Theropsida Seeley, 1895
Synapsids (Greek, 'fused arch'), synonymous with theropsids (Greek,
'beast-face'), are a group of animals that includes mammals and every
animal more closely related to mammals than to other living
amniotes. They are easily separated from other amniotes by having a
temporal fenestra, an opening low in the skull roof behind each eye,
leaving a bony arch beneath each; this accounts for their name.
Primitive synapsids are usually called pelycosaurs or pelycosaur-grade
synapsids; more advanced mammal-like ones, therapsids. The
non-mammalian members are described as mammal-like reptiles in
classical systematics; they can also be called stem mammals or
proto-mammals. Synapsids evolved from basal amniotes and are one of
the two major groups of the later amniotes; the other is the
sauropsids, a group that includes modern reptiles and birds. The
distinctive temporal fenestra developed in the ancestral synapsid
about 312 million years ago, during the Late
Synapsids were the largest terrestrial vertebrates in the Permian
period, 299 to 251 million years ago, although some of the larger
pareiasaurs at the end of
Permian could match them in size. As with
other groups then extant, their numbers and variety were severely
reduced by the Permian–
Triassic extinction. By the time of the
extinction at the end of Permian, all the older forms of synapsids
(known as pelycosaurs) were already gone, having been replaced by the
more advanced therapsids. Though the dicynodonts and Eutheriodontia,
the latter consisting of
Eutherocephalia (Therocephalia) and
Epicynodontia (Cynodontia), continued into the
Triassic period as the
only known surviving therapsids, archosaurs became the largest and
most numerous land vertebrates in the course of this period. The
cynodont group Probainognathia, which includes Mammaliaformes, were
the only synapsids who outlasted the Triassic. After the
Paleogene extinction event, the synapsids (in the form of
mammals) again became the largest land animals.
1 Linnaean and cladistic classifications
1.1 Synapsids as a reptilian subclass
1.2 The "mammal-like reptiles"
1.3 Primitive and advanced synapsids
2.1 Temporal openings
2.5 Skin and fur
2.5.1 Mammary glands
3 Evolutionary history
5 See also
7 Further reading
8 External links
Linnaean and cladistic classifications
Synapsids as a reptilian subclass
Synapsids were originally defined at the turn of the 20th century as
one of the four main subclasses of reptiles, on the basis of their
distinctive temporal openings. These openings in the cheek bones
allowed the attachment of larger jaw muscles, hence a more efficient
bite. Synapsids were considered to be the reptilian lineage that led
to mammals; they gradually evolved increasingly mammalian features,
hence the name "mammal-like reptiles", which became a broad,
traditional description for all
The "mammal-like reptiles"
The traditional classification of synapsids as reptiles is continued
by some palaeontologists (Colbert & Morales 2001). In the 1990s,
this approach was complemented by a cladistic one, according to which
the only valid groups are those that include common ancestors and all
of their descendants: these are known as monophyletic groups, or
Phylogenetically, synapsids are the entire synapsid/mammal branch of
the tree of life, though in practice the term is most often used when
referring to the reptile-grade synapsids. The term "mammal-like
reptiles" represents a paraphyletic grade, but is commonly used both
colloquially and in the technical literature to refer to all
non-mammalian synapsids. The actual monophyly of Synapsida is not
in doubt, however, and the expressions "Synapsida contains the
mammals" and "synapsids gave rise to the mammals" both express the
same phylogenetic hypothesis.
Primitive and advanced synapsids
The synapsids are traditionally divided into a primitive group and an
advanced group, known respectively as pelycosaurs and therapsids.
'Pelycosaurs' make up the six most primitive families of synapsids.
They were all rather lizard-like, with sprawling gait and possibly
horny scutes. The therapsids contain the more advanced synapsids,
having a more erect pose and possibly hair, at least in some forms. In
traditional taxonomy, the Synapsida encompasses two distinct grades
successively closer to mammals: the low-slung pelycosaurs have given
rise to the more erect therapsids, who in their turn have given rise
to the mammals. In traditional vertebrate classification, the
Therapsida were both considered orders of the
In phylogenetic nomenclature, the terms are used somewhat differently,
as the daughter clades are included. Most papers published during the
21st century have treated "Pelycosauria" as an informal grouping of
Therapsida has remained in use as a clade
containing both the traditional therapsid families and mammals.
