Synapsida (Extinct Advanced article
Mammal Relatives) Article Contents
• Basic Features
Hans-Dieter Sues, • DiversityNational Museum of Natural History, Washington, District of
• Habitats and Abundance
Columbia, USA • Habits and Life Histories
• Fossil History and Phylogeny
Based in part on the previous version of this eLS article ‘Synapsida
(Mammal-like Reptiles)’ (2001) by Hans-Dieter Sues.
Published online: 30th September 2020
In the phylogenetic system, the Synapsida include Diversity
Mammalia and all other amniotes more closely
related to Mammalia than to Reptilia. The most The oldest synapsids currently known are from the Late Carbonif-
characteristic feature is the presence of a sin- erous (Pennsylvanian) of Nova Scotia (Canada), over 310 million
gle large opening behind the orbit on either years ago (Angielczyk and Kammerer, 2018). By the early Per-
side of the skull. This opening is small in basal mian, nonmammalian synapsids were already the most diverse
synapsids but increases in size in later, more group of terrestrial vertebrates, and they reached the acme of
derived forms including mammals. Nonmam- their diversity and abundance in the late Permian. The number
malian synapsids form an extensive series of of synapsid taxa dropped significantly during the mass extinction
at the end of that period when several major lineages of therapsids
taxa that are successively more closely related to
vanished. During the Triassic, some groups of therapsids diversi-
mammals and thus are now often referred to as fied again, but diapsid reptiles soon came to outnumber synapsids
stem-mammals or ancient mammal relatives. They in communities of land-dwelling vertebrates. In the latter part of
range from the Late Carboniferous to the Early the Triassic, the closest relatives of mammals such as Morganu-
Cretaceous. Crown-groupmammals (which include codon first appeared (Kielan-Jaworowska et al., 2004). Mammals
 all present-day mammals) are first known from that are part of the group that includes all extant species first 
the Middle Jurassic. Synapsida and Reptilia are appeared in the Middle Jurassic. See also: Extinction: End-Per-
now considered two entirely distinct evolutionary mian Mass Extinction; Biotic Recoveries after Extinction
lineages of Amniota with a common ancestor in
the Carboniferous. Habitats and Abundance
Nonmammalian synapsids are known only from continental set-
Basic Features tings. Most of them were clearly land-dwellers. Fossils of theseanimals have been discovered on every present-day continent
including Antarctica as well as in Greenland, Japan and Mada-
The most diagnostic feature of Synapsida, which is retained in gascar. ‘Pelycosaurs’ and therapsids probably occupied all major
extant mammals, is the presence of a single temporal open- habitats on land, but appear most abundant in lowland floodplain
ing behind the orbit on either side of the skull (Figure 1). settings. However, this distribution reflects, at least in part, the
This opening is typically bounded by the jugal, postorbital and fact that skeletal remains of vertebrates are much more likely
squamosal bones. In more derived nonmammalian synapsids to be preserved in sedimentary rocks deposited by rivers and in
and in mammals, this opening becomes greatly enlarged and lakes. By far the greatest diversity of therapsids is known from
the jaw-closing muscles insert along the margins of and on the the Permo–Triassic continental strata of the Beaufort Group of
connective tissue (fascia) covering the opening. Early-diverging the Karoo in South Africa; some species are documented by
synapsids (pelycosaurs) are readily distinguished from other thousands of fossils (Rubidge and Sidor, 2001; Angielczyk and
early amniotes by various features of the skull, especially the Kammerer, 2018). The Karoo therapsids apparently lived in tem-
forward-sloping, plate-like occiput and the presence of a single perate, fairly moist climates at relatively high palaeolatitudes. See
postparietal bone. also: Fossil Record: Quality
eLS subject area: Evolution & Diversity of Life Habits and Life Histories
How to cite:
Sues, Hans-Dieter. Synapsida (Extinct Mammal Relatives), eLS, As for most extinct vertebrates, there is scant evidence concern-
Vol 1: 394–397, 2020. ing the life histories of nonmammalian synapsids. They prob-
DOI: 10.1002/9780470015902.a0029199 ably hatched from possibly soft-shelled eggs as this mode of
This is a US Government work and is in the public domain in the United States of America.
