Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniote
s, the other being the sauropsids
, the group that includes
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocepha ...
and birds. The group includes
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
s and every animal more closely related to mammals than to sauropsids. Unlike other amniotes, synapsids have a single
An infratemporal fenestra, also called the lateral temporal fenestra or simply temporal fenestra, is an opening in the skull behind the orbit in some animals. It is ventrally bordered by a zygomatic arch. An opening in front of the eye sockets, ...
, an opening low in the
The skull roof, or the roofing bones of the skull, are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium.
In compara ...
behind each eye
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a ...
, leaving a bony arch
beneath each; this accounts for their name.
The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous
[ when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.
Traditionally, non-mammalian synapsids were believed to have evolved from reptiles, and therefore described as mammal-like reptiles in classical systematics, and primitive synapsids were also referred to as pelycosaurs, or pelycosaur- grade synapsids. These ] paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ... terms have now fallen into disfavor and are only used informally (if at all) in modern literature, because it is now known that synapsids are not reptiles, nor are they part of reptilian lineage in a cladistical sense. They are now more correctly referred to as stem mammals, and sometimes as proto-mammals, or paramammals.
Synapsids were the largest terrestrial vertebrates in the Permian
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ... period, 299 to 251 million years ago, equalled only by some large pareiasaurs at the end of the Permian. Most lineages of pelycosaur-grade synapsids were replaced at the end of Early Permian by the more advanced therapsids. Synapsid numbers and variety were severely reduced by the Permian–Triassic extinction. Only the therapsid dicynodonts and eutheriodonts (consisting of Therocephalia and Cynodontia) are known to have continued into the Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest perio ... period. The cynodont group Probainognathia
Probainognathia is one of the two major subgroups of the clade Eucynodontia, the other being Cynognathia. The earliest forms were carnivorous and insectivorous, though some groups eventually also evolved herbivorous diets. The earliest and mo ..., which includes Mammaliaformes
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent ... (mammals and their closer ancestors), were the only synapsids to survive beyond the Triassic.
During the Triassic, the sauropsid archosaur
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...s became the largest and most numerous land vertebrates, and gave rise to the dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is .... When all non-avian dinosaurs were wiped out by the Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With t ..., the mammalian synapsids diversified again to become the largest land and marine animals on Earth.
Linnaean and cladistic classifications
At the turn of the 20th century synapsids were thought to be one of the four main subclasses of
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchoce ...s. However, this notion was disproved upon closer inspection of skeletal remains, as synapsids are differentiated from reptiles by their distinctive temporal openings. These openings in the skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ... bones allowed the attachment of larger jaw muscles, hence a more efficient bite.
Synapsids were subsequently considered to be a later reptilian lineage that became mammals by gradually evolving increasingly mammalian features, hence the name "mammal-like reptiles" (also known as pelycosaurs). These became the traditional terms for all Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ... (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within Sauropsida
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early s ... (sauropsids), the sister group to synapsids, thus making synapsids their own taxonomic group. [
As a result, the ] paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ... terms "mammal-like reptile" and "pelycosaur" are seen as outdated and disfavored in technical literature, and the term stem mammal (or sometimes protomammal or paramammal) is used instead. Phylogenetically, it is now understood that synapsids comprise an independent branch of the tree of life
The tree of life is a fundamental archetype in many of the world's mythological, religious, and philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The Assyrian Sacred Tree: A Histor .... The monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ... of Synapsida is not in doubt, and the expressions such as "Synapsida contains the mammals" and "synapsids gave rise to the mammals" both express the same phylogenetic hypothesis. This terminology reflects the modern cladistical approach to animal relationships, according to which the only valid groups are those that include all of the descendants of a common ancestor: these are known as monophyletic
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ... groups, or clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...s.
Additionally, Reptilia (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within Sauropsida
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early s ..., the sister group of Synapsida within Amniota.
