Synapsids Temporal range: Mississippian—Present, 320–0 Ma PreЄ Є O S D C P T J K Pg N
Dimetrodon grandis skeleton, National Museum of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
clade:	Amniota
clade:	Synapsida Osborn, 1903
Subgroups
†Caseasauria †Caseidae †Eothyrididae Eupelycosauria †Edaphosauridae Sphenacodontia †Varanopidae
Synonyms
Theropsida Bakker, 1986

Synapsids ('fused arch') are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes.^[1] 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.^[2] Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics,^[3][4] but are referred to as "stem mammals" (or sometimes "protomammals") under cladistic terminology.^[5] Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other being the sauropsids (reptiles and birds). Their distinctive temporal fenestra developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous period.

Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago. As with almost all groups then extant, their numbers and variety were severely reduced by the Permian-Triassic extinction. Though some species survived into the Triassic period, archosaurs became the largest and most numerous land vertebrates in the course of this period. Few of the nonmammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, as a phylogenetic unit, they included the mammals as descendants, and in this sense synapsids are still very much a living group of vertebrates. After the Cretaceous–Paleogene extinction event, the synapsids (in the form of mammals) again became the largest land animals.

Linnaean and cladistic classifications[link]

Synapsids as a reptilian subclass[link]

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 attachment of larger jaw muscles, hence a more efficient bite. Synapsids were considered to be the reptilian lineage that led to mammals via gradually evolved, increasingly mammalian features, hence the name "mammal-like reptiles" which became a broad, traditional description for all nonmammalian synapsids.^[3][4]

The "mammal-like reptiles"[link]

The traditional classification of synapsids as reptiles is continued by some palaeontologists (Colbert & Morales 2001). 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. Because mammals are directly descended from the synapsids, mammals are included under Synapsida as a clade.^[6]

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.^[7] 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[link]

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.^[8] 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.^[2][3] At least one paleontologist, Benton, still follows such traditional classification; Benton classifies the synapsids as a paraphyletic class.^[4]

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, if it is used at all, being virtually synonymous with Synapsida itself. 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[link]

Temporal openings[link]

The synapsids are distinguished by a single hole behind each eye.

Synapsids evolved a temporal fenestra behind each eye orbit on the lateral surface of the skull. It may have evolved to provide 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 opening in the skull left the inner cranium only covered 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[link]

Eothyris, an early synapsid with multiple canines

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[link]

Most paleontologists hold fossilized jaw remains to be the distinguishing feature used to classify synapsids and reptiles. 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 (the dentary), whereas the lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones (including the dentary, articular, 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 mammals possess the malleus, incus and stapes, mammal-like reptiles (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 skull bone).^[9]

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 skull 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.

Palate[link]

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.^[10]

Skin[link]

Synapsid combination of hair and scutes, detail of a rat's tail

The actual skin of the synapsids has been subject to some discussion. Basal reptilian skin is rather thin, and lacks the thick dermal layer that produces leather in mammals.^[11] Exposed parts of reptiles are protected by horny scales or scutes. Mammal hide has a thick, fibrous dermis and rarely exhibits scutes. A hallmark of mammals is the presence of copious glands and hair follicles.

When the change from reptilian to mammalian skin took place is not known, though fossilized rows of osteoderms indicate horny armour on the neck and back of pelycosaurs, and skin impressions indicate some retained rectangular scutes on their undersides.^[12] 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 (like mammalian hair or avian feathers).^[13] The remaining upper surface of the pelycosaurs may have borne scutes, too, or may have been glandular and leathery like that of a mammal.

It is currently unknown at what stage the synapsids acquired mammalian characteristics such as body hair and mammary glands, as the fossils only rarely provide direct evidence for soft tissues. An exceptionally well-preserved skull of Estemmenosuchus, a therapsid from the Upper Permian shows smooth, hairless skin with what appears to be glandular depressions.^[14] The oldest known fossil showing unambiguous imprints of hair is the Callovian (late middle Jurassic) Castorocauda, an early mammal.^[15] The more advanced therapsids could have had a combination of naked skin, whiskers and scutes. A full pelage likely did not evolve until the therapsid-mammal transition.^[16] The more advanced, smaller therapsids could have had a combination of hair and scutes, a combination still found in some modern mammals, such as rodents and the opossum.

Metabolism[link]

Sail-back pelycosaurs like Edaphosaurus indicate an early trend toward temperature regulation in synapsids.

The first pelycosaurs had the usual reptilian cold-blooded metabolism by all indications, including a sprawling gait and a low slung body.^[2] 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 sphenacodontids (e.g. Dimetrodon).