Volume 1, Issue 2, September 2020 eLS © 2020, John Wiley & Sons, Ltd. www.els.net 394


Synapsida (Extinct Mammal Relatives)
lineages. During the late Palaeozoic, nonmammalian synap-
sids included both the dominant carnivores and herbivores in
communities of land-dwelling vertebrates. One herbivorous
lineage, the dicynodont therapsids, became especially diverse
and abundant and included some large-bodied forms as well
as small, superficially rather mole-like animals specialised for
subterranean digging. Some therapsids built and lived together
in burrows.
Fossil History and Phylogeny
Nonmammalian synapsids have a long, rich fossil history that
(a) extends from the Late Carboniferous (Pennsylvanian) to the Early
Cretaceous, spanning some 200 million years. Indeed, they have
one of the most extensive fossil records of any major group of
vertebrates, and this record illustrates in remarkable detail the
evolutionary transition from nonmammalian amniotes to mam-
mals. Traditionally, palaeontologists distinguished three ‘grades’
of body plan among Synapsida – the basal ‘Pelycosauria’, the
intermediate Therapsida, and the derivedMammalia (Angielczyk
and Kammerer, 2018; Figure 1). ‘Pelycosaurs’ were the first to
appear in the fossil record (Late Carboniferous), and mammals
are the geologically youngest group to appear (Middle Jurassic).
(b) Since the 1980s, the application of phylogenetic methods has
elucidated the evolutionary history of Synapsida. However, the
relationships of many groups, especially the oldest therapsids,
are still poorly understood. See also: Fossil Record; Fossils in
 Phylogenetic Reconstruction 
‘Pelycosaurs’
The Late Carboniferous and Permian ‘pelycosaurs’ represent
a series of basal synapsids that are successively more closely
related to therapsids (which include mammals as a derived clade)
(c) and thus are no longer considered a monophyletic group in
phylogenetic classifications (Angielczyk and Kammerer, 2018).
Figure 1 Evolution of the key feature of Synapsida – the temporal opening The most basal clade, Caseasauria, includes the Caseidae, which
(highlighted in light blue) –from small temporal opening to fully developed represented the first major radiation of plant-eating tetrapods
temporal fossa (which is confluent with the orbit in many mammals). on land and are known from the Late Carboniferous to middle
(a) sphenacodontid ‘pelycosaur’ Dimetrodon; (b) gorgonopsian therapsid Permian of Europe and North America. The skull of caseids is
Sauroctonus; (c) opossum (Didelphis). Not to scale. Modified from Bystrov
(1957). unusually small relative to the massive body, especially in more
derived forms such as the early Permian Cotylorhynchus. The
snout has a pointed end that extends forward beyond the tooth
reproduction is primitive for amniotes and still retained by the rows andmuch enlarged external narial openings. The teeth of the
most primitive living mammals, the monotremes. Known juve- more derived forms have spatulate crowns with apical denticles.
niles of ‘pelycosaurs’ and therapsids appear to differ little in In all other ‘pelycosaurs’, the snout is deep dorsoventrally and
skeletal structure from adult conspecifics. It is likely that, as in rather narrow transversely, and the frontal bone enters broadly
extant nonmammalian amniotes, most of them received little if into the dorsal margin of the orbit. The Varanopidae comprise
any parental care and started fending for themselves soon after lightly built carnivorous forms with highly specialised skulls and
hatching. Nothing is known about the lifespan of nonmammalian rather long, slender limbs and range into the mid-Permian; they
synapsids. Most early synapsids were likely ectotherms, depen- include the only ‘pelycosaurs’ known so far from South Africa.
