Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant reptiles and birds). Synapsids have a single lower temporal fenestra, an opening low in the skull roof behind each eye socket. These openings create a bony arch that allows for the attachment of larger jaw muscles and a more efficient bite. While previously argued to be distinctive to synapsids, single temporal fenestrae are also found in many primitive sauropsids. It has recently been argued that the last common ancestor of synapsids and sauropsids shared a single lower temporal fenestra.
The basal amniotes (reptiliomorphs) from which synapsids evolved were historically simply called "reptiles". Therefore, non-mammalian synapsids were then described as mammal-like reptiles in classical systematics, and non-therapsid synapsids were also referred to as pelycosaurs or pelycosaur-grade synapsids. These paraphyletic terms have fallen out of favor and are only used informally (if at all) in modern literature, as it is now known that all extant reptiles are more closely related to each other and birds than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida or even just an under-clade thereof. In a cladistic sense, synapsids are in fact a monophyletic sister taxon of sauropsids, rather than a part of the sauropsid lineage. Modern alternatives sometimes used include stem mammals, proto-mammals, paramammals or pan-mammals. Most lineages of pelycosaur-grade synapsids were replaced by the more advanced therapsids, which evolved from sphenacodontian pelycosaurs, at the end of the Early Permian, a transition known as Olson's Gap/Extinction.
The earliest synapsids appeared towards the end of the Carboniferous, around 315 to 307 million years ago. Synapsids were the largest terrestrial vertebrates in the Permian period (299 to 251 mya), rivalled only by some large pareiasaurian parareptiles such as Scutosaurus. They were the dominant land predators of the late Paleozoic and early Mesozoic, with eupelycosaurs such as Dimetrodon, Titanophoneus and Inostrancevia being the apex predators during the Permian, and theriodonts such as Moschorhinus during the Early Triassic. Synapsid population and diversity were severely reduced by the Capitanian mass extinction event and the Permian–Triassic extinction event, and only two groups of therapsids, the dicynodonts and eutheriodonts (consisting of therocephalians and cynodonts) are known to have survived into the Triassic. These therapsids rebounded as disaster taxa during the early Mesozoic, with the dicynodont Lystrosaurus making up as much as 95% of all land species at one time, but declined again after the Smithian–Spathian boundary event with their dominant niches largely taken over by the rise of archosaurian sauropsids, first by the pseudosuchians and then by the pterosaurs and dinosaurs. The cynodont group Probainognathia, which includes the group Mammaliaformes, were the only synapsids to survive beyond the Triassic, During the Jurassic, cynodonts lived mostly nocturnally to avoid competition with dinosaurs. After the Cretaceous–Paleogene extinction event wiped out all non-avian dinosaurs and pterosaurs, synapsids (as mammals) rose to dominance once again during the Cenozoic, with roughly 6,800 species living currently.
Linnaean and cladistic classifications
At the turn of the 20th century, synapsids were thought to be one of the four main subclasses of reptiles. 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 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 (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within Sauropsida (sauropsids), the sister group to synapsids, thus making synapsids their own taxonomic group.
As a result, the paraphyletic 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 monophyly 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 cladistic 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 groups, or clades.
Additionally, Reptilia (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within Sauropsida, 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.Benton, Michael J. (2005). Vertebrate Paleontology, 3rd ed. Oxford: Blackwell Science Ltd. . p. 120. Stem mammals were all rather lizard-like, with sprawling gait and possibly horny scutes, 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" In phylogenetic nomenclature, 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 include modern mammals, in practical usage, those two terms are used almost exclusively when referring to the more basal members that lie outside of Mammaliaformes.
Characteristics
Skull 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. 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.
While historically argued to be distinctive to synapsids, recent research has demonstrated that many primitive sauropsids also have a single lower temporal fenestra and it has also been suggested that many thought to be sauropsids with anapsid skulls completely lacking temporal fenestrae are not reptiles but stem-amniotes, implying that this trait was possibly present in the last common ancestor of synapsids and sauropsids, and not a distinctive trait of Synapsida.
Synapsids ancestrally had an opening on the skull roof (pineal foramen) for the parietal eye, a light sensitive organ still found in some amphibians and reptiles used for calibrating thermoregulation and the circadian rhythm among other functions. The functionality of the eye was lost and the pineal foramen closed over in the cynodont line ancestral to mammals (Probainognathia).
Teeth
Many synapsids including living mammals are characterised by heterodont teeth that vary in size and/or shape along the tooth row. This was relatively incipient in many early "pelycosaur" synapsids, but became pronounced in advanced sphenacodontian synapsids, particularly in therapsids, where the teeth became differentiated into incisor, canine and postcanine teeth. The dicynodonts are unique among synapsids for losing most (or all in the case of some dicynodonts) of their teeth in favour of a horny, keratinous turtle-like beak. Tooth cusps first emerged within the cynodont ancestors of mammals. Most non-mammalian synapsids are thought to have continuously replaced their teeth throughout life (polyphyodonty), with advanced cynodonts having evolved limited cycles of tooth replacement, culminating in the emergence of the two sets of teeth (diphyodonty) in Mammaliformes (or possibly the slightly broader grouping Mammaliamorpha).
Jaw and ears
Like their reptilian counterparts, the lower jaws of most non-mammalian synapsids were made up of several major bones including the dentary (the tooth-bearing bone), as well as the articular, angular, surangular and splenial. In cynodonts, these jaw bones other than the dentary became reduced in size and either lost or, in the case of the angular and articular, gradually moved into forming bones of the ear. While modern mammals possess the malleus, incus, tympanic, and stapes, basal synapsids (like all other tetrapods) possess only a stapes. The malleus is derived from the articular and the tympanic (also widely known as the ectotympanic) is derived from the angular bones, while the incus is derived from the quadrate (a cranial bone).
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 fossa. In contrast, all other jawed vertebrates, including reptiles and non-cynodont 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. Non-mammalian cynodonts show articulations transitional between these two conditions.
Palate Synapsids ancestrally possess palatal teeth present on the bones of the roof of the mouth. These were lost in the cynodont ancestors of mammals.
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 (bony armor embedded in the skin), scutes (protective structures of the dermis often with a horny covering), hair or fur, and scale-like structures (often formed from modified hair, as in pangolins and some rodents). While the skin of reptiles is rather thin, that of mammals has a thick dermal layer.