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Theropsida
Theropsida
Seeley , 1895

SYNAPSIDS (Greek , 'fused arch'), synonymous with THEROPSIDS (Greek, 'beast-face'), are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes . 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. Primitive synapsids are usually called pelycosaurs or pelycosaur-grade synapsids; more advanced mammal-like ones, therapsids . The non-mammalian members are described as MAMMAL-LIKE REPTILES in classical systematics; they can also be called STEM MAMMALS or PROTO-MAMMALS. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes; the other is the sauropsids , a group that includes modern reptiles and birds . The distinctive temporal fenestra developed in the ancestral synapsid about 312 million years ago, during the Late Carboniferous period .

Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago, although some of the larger pareiasaurs at the end of Permian could match them in size. As with other groups then extant, their numbers and variety were severely reduced by the Permian– Triassic extinction . By the time of the extinction at the end of Permian, all the older forms of synapsids (known as pelycosaurs ) were already gone, having been replaced by the more advanced therapsids . Though the dicynodonts and Eutheriodontia , the latter consisting of Eutherocephalia ( Therocephalia ) and Epicynodontia ( Cynodontia
Cynodontia
), continued into the Triassic period as the only known surviving therapsids, archosaurs became the largest and most numerous land vertebrates in the course of this period. The cynodont group Probainognathia , which includes Mammaliaformes , were the only synapsids who outlasted the Triassic. After the Cretaceous– Paleogene extinction event , the synapsids (in the form of mammals) again became the largest land animals.

CONTENTS

* 1 Linnaean and cladistic classifications

* 1.1 Synapsids as a reptilian subclass * 1.2 The "mammal-like reptiles" * 1.3 Primitive and advanced synapsids

* 2 Characteristics

* 2.1 Temporal openings * 2.2 Teeth * 2.3 Jaw * 2.4 Palate

* 2.5 Skin and fur

* 2.5.1 Mammary glands * 2.5.2 Patagia

* 2.6 Metabolism

* 3 Evolutionary history * 4 Relationships * 5 See also * 6 References * 7 Further reading * 8 External links

LINNAEAN AND CLADISTIC CLASSIFICATIONS

SYNAPSIDS AS A REPTILIAN SUBCLASS

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

THE "MAMMAL-LIKE REPTILES"

The traditional classification of synapsids as reptiles is continued by some palaeontologists (Colbert "> The synapsids are distinguished by a single hole, known as the temporal fenestra , in the skull behind each eye. This schematic shows the skull viewed from the left side. The middle opening is the orbit of the eye; the opening to the right of it is the temporal fenestra.

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. A similar development took place in the diapsids , which evolved two rather than one opening behind each eye. 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.

TEETH

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

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 modern mammals possess the malleus , incus and stapes , basal synapsids (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 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 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 cranium 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

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

The sea otter has the densest fur of modern mammals.

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.

The ancestral skin type of synapsids has been subject to discussion. Among the early synapsids, only two species of small varanopids have been found to possess scutes; fossilized rows of osteoderms indicate horny armour on the neck and back, and skin impressions indicate some possessed rectangular scutes on their undersides and tails. Archaeothyris , one of the oldest synapsids found. Cotylorhynchus (background), Ophiacodon and Varanops .

Archaeothyris and Clepsydrops , the earliest known synapsids, lived in the Pennsylvanian subperiod (323-299 Mya) of the Carboniferous period and belonged to the series of primitive synapsids which are conventionally grouped as pelycosaurs. The pelycosaurs spread and diversified, becoming the largest terrestrial animals in the latest Carboniferous and Early Permian periods, ranging up to 6 metres (20 ft) in length. They were sprawling, bulky, possibly cold-blooded, and had small brains. Some, such as Dimetrodon, had large sails that might have helped raise their body temperature . A few relict groups lasted into the later Permian but, by the middle of the Late Permian, all of the pelycosaurs had either died off or evolved into their successors, the therapsids. Moschops was a tapinocephalian from the Middle Permian of South Africa.

The therapsids, a more advanced group of synapsids, appeared during the Middle Permian and included the largest terrestrial animals in the Middle and Late Permian . They 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 known extinction in Earth\'s history , possibly related to the Siberian Traps volcanic event. Nikkasaurus was 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 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
Erythrosuchus
, were as big as or bigger than the largest therapsids.