dent on external sources of heat to raise their body temperature The Ophiacodontidae, from the Late Carboniferous and early
above ambient temperature. Therapsids more closely related to Permian of North America and Europe, are characterised by a
mammals show bone microstructure suggestive of higher growth proportionately large skull. The best-known form, Ophiacodon,
and thus higher metabolic rates. has a deep, long snout, and the small orbits and temporal opening
Early synapsids were insectivorous or carnivorous, but her- are placed high on the sides of the skull. The Edaphosauridae,
bivory developed repeatedly and independently in several from the Late Carboniferous and early Permian of Europe and
This is a US Government work and is in the public domain in the United States of America.
Volume 1, Issue 2, September 2020 eLS © 2020, John Wiley & Sons, Ltd. www.els.net 395


Synapsida (Extinct Mammal Relatives)
North America, are characterised by the presence of a large ‘sail’ the mandible could be retracted to cut or grind fodder between
supported by greatly elongated neural spines on the cervical and the edges of the upper and lower beaks. Although dicynodonts
dorsal vertebrae that are studded with short lateral protuberances. suffered a massive loss in species diversity at the end of the
Edaphosaurus, the best-known representative, has a proportion- Permian, one lineage evolved very large plant-eating forms that
ately small skull with massive crushing tooth plates on the palate survived until the end of the Triassic.
and lower jaws. It is considered one of the oldest known herbiv- The Theriodontia comprise the most derived group of euther-
orous tetrapods. Edaphosaurids are the sister-group of the most apsids (Rubidge and Sidor, 2001; Angielczyk and Kammerer,
derived ‘pelycosaurs’, the carnivorous Sphenacodontidae. The 2018). They are characterised by the presence of a distinct,
latter are known from the Late Carboniferous and early Permian free-standing coronoid process on the dentary, which served as
of Europe and North America and include the top predators the site of attachment for jaw-closing muscles. The quadrate and
in the early Permian continental ecosystems of these regions, quadratojugal bones are no longer suturally connected to the
includingDimetrodon. Sphenacodontids are closely related to the squamosal but sit loosely in a depression on the anterior surface
Therapsida, with which they share the presence of a posteriorly of that bone. The earliest theriodonts, the middle to late Permian
notched bony sheet (reflected lamina) on the angular bone of the Gorgonopsia, have rather slender limbs that were more verti-
lower jaw and the possession of a deflected retroarticular process cally oriented than those in more basal synapsids. Their preda-
of the articular bone. The skulls of Dimetrodon and its relatives tory habits are reflected by the presence of enormous, blade-like
further resemble those of therapsids in having a tall maxilla canines and large incisors. The skull of more derived theriodonts
that excludes the lacrimal from the margin of the external narial (Eutheriodontia) is characterised by the possession of a wide,
opening and the presence of large canine-like teeth in the upper dorsally completely open temporal fossa and the anteroposterior
and lower jaws.Dimetrodon is best known for its ‘sail’ supported expansion of the epipterygoid bone to form the anterior portion of
by tall, slender neural spines along much of the vertebral column. the sidewall of the braincase. Both features already occur in the
late Permian to Middle Triassic Therocephalia but became most
Therapsids prominently developed in the Cynodontia.
