Synapsid 201968 224281682 2008-07-08T03:47:05Z 66.32.208.224 fixed an awkward wording about monotremes {{Taxobox | name = Synapsids | fossil_range = {{fossil range|320|100}}[[Pennsylvanian|Late]] [[Carboniferous]] - [[Recent]]<br/> | image = Dimetrodon.jpg | image_width = 210px | image_caption = ''[[Dimetrodon]] grandis'' skeleton at the [[National Museum of Natural History]] of U.S.A. | regnum = [[Animalia]] | phylum = [[Chordate|Chordata]] | subphylum = [[Vertebrate|Vertebrata]] | infraphylum = [[Gnathostomata]] | superclassis = [[Tetrapod]]a {{Taxobox_norank_entry | taxon = [[Amniote|Amniota]]}} | classis = '''Synapsida''' [[paraphyly|*]] | classis_authority = [[Henry Fairfield Osborn|Osborn]], 1903 | synonyms = * Theropsida | subdivision_ranks = [[Order (biology)|Orders & Suborders]] | subdivision = * '''Order [[Pelycosaur]]ia [[paraphyletic|*]] ** Suborder [[Caseasauria]] ** Suborder [[Eupelycosauria]] [[paraphyletic|*]] * '''Order [[Therapsida]]''' [[paraphyletic|*]] ** Suborder [[Biarmosuchia]] ** Suborder [[Dinocephalia]] ** Suborder [[Anomodontia]] ** Suborder [[Gorgonopsia]] ** Suborder [[Therocephalia]] ** Suborder [[Cynodontia]] [[paraphyletic|*]] For complete phylogeny, see text. }} '''Synapsids''' ('fused arch'), also known as '''theropsids''' ('beast face'), are a class of [[animal]]s that includes [[mammals]] and everything closer to mammals than to other living [[amniotes]].<ref>Laurin and Reisz 2007.</ref> The non-mammalian members were traditionally described as '''mammal-like reptiles''',<ref name=Carroll397>Carroll 1988: 397.</ref><ref name=Benton122>Benton 2005: 122.</ref> and are sometimes referred to as "stem-mammals".{{Fact|date=March 2008}} Synapsids are one of the two major groups of [[amniote]], the other being the [[sauropsid]]s (or [[reptile]]s in the proper sense). They are distinguished from the latter by a single opening ([[temporal fenestra]]) in their [[skull]] behind each eye, which developed in the ancestral synapsid about 324 [[million years ago]] ([[mya (unit)|mya]]) during the [[Pennsylvanian|late]] [[Carboniferous]] [[Period (geology)|Period]]. Synapsids were the dominant [[terrestrial animal]]s 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–Triassic extinction event|Permian extinction]]. Some species survived into the [[Triassic]] period, but [[Saurischia|saurischian dinosaurs]] 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 [[vertebrate]]s. ==Changing classifications== Synapsids were originally defined at the turn of the 20th century, as one of the four main subclasses of [[reptile]]s, 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 [[evolution|evolved]], increasingly mammalian features, hence the name "mammal-like reptiles" which became a broad, traditional description for all non-mammalian synapsids.<ref name=Carroll397/><ref name=Benton122/> The term is not considered a formal one by most experts; technically speaking, the non-mammalian synapsids were closer on the [[evolution]]ary tree to mammals than to reptiles. The traditional classification continued through to the late 1980s (e.g. Carroll 1988). In the 1990s this approach was replaced by a [[cladistics|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 [[clade]]s. 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 [[paraphyletic]]ally-defined Class Synapsida. The non-mammalian synapsids are traditionally divided into a primitive and an advanced group, known respectively as '[[pelycosaur]]s' and [[therapsid]]s. 'Pelycosaurs' are an artificial, paraphyletic grouping of six families which are united only in that they are primitive in relation to therapsids, and do not constitute a clade.<ref>Benton 2005: 120.</ref> 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== [[Image:Skull synapsida 1.png|thumb|left|200px|The synapsids are diagnosed by a single [[temporal fenestrae|hole behind each eye]].]] Synapsids evolved a [[temporal fenestra]] behind each eye [[orbit (anatomy)|orbit]] on the lateral surface of the [[skull]]. It may have evolved to provide new attachment sites for [[jaw]] muscles. Some synapsids (including mammals) also have a [[warm-blooded]] metabolism, even though early synapsids, such as pelycosaurs were most certainly [[cold-blooded]]. Like mammals, the nonmammalian synapsids possessed [[gland]]ular [[skin]] that lacked [[scale (zoology)|scale]]s, though at least the [[pelycosauria|pelycosaurs]] retained the "scales" of more primitive [[tetrapod]]s on their undersides. These scales differed in structure from reptilian scales, an epidermal feature like mammalian hair or avian feathers.<ref name="caroll1969">Carroll, R.L. (1969). "Problems of the origin of reptiles." ''Biological Reviews'', '''44''': 393-432.