Synapsid

Synapsids ('fused arch'), also known as theropsids ('beast face'), are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. In classical systematics, the non-mammalian members are described as mammal-like reptiles, and are sometimes referred to as "proto-mammals" or "stem-mammals" in cladistics. Synapsids are one of the two major groups of amniote, the other being the sauropsids (reptiles and birds). They are distinguished from other amniotes by having a single opening (temporal fenestra) in their skull behind each eye, which developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous Period.

Synapsids were the dominant terrestrial animals in the middle to late Permian period. As with almost all life forms then extant, their numbers and variety were severely reduced by the Permian extinction. Some species survived into the Triassic period, but archosaurs quickly became the dominant animals and few of the non-mammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, they included the prehistoric ancestors of mammals and in this sense, synapsids are still very much a living class of vertebrates.

Changing classifications

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 non-mammalian synapsids.

The traditional classification continued through to the late 1980s (e.g. Carroll 1988). 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 (other) synapsids, mammals are included under Clade Synapsida. However, formal classification has not kept pace with cladistics, so mammals are still often treated as a separate class alongside a paraphyletically-defined Class Synapsida. At the same time, the term "reptiles", which traditionally denoted all cold-blooded amniotes, is now re-defined to include only the sauropsids (a class that unites the anapsids and the diapsids to the exclusion of the synapsids, because these first two groups are more closely related to each other than to the third one). Hence the term "mammal-like reptiles" for the synapsids is considered obsolete under this terminology.

The non-mammalian synapsids are traditionally divided into a primitive and an advanced group, known respectively as 'pelycosaurs' and therapsids. 'Pelycosaurs' make up a paraphyletic grouping of six families which are united only in that they are primitive in relation to therapsids, and do not constitute a clade. They are currently divided between two primary synapsid clades, the Caseasauria and the Eupelycosauria, the latter of which also includes all the more advanced synapsids and therefore the mammals. That is to say, therapsids make up a well-defined clade within the eupelycosaurs, as long as mammals are included in the therapsids.

Characteristics

Temporal openings

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 Diapsides, who 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

Synapsids are characterized by having differentiated teeth. These include the canines, molars, and incisors. The trend towards differentiation is found in earlier stem amniotes in the form of enlargement of the first teeth on the maxilla, forming a form of proto-canines. This trait was subsequently lost in the Sauropsid line, but developed further in the synapsids. Early synapsids could have 2 or even 3 enlarged "canines", but in the therapsides, the pattern had settled to one canine in each upper jaw half. The lower canines developed later.

Jaw

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 dentary, or lower jaw, consists of a single bone in mammals, where the lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones. As they evolved, these jaw bones were reduced in size and gradually moved into the ear, forming the middle ear bones.

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 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

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

The actual skin of the synapsids has been subject to some discussion. Basal reptilian skin is rather thin, and lack the thick dermal layer that produces leather in mammals. Exposed parts of reptiles are protected by horny scales or scutes. Mammal hide has copious glands and rarely exhibit scutes.

When the change from to reptilian to mammalian type skin took place is not known, though fossilized skin impressions indicate that at least the pelycosaurs retained the scutes of more primitive tetrapods on their undersides. The pelycosaur scutes probably were non-overlapping 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 epidermal feature (like mammalian hair or avian feathers). The upper surface of the animals may have had glandular leathery skin like a mammal, and the pelycosaurs may thus perhaps be visualized as being "naked lizards", both furless and scaleless, as their overall aspect was more like a modern lizard than a modern mammal. The features distinguishing pelycosaurs from other reptiles are relatively fine ones of internal structure.

It is currently unknown at what stage the synapsids acquired mammalian characteristics like body hair and mammary glands, as the fossils only rarely provide direct evidence for soft tissues. Much, however, can be inferred from differences in skeletal structure. The more advanced therapsids could have had a combination of naked skin, scutes and hair, a combination stil found in some modern mammals.

Metabolism

Some synapsids (including mammals) also have a warm-blooded metabolism, even though early synapsids, such as pelycosaurs, are believed to have been cold-blooded. On the other hand, the presence of a secondary palate, erect posture and other indicators of high metabolic rate among the advanced cynodonts strongly suggests that many mammalian features, including an effective insulating layer of body hair, had evolved by this stage. This is now confirmed by impressions of fur in rocks directly underlying some fossil therapsids.

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