Difference between revisions of "Therapsid" - New World Encyclopedia

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{{Taxobox_begin | color = pink | name = Therapsids}}
 
{{Taxobox_begin | color = pink | name = Therapsids}}
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{{Taxobox_image | image = [[Image:Phtinosuchus1ZICA.png|240px]]|caption=''[[Phthinosuchus]]'', an early therapsid|}}
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{{Taxobox_image | image = [[Image:Phtinosuchus1ZICA.png|240px]]|caption=''Phthinosuchus'', an early therapsid|}}
 
{{Taxobox_begin_placement | color = pink}}
 
{{Taxobox_begin_placement | color = pink}}
 
{{Taxobox_regnum_entry | taxon = [[Animal|Animalia]]}}
 
{{Taxobox_regnum_entry | taxon = [[Animal|Animalia]]}}
 
{{Taxobox_phylum_entry | taxon = [[Chordate|Chordata]]}}
 
{{Taxobox_phylum_entry | taxon = [[Chordate|Chordata]]}}
 
{{Taxobox_subphylum_entry | taxon = [[Vertebrate|Vertebrata]]}}
 
{{Taxobox_subphylum_entry | taxon = [[Vertebrate|Vertebrata]]}}
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{{Taxobox_superclassis_entry | taxon = [[Tetrapod|Tetrapoda]]}}
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{{Taxobox_superclassis_entry | taxon = Tetrapoda}}
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{{Taxobox_norank_entry | taxon = [[Amniote|Amniota]]}}
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{{Taxobox_norank_entry | taxon = [[Amniote|Amniota]]*}}
 
{{Taxobox_classis_entry | taxon = [[Synapsid]]a}}
 
{{Taxobox_classis_entry | taxon = [[Synapsid]]a}}
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{{Taxobox ordo entry | taxon = '''Therapsida'''}}<br/>{{Taxobox authority | author = [[Robert Broom|Broom]] | date = 1905}}
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{{Taxobox ordo entry | taxon = '''Therapsida'''}}<br/>{{Taxobox authority | author = [[Robert Broom|Broom]]* | date = 1905}}
 
{{Taxobox_end_placement}}
 
{{Taxobox_end_placement}}
 
{{Taxobox_section_subdivision | color = pink | plural_taxon = Suborders}}
 
{{Taxobox_section_subdivision | color = pink | plural_taxon = Suborders}}
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[[Biarmosuchia]] <br/>
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[[Biarmosuchia]]* <br/>
−
[[Dinocephalia]] <br/>
+
[[Dinocephalia]]* <br/>
−
[[Eotheriodontia]] <br/>
+
[[Eotheriodontia]]* <br/>
−
[[Anomodont]]ia <br/>
+
[[Anomodont]]*ia <br/>
−
[[Gorgonopsia]]<br/>
+
[[Gorgonopsia]]*<br/>
−
[[Therocephalia]]<br/>
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[[Therocephalia]]*<br/>
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[[Cynodont]]ia
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[[Cynodont]]*ia
 
{{Taxobox_end}}
 
{{Taxobox_end}}
  
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'''Therapsids''', previously known as the "[[mammal-like reptile]]s", are an [[Order (biology)|order]] of [[synapsid]]s. Traditionally, synapsids were referred to as [[reptile]]s. However, when the term is used [[cladistics|cladistically]], the [[taxon]] also includes the [[mammal]]s, which are descended from the [[cynodont]] therapsids.
+
'''Therapsids''', which have been called "mammal-like reptiles," are a group of amniote [[synapsid]]s. ''Amniotes'' are [[animal]]s whose embryos are surrounded by an amniotic membrane that encases it in amniotic fluid. [[Reptile]]s have traditionally been defined as including all the amniotes except [[bird]]s and [[mammal]]s. ''Synapsids'' are tetrapods (four-legged [[vertebrate]]s) that are characterized by a pair of holes in their skulls behind the [[eye]]s. The synapsids are considered to have eventually evolved into mammals, through [[evolution#theory of descent with modification|descent]] via the [[cynodont]] therapsids. Although synapsids have traditionally been referred to as reptiles, when the term is used [[cladistics|cladistically]], the taxon also includes the [[mammal]]s because of descent through the therapsids.
 +
 
 +
[[Taxonomy|Taxonomically]], therapsids are an order of synapsids, the '''Therapsida'''.
 +
 
 +
==Classification==
 +
There are different classification schemes that have been developed involving the Order Therapsid. The following is one scenario.
 +
 