Cynodonts, which include mammals as a subgroup in phyloge-
Whereas ‘pelycosaurs’ are mainly known from North America netic classifications, first appear near the end of the Permian in
and Europe, the oldest undisputed therapsids date from the mid- Africa and Europe and diversified during the Triassic (Rubidge
dle Permian of China, Russia, and South Africa (Angielczyk and Sidor, 2001; Angielczyk and Kammerer, 2018). Their denti-
and Kammerer, 2018). The latter forms still closely resemble tion comprises incisors, canines, and increasingly complex post-
 sphenacodontid ‘pelycosaurs’ in much of their skeletal struc- canine (cheek) teeth with cingula and smaller cusps in addition to 
ture, but their skulls have proportionately much larger temporal the main cusp. In some derived, omnivorous or herbivorous cyn-
openings, which become fossae for the jaw-closing muscles, and odonts (Gomphodontia), the crowns of the upper and lower post-
the occiput slopes backwards rather than forwards. The dentition canines are transversely expanded andmet in precise occlusion. A
comprises fewer teeth, and the upper canines are much enlarged. secondary bony palate formed bymid-line fusion of the premaxil-
The shoulder and pelvic girdles are less massive and the limbs are lary, maxillary and palatine bones and separated the nasal passage
more slender than those in sphenacodontids such as Dimetrodon. from the oral cavity. The zygomatic arches curve outwards, and
The bones of the shoulder girdle become reduced in size and, in a superficial slip of the jaw-closing musculature, which devel-
most therapsids, mobility at the shoulder joint increased. oped into the masseter muscles in mammals, passed through the
More derived therapsids (Eutherapsida) show even greater space between the zygomatic arch and the coronoid process of
expansion of the temporal openings. The most primitive group the dentary to insert on the lateral surface of that bone (Lauten-
of eutherapsids are the Dinocephalia (Angielczyk and Kam- schlager et al., 2017). The dentary is the largest bone of the lower
merer, 2018). They are large-bodied animals with massively built jaw. The quadrate, quadratojugal and postdentary bones are much
skeletons. The enlarged upper and lower incisors met in a pre- reduced, and the latter were only loosely attached to the dentary.
cise, interlocking fashion. Dinocephalians include both carni- They probably functioned as part of a sound-transmitting mecha-
vores (Anteosauridae) and herbivores (Tapinocephalidae). The nism involving a tympanicmembrane at the back of the lower jaw.
latter are particularly noteworthy for the great thickening of the In mammals, these bones became completely separated from the
bones forming the skull roof, which may have been used as a lower jaw and integrated into the middle ear. Whereas the fore-
battering ram in head butting. limb of nonmammalian cynodonts retained a sprawling posture
Another major group of eutherapsids are the Anomodontia, similar to that in earlier synapsids, the femur was already held in
which include the dominant terrestrial herbivores during the latter a much more vertical position.
part of the Permian and part of the Triassic (Fröbisch, 2007). The A recently discovered group of small-bodied derived cyn-
most diverse group of anomodonts were the Dicynodontia. They odonts, the Middle to Late Triassic Brasilodontidae, is
are distinguished by their highly modified skull, which has a short noteworthy for the presence of certain mammalian features
snout and a long temporal region behind the orbits. The tips of the like postcanine teeth with divided roots and the presence of
snout andmandible usually lack teeth. Instead, the jawbones form a promontory on the prootic (which encloses the cochlea of
sharp cutting edges, which were probably covered by a keratinous the inner ear) (Bonaparte, 2012). Traditionally, mammals were
beak in life. In most dicynodonts, the upper canines are the only defined based on features such as the development of a jaw joint
remaining teeth. The facet for the jaw joint on the articular bone between the dentary and squamosal bones (Kielan-Jaworowska
is nearly twice as long as that on the quadrate; this indicates that et al., 2004), but these features have a much wider distribution
This is a US Government work and is in the public domain in the United States of America.
Volume 1, Issue 2, September 2020 eLS © 2020, John Wiley & Sons, Ltd. www.els.net 396


Synapsida (Extinct Mammal Relatives)
than traditionally assumed. Following Rowe (1988), most References
researchers now distinguish between Mammaliaformes and
Mammalia. Mammaliaformes comprises the last common ances- Angielczyk KD and Kammerer CF (2018) Non-mammalian synap-
tor of Morganucodon and Mammalia and all its descendants. sids: the deep roots of the mammalian family tree. In: Zachos FE
Mammalia, the crown group, comprises the last common ancestor and Asher RJ (eds) Mammalia: Mammalian Evolution, Diversity
of monotremes, marsupials and placentals and all its descendants. and Systematics, pp 117–198. de Gruyter: Berlin.
