Synapsids, synonymous with theropsids, 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 Carboniferous period. 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pelycosaurs were already gone, having been replaced by the more advanced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 and Epicynodontia, 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, was the only synapsids
who outlasted the Triassic. After the Cretaceous–Paleogene extinction event,
the synapsids again became the largest land animals.
The only extant synapsids today are mammals.
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 nonmammalian synapsids.
= The "mammal-like reptiles"= The traditional classification of
synapsids as reptiles is continued by some palaeontologists. 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 clades.
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 Pelycosauria and Therapsida were both considered orders of the subclass
Synapsida. 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 primitive members. 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 Mammaliaformes.
Characteristics = Temporal openings=
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. = Teeth=
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. = Jaw=
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, whereas the lower jaw of modern and prehistoric reptiles consists
of a conglomeration of smaller bones. 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, mammal-like reptiles
possess only a stapes. The malleus is derived from the articular, while the
incus is derived from the quadrate. 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 that does not connect to the squamosal, but connects to the quadrate
with a receding articular bone. = Palate=
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 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. 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, scutes, hair or fur, and scale-like structures.
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. 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. The oldest known fossil showing unambiguous imprints of hair is
the Callovian Castorocauda, a non-mammalian mammaliaform. 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 metabolism. = Metabolism=
The first pelycosaurs had the usual reptilian cold-blooded metabolism by all
indications, including a sprawling gait and a low slung body. However, there
appears to have been an early trend towards a form of temperature regulation
in several pelycosaur lines, as indicated by the large "sails" in both
edaphosaurids and some sphenacodontids. The sphenacodontids gave rise to the
therapsids, which lacked the sail and may have controlled their body
temperatures using metabolic heat. The legs and feet of the early therapsid
groups point to a more erect posture, traditionally interpreted as a sign of
more efficient metabolism. The presence of large turbinates acting as moisture
traps in the nasal passage found in therocephalian and cynodont therapsids,
but not in pelycosaurs, is additional evidence for the shift in metabolism in
these groups. In the later cynodonts, the presence of a secondary palate,
erect posture and other indicators of high metabolic rate suggests many
mammalian features had evolved by this stage. The high metabolism of the
advanced forms forced the evolution of hair only when mouse-sized animals
evolved in the synapsid-mammal transition.
Evolutionary history Archaeothyris and Clepsydrops, the
earliest known synapsids, lived in the Pennsylvanian subperiod 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 in length. They were sprawling, bulky,
and cold-blooded, and had small brains. Some, such as Dimetrodon, had large
sails that may 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. 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, to large,
bulky herbivores a ton or more in weight. 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, which may have been related to the Siberian Traps volcanic
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. 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.
After the Permian extinction, the synapsids did not count more than three
surviving clades. The first were the therocephalians, which only lasted the
first 20 million years of the Triassic period. The second survivors were
specialised, beaked herbivores known as dicynodonts, contained some members that
reached large size. 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. Unlike the dicynodonts, which remained
large, the cynodonts became progressively smaller and more
mammal-like as the Triassic progressed, though some forms like Trucidocynodon
remained large sized. The first mammaliaforms evolved from the cynodonts
during the early Norian 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 and allowing sophisticated
hearing. Whether through climate change,
vegetation change, ecological competition, or a combination of
factors, most of the remaining large cynodonts and dicynodonts had
disappeared by the Rhaetian age, even before the Triassic-Jurassic extinction
event that killed off most of the large nondinosaurian archosaurs. The remaining
Mesozoic synapsids were small, ranging from the size of a shrew to the
badger-like mammal Repenomamus. During the Jurassic and Cretaceous, the
remaining nonmammalian cynodonts were small, such as Tritylodon. No cynodont
grew larger than a cat. Most Jurassic and Cretaceous cynodonts were
herbivorous, though some were carnivorous. The family Tritheledontidae
first appeared near the end of the Triassic. They were carnivorous and
persisted well into the Middle Jurassic. The other, Tritylodontidae, first
appeared at the same time as the tritheledonts, but they were
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.
Today, the 5,500 species of living synapsids, known as the mammals, include
both aquatic and flying species, and the largest animal ever known to have
existed. Humans are synapsids, as well. Unique among the synapsids, however,
most mammals are viviparous and give birth to live young rather than laying
eggs, with the exception of the monotremes.
Triassic and Jurassic ancestors of living mammals, along with their close
relatives, had high metabolic rates. This meant consuming food in much
greater quantity. To facilitate rapid digestion, these synapsids evolved
mastication 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. Relationships
Below is a cladogram of the most commonly accepted phylogeny 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
placed the family Varanopidae with Caseasauria as the most basal ofshoot of
the synapsid lineage. Reisz 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 analysis of Benson 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 modified from the analysis of Benson:
See also Anapsida
Diapsida Euryapsida
List of synapsids Mammal classification
Prehistoric mammal Timeline of evolution
Vertebrate paleontology References
Further reading Colbert, E. H.. Evolution of the
Vertebrates. New York: John Wiley & Sons Inc. ISBN 0-471-16466-6.
External links Synapsida - Pelycosauria - at Palaeos
Transitional Vertebrate Fossils - includes description of important
transitional genera in the evolutionary sequence linking primitive synapsids to
mammals