Primitive and advanced synapsids
The synapsids are traditionally divided for convenience, into therapsids, an advanced group of synapsids and the branch within which mammals evolved, and stem mammals, (previously known as pelycosaurs), comprising the other six more primitive families of synapsids. Stem mammals were all rather lizard-like, with sprawling gait and possibly horny
A scute or scutum ( Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior ...s, while therapsids tended to have a more erect pose and possibly hair, at least in some forms. In traditional taxonomy, the Synapsida encompasses two distinct grades: the low-slung stem mammals have given rise to the more erect therapsids, who in their turn have given rise to the mammals. In traditional vertebrate classification, the stem mammals and therapsids were both considered orders of the subclass Synapsida.
Practical versus phylogenetic usage of “synapsid” and “therapsid”
Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ..., the terms are used somewhat differently, as the daughter clades are included. Most papers published during the 21st century have treated "Pelycosaur" as an informal grouping of primitive members. Therapsida has remained in use as a clade containing both the traditional therapsid families and mammals.
Although Synapsida and Therapsida includes modern mammals, in practical usage, those two terms are used almost exclusively when referring to the more basal members that lie outside of Mammaliaformes
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent ....
Synapsids evolved a
An infratemporal fenestra, also called the lateral temporal fenestra or simply temporal fenestra, is an opening in the skull behind the orbit in some animals. It is ventrally bordered by a zygomatic arch. An opening in front of the eye sockets, ... behind each eye orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a ... 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
The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the o ... 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.
Synapsids are characterized by having differentiated teeth. These include the canines,
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone t ..., and incisor
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wh ...s. The trend towards differentiation is found in some labyrinthodont
"Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). Traditionally consi ...s and early anapsid reptilians in the form of enlargement of the first teeth on the maxilla
The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ..., forming a form of protocanines. This trait was subsequently lost in the diapsid 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
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bon ..., 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
The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals.
In most vertebrates, the articular bone is connected to two oth ..., 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
The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations fro ..., incus
The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ... and stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foo ..., 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 cranial bone). [Salentijn, L. ''Biology of Mineralized Tissues: Prenatal Skull Development'', Columbia University College of Dental Medicine post-graduate dental lecture series, 2007]
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
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, ... 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
The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal .... In early synapsids, a secondary palate began to form on the sides of the maxilla
The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ..., still leaving the mouth and nostril connected.
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
In anatomy, the palatine bones () are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxillae, they comprise the hard palate. (''Palate'' is derived from the Latin .... 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
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),
A scute or scutum ( Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior ...s (protective structures of the dermis often with a horny covering), hair or fur, and scale-like structures (often formed from modified hair, as in pangolin
Pangolins, sometimes known as scaly anteaters, are mammals of the order Pholidota (, from Ancient Greek ϕολιδωτός – "clad in scales"). The one extant family, the Manidae, has three genera: '' Manis'', '' Phataginus'', and '' Smu ...s and some rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are ...s). While the skin of reptiles is rather thin, that of mammals has a thick dermal
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided in ... 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 scute
A scute or scutum ( Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior ...s; 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
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided in ... structures with a horny overlay, like those found in modern crocodile
Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant me ...s and turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...s. 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 squamate
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, i ...s.
It is currently unknown exactly when mammalian characteristics such as body hair
Body hair, or androgenic hair, is the terminal hair that develops on the human body during and after puberty. It is differentiated from the head hair and less visible vellus hair, which is much finer and lighter in color. The growth of androg ... and mammary gland
A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in prim ...s 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
In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 166.1 ± 4.0 Ma (million years ago) and 163.5 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding th ... (late middle Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...) '' 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.
See also the news item at
[ 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 metabolism.] The oldest examples of nocturnality in synapsids is believed to have been in species that lived more than 300 million years ago.
