For complete phylogeny, see text.
Synapsids ('fused arch') also known as Theropsids ('beast face'), and traditionally described as 'mammal-like reptiles', are a group of amniotes (the other being the sauropsids) that developed one opening in their skull (temporal fenestra) behind each eye, about 320 million years ago (mya) during the late Carboniferous Period.
At the turn of the 20th century, the Synapsids were originally defined as one of the five main subclasses of reptiles, on the basis of their distinctive temporal openings. These openings in the cheek bones allowed attachment of larger jaw muscles, hence a more efficient bite. The synapsids represented the reptilian lineage that led to the mammals and gradually evolved increasingly mammalian features, hence the term 'mammal-like reptiles'.
The traditional classification continued through to the late 1980s (see e.g. Carroll 1988). In the 1990s this approach was replaced by a cladistic one, according to which the only valid groups are those that include common ancestors and all their descendants. Because synapsids evolved into mammals, mammals are included under Clade Synapsida.
A recent, compromising position (see Benton 2004) has the class Synapsida as intentionally paraphyletic, constituting a grade of animals from the earliest split with sauropsids to the arbitrary division with its daughter class, Mammalia.
Archaeothyris and Clepsydrops were the earliest known synapsids. They belonged to a group called pelycosaurs and they lived in Pennsylvanian time of the Carboniferous Period. The pelycosaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals, in the latest Carboniferous and Early Permian Periods. They are currently divided into two clades, the Caseasauria and the Eupelycosauria. They were sprawling, bulky, cold-blooded and had small brains. They were the largest land animals of their time, ranging up to 3 m (10 ft) in length. Many, like Dimetrodon, had large sails that may have helped raise their body temperature. A few relict groups lasted into the later Permian.
The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian and went on to become the dominant large terrestrial animals during the latter half. They have dominated the world twice: once in the Permian and once in the Cenozoic, the current era. They were by far the most diverse and abundant animals of the Middle and Late Permian, including a diverse range of herbivores and carnivores, ranging from small animals the size of a rat (e.g: Robertia), to large bulky herbivores a tonne 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 extinction in Earth's history, which may have been related to the Siberian Traps volcanic event.
Only a few therapsids and no pelycosaurs survived the Permian extinction and 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 (formerly known as thecodonts; this term is not used in modern classifications). Some of these, like Euparkeria, were small and lightly built, while others, like Erythrosuchus, were as big as or bigger than the largest therapsids.
Triassic therapsids included three groups. Specialised, beaked herbivores known as dicynodonts (such as Lystrosaurus and its descendants, the Kannemeyeriidae), contained some members which reached large size (up to a tonne or more). The increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts included the eucynodonts from the Olenekian age, an early representative of which was Cynognathus. Finally, there were the therocephalians, which only lasted into the early part of the Triassic.
Unlike the dicynodonts, which remained large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed. From the most advanced and tiny cynodonts, which were only the size of a shrew, came the first mammal precursors, during the Carnian age of the Late Triassic, about 220 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 (belonging to the Traversodontidae) and dicynodonts (of the family Kannemeyeriidae) had disappeared by the Norian age, even before the Triassic-Jurassic extinction event that killed off all of the large non-dinosaurian archosaurs. Their places were taken by the diapsid archosaurs known as dinosaurs, which dominated the terrestrial ecosystem for the rest of the Mesozoic Era. The remaining Mesozoic synapsids were small, ranging from the size of a shrew, to the badger-like Repenomamus.
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, and some were carnivorous. The family Trithelodontidae 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 Trithelodonts, 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 that this group survived. New fossil finds have been found in the Cretaceous rocks of Gondwana. This is an example of Lazarus taxon.
Today, the 4,500 species of living synapsids are currently the dominant land animals and include both aquatic (whales) and flying (bats) species, including the largest animal ever known to have existed (the blue whale).
Synapsids evolution into mammals is believed to be triggered by moving to a nocturnal niche, one of the few niches that the increasing dinosaurs didn't dominate. In order to survive at night, proto-mammals had to increase their metabolic rate to keep their body warm. This meant consuming food (generally thought to be insects) more rapidly. To facilitate rapid digestion, proto-mammals evolved mastication (chewing) and specialized teeth that aided chewing. Limbs also evolved to move under the body instead of to the side. This allowed the proto-mammals to be able to change direction quicker in order to catch small prey at a faster rate. Rather than out-running predators, instead proto-mammals adapted the strategy of outmaneuvering predators using this same ability, it is believed. 
- Series Amniota
- CLASS SYNAPSIDA *
- Order Pelycosauria *
- Order Therapsida *
- CLASS MAMMALIA
- CLASS SYNAPSIDA *
Synapsida |-Caseasauria `-Eupelycosauria |-Varanopseidae `-+-Ophiacodontidae `-+-Edaphosauridae `-Sphenacodontia |-Sphenacodontidae `-Therapsida |-Biarmosuchia | `-Eotitanosuchus `-Eutherapsida |-Dinocephalia `-Neotherapsida |-Anomodontia `-Theriodontia |-Gorgonopsia `-Eutheriodontia |-Therocephalia `-Cynodontia |- + -Dvinia | `--Procynosuchus `- Epicynodontia |-Thrinaxodon `-Eucynodontia |- + -Cynognathus | `- + -Tritylodontidae | `- Traversodontidae `-Probainognathia |- + - Trithelodontidae | `--Chiniquodontidae `- + - Prozostrodon `- Mammaliaformes `-Mammalia
- Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd
- Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
- Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
- Laurin, M. and Reisz, R. R., (1997), Tree of Life - Synapsida - Tree of Life Web Project