After the Permian extinction, the synapsids did not count more than three surviving clades. The first comprised the therocephalians, which only lasted the first 20 million years of the Triassic period. The second were specialised, beaked herbivores known as dicynodonts (such as the Kannemeyeriidae ), which contained some members that reached large size (up to a tonne or more). And finally there were the increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts, including the eucynodonts from the Olenekian
Olenekian
age, an early representative of which was Cynognathus. Cynognathus was the largest predatory cynodont of the Triassic .

Unlike the dicynodonts, which were large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed, though some forms like Trucidocynodon remained large. The first mammaliaforms evolved from the cynodonts during the early Norian age of the Late Triassic, about 225 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. Repenomamus was the largest mammal of the Mesozoic .

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 Rhaetian
Rhaetian
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 . Tritylodon was a cynodont that lived in Early Jurassic .

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 , that first appeared near the end of the Triassic, was carnivorous and persisted well into the Middle Jurassic . The other, Tritylodontidae , first appeared at the same time as the tritheledonts, but was 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. Most mammals are viviparous and give birth to live young rather than laying eggs with the exception being the monotremes .

Triassic and Jurassic ancestors of living mammals, along with their close relatives, had high metabolic rates. This meant consuming food (generally thought to be insects) in much greater quantity. To facilitate rapid digestion , these synapsids 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. This helped make it possible to support their higher metabolic demands.

RELATIONSHIPS

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:

SYNAPSIDA

Caseasauria

Eupelycosauria

Varanopidae

Ophiacodontidae

Edaphosauridae

Sphenacodontia
Sphenacodontia

Sphenacodontidae

Therapsida

Biarmosuchia

Eutherapsida

Dinocephalia

Neotherapsida

Anomodontia

Theriodontia

Gorgonopsia

Eutheriodontia

Therocephalia

Cynodontia
Cynodontia

Cynognathia

Probainognathia

MAMMALIA

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 border:0;padding:0 0.2em;border-bottom:1px solid;vertical-align:bottom;text-align:center;">

Tseajaia campi

Limnoscelis paludis
Limnoscelis paludis

Amniota

Captorhinus spp.

Protorothyris archeri

SYNAPSIDA

Ophiacodontidae

Archaeothyris florensis

Varanosaurus acutirostris

Ophiacodon spp.

Stereophallodon ciscoensis

Varanopidae

Archaeovenator hamiltonensis

Pyozia mesenensis

Mycterosaurus longiceps

? Elliotsmithia longiceps (BP/1/5678)

Heleosaurus scholtzi

Mesenosaurus romeri

Varanops brevirostris

Watongia meieri

Varanodon agilis

Ruthiromia elcobriensis

Aerosaurus wellesi

Aerosaurus greenleorum

Caseasauria

Eothyrididae

Eothyris parkeyi

Oedaleops campi

Caseidae

Oromycter dolesorum

Casea broilii

Trichasaurus texensis

Euromycter rutenus (="Casea" rutena)

Ennatosaurus tecton

Angelosaurus romeri

Cotylorhynchus romeri

Cotylorhynchus bransoni

Cotylorhynchus hancocki

Ianthodon schultzei

Edaphosauridae

Ianthasaurus hardestiorum

Glaucosaurus megalops

Lupeosaurus kayi

Edaphosaurus boanerges

Edaphosaurus novomexicanus

Sphenacodontia
Sphenacodontia

Haptodus garnettensis

Pantelosaurus saxonicus

THERAPSIDA

Raranimus dashankouensis

Biarmosuchus tener

Biseridens qilianicus

Titanophoneus potens

Sphenacodontidae

Cutleria wilmarthi

Secodontosaurus obtusidens

Cryptovenator hirschbergeri
Cryptovenator hirschbergeri

Dimetrodon
Dimetrodon
spp.

Sphenacodon spp.

However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids, returned Caseasauria to its position as the sister to all other synapsids. Brocklehurst et al. (2016) demonstrated that many of the postcranial characters used by Benson (2012) to unite Caseasauria with Sphenacodontidae and Edaphosauridae were absent in the newly discovered postcranial material of eothyridids, and were therefore acquired convergently.

SEE ALSO

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

REFERENCES

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