However, in practical usage, the terms are used almost exclusively
when referring to the more basal members that lie outside of
The synapsids are distinguished by a single hole, known as the
temporal fenestra, in the skull behind each eye. This schematic shows
the skull viewed from the left side. The middle opening is the orbit
of the eye; the opening to the right of it is the temporal fenestra.
Synapsids evolved a temporal fenestra behind each eye orbit on the
lateral surface of the skull. It may have provided new attachment
sites for jaw muscles. A similar development took place in the
diapsids, which evolved two rather than one opening behind each eye.
Originally, the openings in the skull left the inner cranium covered
only by the jaw muscles, but in higher therapsids and mammals, the
sphenoid bone has expanded to close the opening. This has left the
lower margin of the opening as an arch extending from the lower edges
of the braincase.
Eothyris, an early synapsid with multiple canines
Synapsids are characterized by having differentiated teeth. These
include the canines, molars, and incisors. The trend towards
differentiation is found in some labyrinthodonts and early anapsid
reptilians in the form of enlargement of the first teeth on the
maxilla, forming a form of protocanines. This trait was subsequently
lost in the sauropsid line, but developed further in the synapsids.
Early synapsids could have two or even three enlarged "canines", but
in the therapsids, the pattern had settled to one canine in each upper
jaw half. The lower canines developed later.
The jaw transition is a good classification tool, as most other
fossilized features that make a chronological progression from a
reptile-like to a mammalian condition follow the progression of the
jaw transition. The mandible, or lower jaw, consists of a single,
tooth-bearing bone in mammals (the dentary), whereas the lower jaw of
modern and prehistoric reptiles consists of a conglomeration of
smaller bones (including the dentary, articular, and others). As they
evolved in synapsids, these jaw bones were reduced in size and either
lost or, in the case of the articular, gradually moved into the ear,
forming one of the middle ear bones: while modern mammals possess the
malleus, incus and stapes, basal synapsids (like all other tetrapods)
possess only a stapes. The malleus is derived from the articular (a
lower jaw bone), while the incus is derived from the quadrate (a
Mammalian jaw structures are also set apart by the dentary-squamosal
jaw joint. In this form of jaw joint, the dentary forms a connection
with a depression in the squamosal known as the glenoid cavity. In
contrast, all other jawed vertebrates, including reptiles and
nonmammalian synapsids, possess a jaw joint in which one of the
smaller bones of the lower jaw, the articular, makes a connection with
a bone of the cranium called the quadrate bone to form the
articular-quadrate jaw joint. In forms transitional to mammals, the
jaw joint is composed of a large, lower jaw bone (similar to the
dentary found in mammals) that does not connect to the squamosal, but
connects to the quadrate with a receding articular bone.
Over time, as synapsids became more mammalian and less 'reptilian',
they began to develop a secondary palate, separating the mouth and
nasal cavity. In early synapsids, a secondary palate began to form on
the sides of the maxilla, still leaving the mouth and nostril
Eventually, the two sides of the palate began to curve together,
forming a U-shape instead of a C-shape. The palate also began to
extend back toward the throat, securing the entire mouth and creating
a full palatine bone. The maxilla is also closed completely. In
fossils of one of the first eutheriodonts, the beginnings of a palate
are clearly visible. The later
Thrinaxodon has a full and completely
closed palate, forming a clear progression.
Skin and fur
The sea otter has the densest fur of modern mammals.
In addition to the glandular skin covered in fur found in most modern
mammals, modern and extinct synapsids possess a variety of modified
skin coverings, including osteoderms (bony armor embedded in the
skin), scutes (protective structures of the dermis often with a horny
covering), hair or fur, and scale-like structures (often formed from
modified hair, as in pangolins and some rodents). While the skin of
reptiles is rather thin, that of mammals has a thick dermal layer.
The ancestral skin type of synapsids has been subject to discussion.
