ancient branches did not vanish; their modern members remain around us.

A bird did not stop being an archosaur when feathers, wings and a beak evolved. It is still an amniote, sauropsid, archosaur, dinosaur, theropod and bird. A human is likewise an amniote, synapsid, therapsid, cynodont, mammal and primate. In cladistic classification, groups contain all descendants of their shared ancestor. Mammals did not “leave” Synapsida; they remain inside it.

What is secure and what remains under study?

Comparative anatomy and phylogenetic analysis establish that early amniotes split into synapsid and sauropsid lines; archosaurs arose within sauropsids; mammals are synapsids; birds and crocodilians are archosaurs; and dinosaurs lie on the bird line. These branches overlapped in the Triassic and later, and they still coexist. Temporal openings help describe skull anatomy but are not enough by themselves to classify every fossil.

Researchers continue to refine the placement of some early amniotes, the age of the earliest split, the origin and modification of skull openings, the position of turtles among sauropsids, the physiology of early synapsids and archosaurs, the timing of hair, and the causes of changing ecological dominance after mass extinctions. None of that uncertainty challenges the basic membership of mammals within Synapsida or birds and crocodilians within Archosauria.

It is inaccurate to say that archosaurs and synapsids were the only ancient terrestrial vertebrate groups; that Archosauria is the direct sister group of Synapsida; that every synapsid was a human ancestor; that every archosaur was a dinosaur; that Dimetrodon was a reptile or dinosaur; or that mammals appeared only after dinosaurs vanished. The right tree keeps the larger sauropsid branch visible between the earliest amniote split and Archosauria.