Morganucodon and its close relatives have already numerous Bonaparte JF (2012) Evolution of the Brasilodontidae. Historical
features found in present-day mammals but, for example, the Biology 25: 643–653.
novel dentary-squamosal jaw joint still existed side-by-side with Bystrov AP (1957) Past, Present, and Future of Man (In Russian).
the primitive articular-quadrate jaw joint. An exceptionally pre- Medgiz: Leningrad.
served skeleton of a Middle Jurassic mammaliaform from China Fröbisch J (2007) The cranial anatomy of Kombuisia frerensis Hot-
provides the oldest known record of fur (although hair probably ton (Synapsida, Dicynodontia) and a new phylogeny of anomod-
evolved earlier).Morganucodon is known from the Late Triassic ont therapsids. Zoological Journal of the Linnean Society 150:
and Early Jurassic of Europe, China, and North America. The 117–144.
oldest known representatives of crown-group mammals in the Kielan-Jaworowska Z, Cifelli RL and Luo Z-X (2004) Mammals
fossil record date from the Middle Jurassic (Luo et al., 2011). from the Age of the Dinosaurs: Origins, Evolution, and Structure.
Columbia University Press: New York.
Glossary Lautenschlager S, Gill P, Luo Z-X, et al. (2017) Morphologicalevolution of the mammalian jaw adductor complex. Biological
Amniote A vertebrate that produces eggs that contain a Reviews 92: 1910–1940.
membrane (amnion) enclosing the developing embryo in a Luo Z-X, Yuan C-X, Meng Q-J, et al. (2011) A Jurassic eutherian
mammal and divergence of marsupials and placentals. Nature 476:
fluid-filled chamber. Amniotes include reptiles, birds, and
442–445.
mammals. Rowe TB (1988) Definition, diagnosis, and origin of Mammalia.
Cingulum (pl. cingula) A band of small cusps encircling the Journal of Vertebrate Paleontology 8: 241–264.
principal cusps on a tooth crown. Rubidge B and Sidor CA (2001) Evolutionary patterns among
Crown-group An assemblage comprising the last common Permo-Triassic therapsids. Annual Review of Ecology and System-
ancestor of all present-day members of a group of organisms atics 32: 449–480.
and all of its extant and extinct descendants.
 Epipterygoid Bone in the anterior portion of the bony sidewall
of the braincase. Further Reading

Fossa Depression or hollow in a bony structure.
Benton MJ (2014) Vertebrate Palaeontology, 4th edn. Wiley Black-
Lacrimal Bone in the front wall of the eye socket (orbit) well: Chichester.
enclosing the tear (lacrimal) duct. Kammerer CF, Angielczyk KD and Fröbisch J (eds) (2014) Early
Naris (pl. nares) Opening of the nasal passage. The external Evolutionary History of the Synapsida. Springer: Dordrecht.
opening is the nostril. Kemp TS (2005) The Origin and Evolution of Mammals. Oxford
Occiput Back of the skull. University Press: Oxford.
Prootic Bone in the wall of the braincase that bounds part of Kielan-Jaworowska Z, Cifelli RL and Luo Z-X (2004) Mammals
the inner ear. from the Age of the Dinosaurs: Origins, Evolution, and Structure.
Zygomatic arch Bony bridge extending from the orbit to the Columbia University Press: New York.
back end of the skull and enclosing the temporal fossa for the Zachos FE and Asher RJ (eds) (2018) Mammalia: Mammalian Evo-
jaw-closing muscles on the side of the skull. lution, Diversity and Systematics. de Gruyter: Berlin.
This is a US Government work and is in the public domain in the United States of America.
Volume 1, Issue 2, September 2020 eLS © 2020, John Wiley & Sons, Ltd. www.els.net 397