Among the early synapsids, only two species of small varanopids have
been found to possess scutes; fossilized rows of osteoderms
indicate horny armour on the neck and back, and skin impressions
indicate some possessed rectangular scutes on their undersides and
tails. The pelycosaur scutes probably were nonoverlapping
dermal structures with a horny overlay, like those found in modern
crocodiles and turtles. These differed in structure from the scales of
lizards and snakes, which are an epidermal feature (like mammalian
hair or avian feathers). Recently, skin impressions from the genus
Ascendonanus suggest that at least varanopsids developed scales
similar to those of squamates.
It is currently unknown exactly when mammalian characteristics such as
body hair and mammary glands first appeared, as the fossils only
rarely provide direct evidence for soft tissues. An exceptionally
well-preserved skull of Estemmenosuchus, a therapsid from the Upper
Permian, preserves smooth skin with what appear to be glandular
depressions, an animal noted as being semi-aquatic. The oldest
known fossil showing unambiguous imprints of hair is the Callovian
(late middle Jurassic)
Castorocauda and several contemporary
haramiyidans, both non-mammalian mammaliaform (see below,
however). More primitive members of the
Cynodontia are also
hypothesized to have had fur or a fur-like covering based on their
inferred warm-blooded metabolism. While more direct evidence of
fur in early cynodonts has been proposed in the form of small pits on
the snout possibly associated with whiskers, such pits are also found
in some reptiles that lack whiskers. There is evidence that some
other non-mammalian cynodonts more basal than Castorocauda, such as
Morganucodon, had Harderian glands, which are associated with the
grooming and maintenance of fur. The apparent absence of these glands
in non-mammaliaformes may suggest that fur did not originate until
that point in synapsid evolution. It is possible that fur and
associated features of true warm-bloodedness did not appear until some
synapsids became extremely small and nocturnal, necessitating a higher
However, recent discoveries on Russian
Permian coprolites showcase
that at least some synapsids did already have fur in this epoch. These
are the oldest impressions of hair on synapsids.
Early synapsids, as far back as their known evolutionary debut in the
Carboniferous period, may have laid parchment-shelled
(leathery) eggs which lacked a calcified layer, as most modern
reptiles and monotremes do. This may also explain why there is no
fossil evidence for synapsid eggs to date. Because they were
vulnerable to desiccation, secretions from apocrine-like glands may
have helped keep the eggs moist. According to Oftedal, early
synapsids may have buried the eggs into moisture laden soil, hydrating
them with contact with the moist skin, or may have carried them in a
moist pouch, similar to that of monotremes, though this would limit
the mobility of the parent. The latter may have been the primitive
form of egg care in synapsids rather than simply burying the eggs, and
the constraint on the parent's mobility would have been solved by
having the eggs "parked" in nests during foraging or other activities
and periodically be hydrated, allowing higher clutch sizes than could
fit inside a pouch (or pouches) at once, and large eggs, which would
be cumbersome to carry in a pouch, would be easier to care for. The
basis of Oftedal's speculation is the fact that many species of
anurans can carry eggs or tadpoles attached to the skin, or embedded
within cutaneous "pouches" and how most salamanders curl around their
eggs to keep them moist, both groups also having glandular skin.
The glands involved in this mechanism would later evolve into true
mammary glands with multiple modes of secretion in association with
hair follicles. Comparative analyses of the evolutionary origin of
milk constituents support a scenario in which the secretions from
these glands evolved into a complex, nutrient-rich milk long before
true mammals arose (with some of the constituents possibly predating
the split between the synapsid and sauropsid lines).
almost certainly able to produce this, which allowed a progressive
decline of yolk mass and thus egg size, resulting in increasingly
altricial hatchlings as milk became the primary source of nutrition,
which is all evidenced by the small body size, the presence of
epipubic bones, and limited tooth replacement in advanced cynodonts,
as well as in mammaliaforms.
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Aerial locomotion first began in non-mammalian haramiyidan cynodonts,
with Arboroharamiya, Xianshou,
bearing equisitely preserved, fur-covered wing membranes that stretch
across the limbs and tail. Their fingers are elongated, similar to
those of bats and colugos and likely sharing similar roles both as
wing supports and to hang on tree branches.
