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The Synapsida ('fused arch') are an important group of that appeared and diversified very early, becoming the dominant life-form on land for the entire Permian period. Following the they were much reduced in numbers and diversity, and found themselves progressively supplanted by the . Synapsids then became more and more mammal-like throughout the Triassic, finally becoming successful as small near the end of the period. They remained under the feet of the dinosaurs until gave them the opportunity to once again inherit the Earth.
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The Synapsids are one of two major evolutionary lineages of , the other being the . The two diverged very early in the history of reptilian evolution, some time during the early part of the late .
Essentially, if very simplistically, the Sauropsids or "lizard faces" constitute an evolutionary sequence starting from a primitive anapsid condition (e.g. the and the ) with a simple skull with no openings in the side for attachment of jaw muscles. The sequence continues through to various archosaurs, including dinosaurs, pterosaurs, and diverse other forms (most of which have various for muscle attachment); and from archosaurs finally to birds -- thus far the culmination of Sauropsid evolution.
The Synapsida, or to give them their alternative name, or "beast (mammal) faces" constitute a different evolutionary lineage that developed a single opening, the synapsid arch, behind each eye, for attachment of jaw muscles, giving a superior bite and permitting adaptive radiation during the late Carboniferous. These forms evolved through the primitive pelycosaur stage, to the therapsids or mammal-like reptiles, and finally the mammals themselves. Pelycosaur, therapsid, and mammal represent three evolutionary grades in a single progressive evolutionary axis (see the cladogram, below). The therapsids, as forms transitional between basal amniote and mammal, can be thought of as occupying the same evolutionary space as the dinosaurs, which are transitional between reptiles and birds, do.
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and were the earliest known synapsids. They belonged to a group called and they lived in time of the Period. The pelycosaurs were the first successful group of , spreading and diversifying until they became the dominant large terrestrial animals, in the latest Carboniferous and Early Periods. They are currently divided into two , the and the . They were sprawling, bulky, and had small brains. They were the largest land animals of their time, ranging up to 3 m (10 ft) in length. Many, like , had large sails that may have helped . A few relict groups lasted into the later .
The , 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 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: ), to large bulky herbivores a tonne or more in weight (e.g: ). After flourishing for many millions of years, these successful animals were all but wiped out by the about 250 Mya, the largest in , which may have been related to the volcanic event.
Only a few therapsids and no pelycosaurs, survived the Permian extinction and went on to be successful in the new early landscape; they include and (later in the early Triassic) . Now, however, they were accompanied by the early (formerly known as , although this term is not used in modern classifications). Some of these (like ) were small and lightly built, while others (like ) were as big as or bigger than the largest therapsids.
Triassic therapsids included three groups, the specialised, beaked herbivores known as (such as Lystrosaurus and its descendants, the ), some of which reached large size (up to a tonne or more); the increasingly mammal-like carnivorous, herbivorous, and insectivorous (including, from the age, the , an early representative of which was Cynognathus) and the , 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 (only the size of a , came the first precursors, during the age of the Late Triassic, about 220 million years ago (mya).
During the evolutionary succession from early therapsid to cynodont to eucynodont to mammal, the main lower jaw bone, the , replaced the adjacent bones, so that the lower jaw gradually became just one large bone, with several of the smaller jaw bones migrating into the and allowing sophisticated hearing.
Whether through climate or vegetation change, ecological competition or a combination of factors, most of the remaining large cynodonts (belonging to the ) and dicynodonts (of the family Kannemeyeriidae) had disappeared by the age, even before the that killed off all of the large . Their places were taken by the diapsid archosaurs known as , which dominated the terrestrial ecosystem for the rest of the Era. The remaining Mesozoic synapsids were small, ranging from the size of a , to the -like .
During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as . No cynodont grew larger than a . Most Jurassic and Cretaceous cynodonts were , and some were carnivorous. The family , first appeared near the end of the Triassic. They were and persisted well into the . The other, , 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 rocks of . This is an example of .
Today, the 4,500 of living synapsids are currently the dominant land animals and include both aquatic () and flying () species, including the largest animal ever known to have existed (the ).
The synapsids have dominated the world twice, once in the and once in the (today).
Synapsids evolution into mammals is believed to be triggered by moving to a (night) , one of the few niches that the increasing didn't dominate. In order to survive at night, 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 , proto-mammals evolved (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 at a faster rate. Rather than out-running , instead proto-mammals adapted the strategy of outmaneuvering predators using this same ability, it is believed.
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Cladistically, the Synapsida are a vertical clade, not a horizontal class or . Hence the term Synapsida refers to a major group in the present cladistic interpretation of tetrapod evolution. But confusion can come about because the term Synapsida is also used in the to refer to all theropsids except mammals.
Stratigraphic Range: as (subclass of ) to ; as Carboniferous to
Original Author: , 1903
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The Synapsids were originally defined, at the turn of the 20th Century, as one of the five main of , on the basis of their distinctive . These openings in the cheek bones were for the purpose of allowing attachment for 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 "".
The traditional classification continued through to the late 1980s (see e.g. Carroll 1988). In the 1990s this approach was replaced by a one, according to which the only valid groups are those that include . Because Synapsids evolved into , the mammals therefore are included under the Clade Synapsida.
A recent, compromise position (see Benton 2004) has the class Synapsida as intentionally , constituting a grade of animals from the earliest split with to the arbitrary division with its daughter class, .
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- (2004), , 3rd ed.
- (1988), , .
- , (1969), , (2nd ed.)
- and , (1997), -
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- - includes description of important genera from reptile to mammal
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Kheper MAK(date), , Palaeos org MAK061001
- Evolutionary history, Taxonomy, References (MAK060501 co-author), copied to Palaeos org MAK061018
- Phylogeny