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ... coprolites from Russia 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 Late Carboniferous period,
may have laid parchment-shelled (leathery) eggs, which lacked a calcified layer, as most modern reptiles and monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brains ...s 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
Apocrine () glands are a type of exocrine gland, which are themselves a type of gland, i.e. a group of cells specialized for the release of secretions. Exocrine glands secrete by one of three means: holocrine, merocrine and apocrine. In apoc ...-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 ( echidna
Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and ...s carry their eggs and offspring via a temporary pouch), 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 salamander
Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...s 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
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early s ... lines). Cynodonts were almost certainly able to produce this, which allowed a progressive decline of yolk mass and thus egg size, resulting in increasingly altricial
In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ... hatchlings as milk became the primary source of nutrition, which is all evidenced by the small body size, the presence of epipubic bone
Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them).
They first occ ...s, and limited tooth replacement in advanced cynodonts, as well as in mammaliaforms.
Aerial locomotion first began in non-mammalian haramiyidan cynodonts, with '' Arboroharamiya'', '' Xianshou'', '' Maiopatagium'' and '' Vilevolodon'' bearing exquisitely preserved, fur-covered wing membranes that stretch across the limbs and tail. Their fingers are elongated, similar to those of bats and
Colugos () are arboreal gliding mammals that are native to Southeast Asia. Their closest evolutionary relatives are primates. There are just two living species of colugos: the Sunda flying lemur (''Galeopterus variegatus'') and the Philippin ...s and likely sharing similar roles both as wing supports and to hang on tree branches.
Within true mammals, aerial locomotion first occurs in volaticotherian eutriconodont
Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the ...s. A fossil '' Volaticotherium
''Volaticotherium antiquum'' (meaning "ancient gliding beast") is an extinct, gliding, insectivorous mammal that lived in Asia during the Jurassic period, around 164 mya. It is the only member of the genus ''Volaticotherium''.
The discovery of ...'' has an exquisitely preserved furry patagium
The patagium (plural: patagia) is a membranous body part that assists an animal in obtaining lift when gliding or flight. The structure is found in extant and extinct groups of flying and gliding animals including bats, birds, some dromae ... 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 lifestyle.
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...n mammals would only achieve powered flight and gliding long after these early aeronauts became extinct, with the earliest-known gliding metatheria
Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well ...ns and bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...s evolving in the Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''palai ....
Recently, it has been found that endothermy was developed as early as ''
''Ophiacodon'' (meaning "snake tooth") is an extinct genus of synapsid belonging to the family Ophiacodontidae that lived from the Late Carboniferous to the Early Permian in North America and Europe. The genus was named along with its type sp ...'' in the late Carboniferous. 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 modern endotherms.
'' Asaphestera'', ''
''Archaeothyris'' is an extinct genus of ophiacodontid synapsid that lived during the Late Carboniferous and is known from Nova Scotia. Dated to 306 million years ago, ''Archaeothyris'', along with a more poorly known synapsid called '' Echine ...'' and '' Clepsydrops'', the earliest-known synapsids, lived in the Pennsylvanian subperiod (323–299 mya) of the Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonif ... period and were one of many types of primitive synapsids that are now informally grouped together as stem mammals or sometimes as protomammals (previously known as pelycosaurs). The early synapsids spread and diversified, becoming the largest terrestrial animals in the latest Carboniferous and Early Permian periods, ranging up to 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
A relict is a surviving remnant of a natural phenomenon.
A relict (or relic) is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas.
Geology and geomorphology
In geology, a r ... groups lasted into the later Permian but, by the middle of the Late Permian, all had either died off or evolved into their successors, the therapsids.
The therapsids, a more advanced group of synapsids, appeared during the Middle 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
''Moschops'' (Greek for "calf face") is an extinct genus of therapsids that lived in the Guadalupian epoch, around 265–260 million years ago. They were heavily built plant eaters, and they may have lived partly in water, as hippopotamuses d ...''). 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
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed an ... in Earth's history, possibly related to the Siberian Traps volcanic event.
Only a few therapsids went on to be successful in the new early Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest perio ... landscape; they include '' Lystrosaurus'' and '' Cynognathus'', the latter of which appeared later in the early Triassic. However, they were accompanied by the early archosaur
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...s (soon to give rise to the dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is ...s). Some of these archosaurs, such as '' Euparkeria'', were small and lightly built, while others, such as '' Erythrosuchus
''Erythrosuchus'' (from el, ἐρυθρός , 'red' and el, σοῦχος , 'crocodile') is an extinct genus of archosauriform reptile from the Triassic of South Africa. Remains have been found from the Cynognathus Assemblage Zone of the Beauf ...'', were as big as or bigger than the largest therapsids.