Within true mammals, aerial locomotion first occurs in volaticotherian
Volaticotherium preserves an exquisitely preserved
furry patagium with delicate wrinkles and that is very extensive,
"sandwiching" the poorly preserved hands and feet and extending to the
base of the tail. Argentoconodon, a close relative, shares a
similar femur adapted for flight stresses, indicating a similar
Therian mammals would only achieve powered flight and gliding long
after these early aeronauts became extinct, with the earliest known
gliding metatherians and bats evolving in the Paleocene.
Recently it has been found that endothermy was present as far back as
the late carboniferous, with Ophiacodon. The presence of
fibrolamellar, a specialised type of bone that can grow quickly while
maintaining a stable structure, shows that
Ophiacodon would have used
its high internal body temperature to fuel a fast growth comparable to
Evolution of mammals
Archaeothyris, one of the oldest synapsids found.
Ophiacodon and Varanops.
Archaeothyris and Clepsydrops, the earliest known synapsids, lived
in the Pennsylvanian subperiod (323-299 Mya) of the Carboniferous
period and belonged to the series of primitive synapsids which are
conventionally grouped as pelycosaurs. The pelycosaurs spread and
diversified, becoming the largest terrestrial animals in the latest
Carboniferous and Early
Permian periods, ranging up to 6 metres
(20 ft) in length. They were sprawling, bulky, possibly
cold-blooded, and had small brains. Some, such as Dimetrodon, had
large sails that might have helped raise their body temperature. A few
relict groups lasted into the later
Permian but, by the middle of the
Late Permian, all of the pelycosaurs had either died off or evolved
into their successors, the therapsids.
Moschops was a tapinocephalian from the Middle
Permian of South
The therapsids, a more advanced group of synapsids, appeared during
Permian and included the largest terrestrial animals in the
Middle and Late Permian. They included herbivores and carnivores,
ranging from small animals the size of a rat (e.g.: Robertia), to
large, bulky herbivores a ton or more in weight (e.g.: Moschops).
After flourishing for many millions of years, these successful animals
were all but wiped out by the Permian-
Triassic mass extinction about
250 mya, the largest known extinction in Earth's history, possibly
related to the
Siberian Traps volcanic event.
Nikkasaurus was an enigmatic synapsid from the Middle
Lystrosaurus was the most common synapsid shortly after the
Triassic extinction event.
Only a few therapsids went on to be successful in the new early
Triassic landscape; they include
Lystrosaurus and Cynognathus, the
latter of which appeared later in the early Triassic. Now, however,
they were accompanied by the early archosaurs (soon to give rise to
the dinosaurs). Some of these, such as Euparkeria, were small and
lightly built, while others, such as Erythrosuchus, were as big as or
bigger than the largest therapsids.
Permian extinction, the synapsids did not count more than
three surviving clades. The first comprised the therocephalians, which
only lasted the first 20 million years of the
Triassic period. The
second were specialised, beaked herbivores known as dicynodonts (such
as the Kannemeyeriidae), which contained some members that reached
large size (up to a tonne or more). And finally there were the
increasingly mammal-like carnivorous, herbivorous, and insectivorous
cynodonts, including the eucynodonts from the
Olenekian age, an early
representative of which was Cynognathus.
Cynognathus was the largest predatory cynodont of the Triassic.
Unlike the dicynodonts, which were large, the cynodonts became
progressively smaller and more mammal-like as the
though some forms like
Trucidocynodon remained large. The first
mammaliaforms evolved from the cynodonts during the early
of the Late Triassic, about 225 mya.
During the evolutionary succession from early therapsid to cynodont to
eucynodont to mammal, the main lower jaw bone, the dentary, replaced
the adjacent bones. Thus, the lower jaw gradually became just one
large bone, with several of the smaller jaw bones migrating into the
inner ear and allowing sophisticated hearing.
Repenomamus was the largest mammal of the Mesozoic.