 +
* '''Series [[Amniota]]*'''
 +
** '''CLASS SYNAPSIDA'''
 +
*** '''Order [[Pelycosaur]]ia'''
 +
**** '''Suborder [[Caseasauria]]*'''
 +
**** '''Suborder [[Eupelycosauria]]*'''
 +
***** Family [[Varanopseidae]]*
 +
***** Family [[Ophiacodontidae]]*
 +
***** Family [[Edaphosauridae]]*
 +
***** Family [[Sphenacodontidae]]*
 +
*** '''Order Therapsida'''
 +
**** '''Suborder [[Biarmosuchia]]*'''
 +
**** '''Suborder [[Dinocephalia]]*'''
 +
**** '''Suborder [[Anomodontia]]*'''
 +
**** '''Suborder [[Gorgonopsia]]*'''
 +
**** '''Suborder [[Therocephalia]]*'''
 +
**** '''Suborder [[Cynodontia]]*'''
 +
***** ''' [[Mammaliformes]]'''
 +
****** '''Class [[Mammal|Mammalia]]'''
 +
or:
 +
* '''Suborder [[Cynodontia]]*'''
 +
** Family [[Probainognathidae]]*
 +
***'''Superfamily [[Chiniquodontoidea]]*'''
 +
'''CLASS [[Mammal|MAMMALIA]]'''
  
 
==Evolutionary history==
 
==Evolutionary history==
 +
The first true "reptiles" or amniotes are categorized as anapsids. '''Anapsids''' (Anapsida) are vertebrates characterized by solid skulls without openings near the temples, but with holes in the skull only for nose, eyes, spinal cord, and so forth. [[Turtle]]s are believed by some to be surviving anapsids.
 +
 +
Shortly after the appearance in the fossil record of the first reptiles, two branches appeared in the fossil record. One led to the Anapsida, which did not develop holes in their skulls, and the other to the '''Diapsida''' (diapsids), which possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. Diapsids ("two arches") are a group of tetrapod animals that appeared in the fossil record about 300 million years ago during the late [[Carboniferous]] period. Living diapsids are extremely diverse, and are considered to include all [[bird]]s, crocodiles, lizards, snakes, and tuataras (and possibly even turtles). While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their assumed ancestry.
 +
 +
The earliest, solid-skulled amniotes are also considered to have given rise to a separate line, the '''Synapsida''' (synapsids). Synapsids have a pair of holes in their skulls behind the eyes (similar to the diapsids); this feature has the advantage of lightening the skull and increasing the space for jaw muscles. The synapsids eventually evolved into [[mammal]]s and thus are often referred to as mammal-like reptiles.
 +
 +
''Archaeothyris'' and ''Clepsydrops'' were the earliest known synapsids. They belonged to a group called [[pelycosaur]]s and they lived in the [[Carboniferous]] Period.  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.
 +
 +
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.
 +
 
Therapsids' evolutionary track began in the Early Permian, when a group of pelycosaurs, the Sphenacodontia, a lineage that gave rise to [[Dimetrodon]] and its family, gave rise to therapsids. The evidence was their anatomical features such as the [[skull]], and the [[vertebra]]e.  Therapsids became the dominant land animals by the Middle [[Permian]], replacing the [[pelycosaur]]s. Therapsid [[Temporal_fenestra|temporal fenestrae]] were larger than those of the pelycosaurs. Endothermy ([[warm-blooded]]ness) in therapsids probably evolved by the Middle or Late [[Permian]] (Dinocephalians and anomodonts were probably [[warm-blooded]]). Therapsids probably had naked skin, like that of [[mammal]]s, rather than [[scale (zoology)|scale]]s as in reptiles and pelycosaurs. Early therapsids did not have [[fur]], which developed in the Middle or Late Permian, in the [[theriodont]]s. Therapsida consists of three major [[clades]], the [[dinocephalia]]ns, the herbivorous [[anomodont]]s and the mostly carnivorous theriodonts, with the carnivorous [[biarmosuchia]]ns as a [[paraphyletic]] assemblage of primitive forms. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian ([[Guadalupian]]) but the anomodont [[dicynodont]]s and the theriodont [[gorgonopsia]]ns and [[therocephalia]]ns flourished, being joined at the very end of the Permian by the first [[cynodont]]s.   
 
Therapsids' evolutionary track began in the Early Permian, when a group of pelycosaurs, the Sphenacodontia, a lineage that gave rise to [[Dimetrodon]] and its family, gave rise to therapsids. The evidence was their anatomical features such as the [[skull]], and the [[vertebra]]e.  Therapsids became the dominant land animals by the Middle [[Permian]], replacing the [[pelycosaur]]s. Therapsid [[Temporal_fenestra|temporal fenestrae]] were larger than those of the pelycosaurs. Endothermy ([[warm-blooded]]ness) in therapsids probably evolved by the Middle or Late [[Permian]] (Dinocephalians and anomodonts were probably [[warm-blooded]]). Therapsids probably had naked skin, like that of [[mammal]]s, rather than [[scale (zoology)|scale]]s as in reptiles and pelycosaurs. Early therapsids did not have [[fur]], which developed in the Middle or Late Permian, in the [[theriodont]]s. Therapsida consists of three major [[clades]], the [[dinocephalia]]ns, the herbivorous [[anomodont]]s and the mostly carnivorous theriodonts, with the carnivorous [[biarmosuchia]]ns as a [[paraphyletic]] assemblage of primitive forms. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian ([[Guadalupian]]) but the anomodont [[dicynodont]]s and the theriodont [[gorgonopsia]]ns and [[therocephalia]]ns flourished, being joined at the very end of the Permian by the first [[cynodont]]s.   
  