After the 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
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided i ... age, an early representative of which was ''Cynognathus''.
Unlike the dicynodonts, which were large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed, though some forms like '' Trucidocynodon'' remained large. The first mammaliaforms evolved from the cynodonts during the early Norian
The Norian is a division of the Triassic Period. It has the rank of an age (geochronology) or stage (chronostratigraphy). It lasted from ~227 to million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.
Stratigraphic ... age 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
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in t ... and allowing sophisticated hearing.
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
The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ... age, even before the Triassic–Jurassic extinction event
The Triassic–Jurassic (Tr-J) extinction event, often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, , and is one of the top five major extinction events of the Phanerozoic eon, profoundly affect ... that killed off most of the large non-dinosaurian archosaurs. The remaining Mesozoic synapsids were small, ranging from the size of a shrew to the badger-like mammal '' Repenomamus
''Repenomamus'' (Latin: "reptile" (reptilis), "mammal" (mammalis)) is a genus of opossum- to badger-sized gobiconodontid mammal containing two species, ''Repenomamus robustus'' and ''Repenomamus giganticus''. Both species are known from fossils ...''.
During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as '' Tritylodon''. No cynodont grew larger than a cat. Most Jurassic and Cretaceous cynodonts were herbivorous
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ..., though some were carnivorous
A carnivore , or meat-eater ( Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and oth .... The family Tritheledontidae, which first appeared near the end of the Triassic, was carnivorous and persisted well into the Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formation .... 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 Cretaceous epoch. 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
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period
The Jurassic ( ) is a geologic period and st ....
Today, the 5,500 species of living synapsids, known as the mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...s, include both aquatic ( whales
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins an ...) and flying ( bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...s) species, and the largest animal ever known to have existed (the blue whale
The blue whale (''Balaenoptera musculus'') is a marine mammal and a baleen whale. Reaching a maximum confirmed length of and weighing up to , it is the largest animal known to have ever existed. The blue whale's long and slender body can b ...). Humans are synapsids, as well. Most mammals are viviparous
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ... and give birth to live young rather than laying eggs with the exception being the monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brains ...s.
Triassic and 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
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intes ..., these synapsids evolved mastication
Chewing or mastication is the process by which food is crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication process, ... (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
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ... of the most commonly accepted phylogeny
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological sp ... of synapsids, showing a long stem lineage including Mammalia and successively more basal clades such as Theriodontia, Therapsida and Sphenacodontia:
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 Varanopidae with Caseasauria as 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, Benson's analysis (2012) placed a clade containing Ophiacodontidae and 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
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ... modified from Benson's analysis (2012):
However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids, returned 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 with Sphenacodontidae
Sphenacodontidae (Greek: "wedge point tooth family") is an extinct family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. The most recent one, ''Dimetrodon angelensis'', is from the late Kungurian or ... and Edaphosauridae were absent in the newly discovered postcranial material of eothyridids, and were therefore acquired convergently.
* Lists of synapsids
* Mammal classification
List of prehistoric mammals
This is an incomplete list of prehistoric mammals. It does not include extant mammals or recently extinct mammals. For extinct primate species, see: list of fossil primates.Mikko's Phylogeny Archiv
*Genus †'' Adelobasileu ...
* Timeline of the evolutionary history of life
The timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth. Dates in this article are consensus estimates based on scientific evidence, m ...
* Vertebrate paleontology
Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord. It also tries to connect, by ...
Synapsida - Pelycosauria
- at Palaeos
- includes description of important transitional genera in the evolutionary sequence linking primitive synapsids to mammals
Tetrapod unranked clades
Extant Pennsylvanian first appearances
Taxa named by Henry Fairfield Osborn