Whether through climate change, vegetation change, ecological
competition, or a combination of factors, most of the remaining large
cynodonts (belonging to the Traversodontidae) and dicynodonts (of the
family Kannemeyeriidae) had disappeared by the
Rhaetian age, even
before the Triassic-
Jurassic extinction event that killed off most of
the large nondinosaurian archosaurs. The remaining
were small, ranging from the size of a shrew to the badger-like mammal
Tritylodon was a cynodont that lived in Early Jurassic.
Jurassic and Cretaceous, the remaining nonmammalian
cynodonts were small, such as Tritylodon. No cynodont grew larger than
a cat. Most
Cretaceous cynodonts were herbivorous, though
some were carnivorous. The family Tritheledontidae, that first
appeared near the end of the Triassic, was carnivorous and persisted
well into the Middle Jurassic. The other, Tritylodontidae, first
appeared at the same time as the tritheledonts, but was herbivorous.
This group became extinct at the end of the Early
Dicynodonts are thought to have become extinct near the end of the
Triassic period, but there is evidence this group survived. New fossil
finds have been found in the
Cretaceous rocks of Gondwana[citation
Today, the 5,500 species of living synapsids, known as the mammals,
include both aquatic (whales) and flying (bats) species, and the
largest animal ever known to have existed (the blue whale). Humans are
synapsids, as well. Most mammals are viviparous and give birth to live
young rather than laying eggs with the exception being the monotremes.
Jurassic ancestors of living mammals, along with their
close relatives, had high metabolic rates. This meant consuming food
(generally thought to be insects) in much greater quantity. To
facilitate rapid digestion, these synapsids evolved mastication
(chewing) and specialized teeth that aided chewing. Limbs also evolved
to move under the body instead of to the side, allowing them to
breathe more efficiently during locomotion. This helped make it
possible to support their higher metabolic demands.
Below is a cladogram of the most commonly accepted phylogeny of
synapsids, showing a long stem lineage including
successively more basal clades such as Theriodontia, Therapsida, and
Most uncertainty in the phylogeny of synapsids lies among the earliest
members of the group, including forms traditionally placed within
Pelycosauria. As one of the earliest phylogenetic analyses, Brinkman
& Eberth (1983) placed the family
the most basal offshoot of the synapsid lineage. Reisz (1986) removed
Varanopidae from Caseasauria, placing it in a more derived position on
the stem. While most analyses find
Caseasauria to be the most basal
synapsid clade, the Benson's analysis (2012) placed a clade containing
Varanopidae as the most basal synapsids, with
Caseasauria occupying a more derived position. Benson attributed this
revised phylogeny to the inclusion of postcranial characteristics, or
features of the skeleton other than the skull, in his analysis. When
only cranial or skull features were included,
Caseasauria remained the
most basal synapsid clade. Below is a cladogram modified from the
analysis of Benson (2012):
Elliotsmithia longiceps (BP/1/5678)
Euromycter rutenus (="Casea" rutena)
However, more recent examination of the phylogeny of basal synapsids,
incorporating newly described basal caseids and eothyridids,
Caseasauria to its position as the sister to all other
synapsids. Brocklehurst et al. (2016)  demonstrated that many of
the postcranial characters used by Benson (2012) to unite Caseasauria
Edaphosauridae were absent in the newly
discovered postcranial material of eothyridids, and were therefore
List of synapsids
Timeline of evolution
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Wikispecies has information related to Synapsida
Synapsida - Pelycosauria - at Palaeos
Vertebrate Fossils - includes description of important
transitional genera in the evolutionary sequence linking primitive
synapsids to mammals
Extant chordate classes
Ascidiacea (sea squirts)
Thaliacea (pyrosomes, salps, doliolids)
(Vertebrates + Myxini)
(fish + Tetrapods)
Agnatha (jawless fish)
Chondrichthyes (cartilaginous fish: sharks, rays, chimaeras)
Actinopterygii (ray-finned fish)
Squamata (scaled reptiles)²
¹subclasses of Sarcopterygii
²orders of class Reptilia (reptiles)
³traditionally placed in Anapsida
italic are paraphyletic groups