</ref> It is currently unknown at what stage they acquired mammalian characteristics like [[body hair]] and [[mammary gland]]s, as the [[fossil]]s only rarely provide direct evidence for soft tissues. Much, however, can be inferred from differences in [[skeleton|skeletal structure]]. Thus the more primitive synapsids can be better visualized as being "naked [[lizard]]s", both [[fur]]less and scaleless, as their overall aspect was more like a modern lizard than a modern mammal, and the distinguishing features are relatively fine ones of internal structure. 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.<ref name=SynRep>Hoyt 1997.</ref> Synapsids are the first tetrapods to have differentiated teeth. These include the canines, molars, and incisors. Early synapsids had multiple jaw bones. As they evolved, these jaw bones were reduced in size and gradually moved into the ear, forming the middle ear bones. Most [[paleontologist]]s hold fossilized [[jaw]] remains to be the distinguishing feature used to classify synapsids and reptiles. The jaw transition is a good [[Scientific classification|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. 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. 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 [[eutheriodont]]s, the beginnings of a palate are clearly visible. The later ''[[Thrinaxodon]]'' has a full and completely closed palate, forming a clear progression.<ref>Hopson 1987.</ref> ==Evolutionary history== {{main|Evolution of mammals}} [[Image:Archaeothyris BW.jpg|thumb|left|175px|''[[Archaeothyris]]'', one of the oldest synapsids found.]] ''[[Archaeothyris]]'' and ''[[Clepsydrops]]'' are the earliest known synapsids.<ref>Lambert 2001: 68-69.</ref> They 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 [[amniote]]s, spreading and diversifying until they became the dominant large terrestrial animals in the latest Carboniferous and Early [[Permian]] Periods. They were sprawling, bulky, [[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, like ''[[Dimetrodon]]'', had large sails that may have helped [[thermoregulation|raise their body temperature]]. A few [[relict]] groups lasted into the later Permian, but most of the 'pelycosaurs' became extinct before the [[Permian-Triassic extinction event|end of Permian]]. [[Image:Sphenacodon BW.jpg|thumb|right|175px|''[[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 dominant large terrestrial animals during the latter half. They have dominated the world twice: once in the [[Permian]] and once in the [[Cenozoic]], the current era. They were by far the most diverse and abundant animals 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 tonne 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 extinction event|Permian-Triassic mass extinction]] about 250 Mya, the largest [[extinction event|extinction]] in [[History of Earth|Earth's history]], which may have been related to the [[Siberian Traps]] volcanic event. [[Image:Nikkasaurus1DB.jpg|thumb|left|175px|''[[Nikkasaurus]]'' - an enigmatic synapsid from the Middle Permian of Russia.]] [[Image:Lystrosaurus BW.jpg|thumb|right|175px|''[[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 [[archosaur]]s (formerly known as [[thecodont]]s; this term is not used in modern classifications). Some of these, like ''[[Euparkeria]]'', were small and lightly built, while others, like ''[[Erythrosuchus]]'', were as big as or bigger than the largest therapsids. Triassic therapsids included three groups. Specialised, beaked herbivores known as [[dicynodont]]s (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 [[cynodont]]s included the [[eucynodonts]] from the [[Olenekian]] age, an early representative of which was ''Cynognathus''. Finally, there were the [[therocephalia]]ns, which only lasted into the early part of the Triassic. [[Image:Cynognathus BW.jpg|thumb|left|250px|''[[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 [[Crurotarsi|non-dinosaurian archosaurs]]. Their places were taken by the archosaurs known as [[dinosaur]]s, which dominated the terrestrial ecosystem for the rest of the [[Mesozoic]] Era. 