The sphenacodontids gave rise to the therapsids, which may have inherited the temperature regulation. The legs and feet of the early therapsid groups point to a more erect posture, traditionally interpreted as a sign of more efficient metabolism.^[17] The presence of large turbinatae acting as moisture traps in the nasal passage found in therapsids, but not in pelycosaurs, confirms the shift in metabolism.^[16] None of them shows any sign of a sail, indicating any temperature regulation would have relied on the creatures' own metabolism rather than external heat. 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. Being rather sizeable creatures, the balmy temperatures of the Triassic likely did allow them to rely on inertial homothermy to keep temperatures steady. The high metabolism of the advanced forms only forced the evolution of hair when mouse-sized animals evolved in the synapsid-mammal transition.^[16]

Evolutionary history[link]

Archaeothyris, one of the oldest synapsids found.

Archaeothyris and Clepsydrops, the earliest known synapsids,^[18] 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 were the first successful group of amniotes, spreading and diversifying until they became the largest terrestrial animals in the latest Carboniferous and Early Permian Periods. They were sprawling, bulky, and 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, such as Dimetrodon, had large sails that may have helped raise their body temperature. A few relict groups lasted into the later Permian, but most of the pelycosaurs became extinct before the end of Permian.

Sphenacodon was a carnivorous pelycosaur that was closely related to Dimetrodon and the therapsids.

The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian and went on to become the largest terrestrial animals during the latter half. They were by far the most diverse and abundant tetrapods of the Middle and Late Permian, and 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). 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.

Nikkasaurus - an enigmatic synapsid from the Middle Permian of Russia

Lystrosaurus was the most common synapsid shortly after the Permian–Triassic extinction event.

Only a few therapsids (and some relict 'pelycosaur' taxa) 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 (soon to give rise to the dinosaurs). 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.

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.

Cynognathus was the largest predatory cynodont 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 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 (whales) and flying (bats) species, and the largest animal ever known to have existed (the blue whale). Humans are synapsids, as well. Unique among the synapsids, however, most mammals are viviparous and give birth to live young rather than lay eggs, the exception being the monotremes.

Synapsids' evolution into mammals is believed to have been triggered by moving to a nocturnal niche. Protomammals with higher metabolic rates were able to keep their bodies warm at night, and were more likely to survive. This meant consuming food (generally thought to be insects) in much greater quantity. To facilitate rapid digestion, protomammals evolved mastication (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^[19] and to be able to change direction more quickly to catch small prey at a faster rate. This helped make it possible to support their higher metabolic demands. Rather than outrunning predators, protomammals are believed to have adapted the strategy of outmaneuvering predators using their improved locomotor capabilities.^[20]

Relationships[link]

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:^[21][22]

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 ofshoot 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, the analysis of Benson (in press) 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 (in press):^[23]

Notes[link]

See also[link]

• Anapsida
• Diapsida
• Euryapsida
• List of synapsids
• Mammal classification
• Prehistoric mammal
• Vertebrate paleontology
• Timeline of evolution

Wikispecies has information related to: Synapsida

References[link]

• Benton, Michael J. (2005). Vertebrate Paleontology, 3rd ed. Oxford: Blackwell Science Ltd. ISBN 0-632-05637-1.
• Bramble, D. M.; Jenkins, F. A. (1993). "Mammalian locomotor-respiratory integration: Implications for diaphragmatic and pulmonary design". Science 262 (5131): 235–240. DOI:10.1126/science.8211141. PMID 8211141. 
• Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York: W.H. Freeman & Co. ISBN 0-7167-1822-7.
• Hopson, James A. (1987). "The Mammal-Like Reptiles: A Study of Transitional Fossils". The American Biology Teacher 49 (1): 16–26. 
• Hoyt, Donald F. (1997). Synapsid Reptiles. Zoo 138 Vertebrate Zoology Home Page.
• Lambert, David (2001). Dinosaur Encyclopedia. ISBN 0-7894-7935-4.
• Laurin, Michel, and Robert R. Reisz (2007). Synapsida: Mammals and their extinct relatives. Version 6 April 2007. The Tree of Life Web Project.

Further reading[link]

• Colbert, E. H. (1969). Evolution of the Vertebrates (2nd ed.). New York: John Wiley & Sons Inc. ISBN 0-471-16466-6. 

External links[link]

• Synapsids - at Macroevolution.net
• Synapsida - Pelycosauria - at Palaeos
• Transitional Vertebrate Fossils - includes description of important transitional genera in the evolutionary sequence linking primitive synapsids to mammals