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 non-mammalian 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 [[Trithelodontidae]] 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 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 [[Cretaceous]] rocks of [[Gondwana]]. This is an example of a [[Lazarus taxon]]. Today, there are 5,400 [[species]] of living synapsids known as the [[mammal]]s, including both aquatic ([[whale]]s) and flying ([[bat]]s) species, and the largest animal ever known to have existed (the [[blue whale]]). [[Humans]] are synapsids as well. Uniquely among the synapsids, however, most mammals are [[viviparous]] and give birth to live young rather than laying eggs, the exception being the [[monotreme]]s. Synapsids' evolution into mammals is believed to have been triggered by moving to a [[nocturnal]] [[ecological niche|niche]], one of the few niches that the increasingly prominent [[dinosaur]]s didn't dominate. [[Mammaliaformes|Proto-mammals]] 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]], proto-mammals evolved [[mastication]] (chewing) and specialized teeth that aided chewing. Limbs also evolved to move under the body instead of to the side, allowing proto-mammals to breathe more efficiently during locomotion<ref>Bramble and Jenkins 1994.</ref> and also to be able to change direction more quickly in order to catch small [[prey]] at a faster rate. This helped make it possible to support their higher metabolic demands. It is believed that, rather than out-running [[predators]], proto-mammals adapted the strategy of outmaneuvering predators using their improved locomotor capabilities.<ref name=SynRep/> ==Taxonomy== ===Classification=== * '''Series [[Amniota]]''' ** '''CLASS SYNAPSIDA''' [[paraphyletic|*]] *** '''Order [[Pelycosaur]]ia''' [[paraphyletic|*]] **** '''Suborder [[Caseasauria]]''' **** '''Suborder [[Eupelycosauria]]''' [[paraphyletic|*]] ***** Family [[Varanopseidae]] ***** Family [[Ophiacodontidae]] ***** Family [[Edaphosauridae]] ***** Family [[Sphenacodontidae]] [[paraphyletic|*]] *** '''Order [[Therapsida]]''' [[paraphyletic|*]] **** '''Suborder [[Biarmosuchia]]''' [[paraphyletic|*]] **** '''Suborder [[Dinocephalia]]''' **** '''Suborder [[Anomodontia]]''' **** '''Suborder [[Gorgonopsia]]''' **** '''Suborder [[Therocephalia]]''' **** '''Suborder [[Cynodontia]]''' [[paraphyletic|*]] ***** Family [[Probainognathidae]] ***** '''Superfamily [[Chiniquodontoidea]]''' [[paraphyletic|*]] ** '''CLASS [[Mammal|MAMMALIA]]''' ===Phylogeny=== Synapsida |-[[Caseasauria]] `-[[Eupelycosauria]] |-[[Varanopseidae]] `-+-[[Ophiacodontidae]] `-+-[[Edaphosauridae]] `-[[Sphenacodontia]] |-[[Sphenacodontidae]] `-[[Therapsida]] |-[[Biarmosuchia]] | `-''[[Eotitanosuchus]]'' `-Eutherapsida |-[[Dinocephalia]] `-Neotherapsida |-[[Anomodontia]] `-[[Theriodontia]] |-[[Gorgonopsia]] `-Eutheriodontia |-[[Therocephalia]] `-[[Cynodontia]] |-+-''[[Dvinia]]'' | `--[[Procynosuchidae]] `-[[Epicynodontia]] |-''[[Thrinaxodon]]'' `-[[Eucynodontia]] |-+-''[[Cynognathus]]'' | `-+-[[Tritylodontidae]] | `- [[Traversodontidae]] `-[[Probainognathia]] |-+- [[Trithelodontidae]] | `--[[Chiniquodontidae]] `-+- ''[[Prozostrodon]]'' `- [[Mammaliaformes]] `-[[Mammalia]] ==Notes== <references/> ==References== * [[Michael J. Benton|Benton, Michael J.]] (2005). [[Vertebrate Paleontology (Benton)|Vertebrate Paleontology]], 3rd ed. Oxford: [[Blackwell's|Blackwell Science Ltd]]. ISBN 0-632-05637-1. * Bramble, D.M., and F.A. Jenkins (1994). Mammalian locomotor-respiratory integration: Implications for diaphragmatic and pulmonary design. ''Science'' 262: 235-240. * [[Robert L. Carroll|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). [http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML 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). [http://tolweb.org/Synapsida/14845/2007.04.06 Synapsida: Mammals and their extinct relatives]. Version 06 April 2007. ''The [[Tree of Life Web Project]]''. ==Further reading== * {{cite book|last=Colbert|first=E. H.|coauthors=|authorlink=Edwin H. Colbert|title=[[Evolution of the Vertebrates]]|edition=2nd ed.|publisher=[[John Wiley & Sons|John Wiley & Sons Inc]]|location=New York|year=1969|isbn=0471164666|series=}} ==See also== * [[Anapsid]]a * [[Diapsid]]a * [[Euryapsida]] * [[Mammal classification]] * [[Prehistoric mammal]] * [[Vertebrate paleontology]] * [[Timeline of evolution]] {{Wikispecies|Synapsida}} ==External links== * [http://www.palaeos.com/Vertebrates/Units/Unit390/000.html Synapsida - Pelycosauria] - at Palaeos * [http://www.talkorigins.org/faqs/faq-transitional/part1b.html Transitional Vertebrate Fossils] - includes description of important [[transitional fossils|transitional]] genera in the evolutionary sequence linking primitive synapsids to mammals [[Category:Paleontology]] [[Category:Pelycosaurs]] [[Category:Synapsids]] [[Category:Therapsids]] [[ca:Sinàpsid]] [[cs:Synapsida]] [[da:Pattedyrlignende krybdyr]] [[de:Synapsiden]] [[es:Synapsida]] [[fr:Reptile mammalien]] [[fr:Synapsida]] [[gl:Synapsida]] [[it:Synapsida]] [[he:סינאפסידה]] [[ka:თერომორფები]] [[hu:Emlősszerűek]] [[nl:Zoogdierachtige reptielen]] [[nl:Synapsiden]] [[ja:単弓類]] [[no:Synapsider]] [[pl:Synapsydy]] [[pt:Sinápsida]] [[ru:Синапсиды]] [[simple:Synapsid]] [[sk:Synapsida]] [[fi:Synapsidit]] [[sv:Synapsida]] [[vi:Lớp Một cung bên]] [[zh:似哺乳爬行動物]] [[zh:合弓綱]]