−
Like all land animals, the therapsids were seriously affected by the [[Permian extinction|end Permian extinction event]], with the very successful gorgonopsians dying out altogether and the remaining groups being represented by only one or two families of a few species, each surviving into the [[Triassic]]. Of these, the dicynodonts, now represented by a single family of large stocky [[herbivore]]s, the [[Kannemeyeridae]], and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide, throughout the Early and Middle Triassic. They died out across much of [[Pangea]] at the end of the [[Carnian]] (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.
+
Therapsids 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: ''[[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. 
 +
 
 +
Only a few therapsids and no pelycosaurs, survived the Permian extinction and went on to be successful in the new early [[Triassic]] landscape. The very successful gorgonopsians died out altogether and the remaining groups were represented by only one or two families of a few species, each surviving into the [[Triassic]]. Of these, the dicynodonts, now represented by a single family of large stocky [[herbivore]]s, the [[Kannemeyeridae]], and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide, throughout the Early and Middle Triassic. They died out across much of [[Pangea]] at the end of the [[Carnian]] (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.
  
 
Some exceptions were the still further derived [[eucynodonts]]. At least three groups of them survived.   
 
Some exceptions were the still further derived [[eucynodonts]]. At least three groups of them survived.   
Line 34: Line 75:
  
 
[[Dicynodonts]] are thought to have become extinct before the end of the [[Triassic]], but there is evidence that they survived the extinction. Their fossils have been found in [[Gondwana]]. Other animals that were common in the Triassic also took refuge here, such as the [[Temnospondyl]]s. This is an example of [[Lazarus taxon]].
 
[[Dicynodonts]] are thought to have become extinct before the end of the [[Triassic]], but there is evidence that they survived the extinction. Their fossils have been found in [[Gondwana]]. Other animals that were common in the Triassic also took refuge here, such as the [[Temnospondyl]]s. This is an example of [[Lazarus taxon]].
−
 
−
==Taxonomy and Phylogeny==
 
−
* '''Class [[Synapsida]]'''
 
−
* '''ORDER THERAPSIDA''' [[paraphyly|*]]
 
−
** '''Suborder [[Biarmosuchia]]''' [[paraphyly|*]]
 
−
*** Family [[Biarmosuchus tener|Biarmosuchidae]]
 
−
**** ''[[Biarmosuchus tener|Biarmosuchus]]''
 
−
*** Family [[Eotitanosuchus olsoni|Eotitanosuchidae]]
 
−
**** ''[[Eotitanosuchus olsoni|Eotitanosuchus]]''
 
−
** '''Eutherapsida'''
 
−
*** '''Suborder [[Dinocephalia]]'''
 
−
**** Family [[Estemmenosuchidae]]
 
−
***** ''[[Estemmenosuchus]]''
 
−
**** '''[[Anteosauria]]'''
 
−
***** Family [[Syodontidae]]
 
−
***** Family [[Brithopodidae]]
 
−
***** Family [[Anteosauridae]]
 
−
**** '''[[Tapinocephalia]]'''
 
−
***** Family [[Titanosuchidae]]
 
−
***** Family [[Tapinocephalidae]]
 
−
*** '''Neotherapsida'''
 
−
****'''Suborder [[Anomodont]]ia''' [[paraphyly|*]]
 
−
***** Superfamily [[Venyukoviamorpha]]
 
−
****** Family [[Venyukoviidae]]
 
−
******* ''[[Suminia getmanovi|Suminia]]
 
−
***** '''Infraorder [[Dicynodont]]a
 
−
**** '''[[Theriodont]]ia''' [[paraphyly|*]]
 
−
***** '''Suborder [[Gorgonopsia]]'''
 
−
****** Family [[Gorgonopsia|Gorgonopsidae]]
 
−
***** '''Eutheriodontia'''
 
−
****** '''Suborder [[Therocephalia]]'''
 
−
****** '''Suborder [[Cynodontia]]''' [[paraphyly|*]]
 
−
******* '''Superorder [[Mammaliformes]]''' [[paraphyly|*]]
 
−
******** '''Class [[Mammal|Mammalia]]'''
 
  
 
== See also ==
 
== See also ==

Revision as of 01:51, 22 December 2006

Therapsids
Phtinosuchus1ZICA.png
Phthinosuchus, an early therapsid
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Tetrapoda
(unranked) Amniota
Class: Synapsida
Order: Therapsida
Broom, 1905
Suborders

Biarmosuchia
Dinocephalia
Eotheriodontia
Anomodontia
Gorgonopsia
Therocephalia
Cynodontia

Therapsids, which have been called "mammal-like reptiles," are a group of amniote synapsids. Amniotes are animals whose embryos are surrounded by an amniotic membrane that encases it in amniotic fluid. Reptiles have traditionally been defined as including all the amniotes except birds and mammals. Synapsids are tetrapods (four-legged vertebrates) that are characterized by a pair of holes in their skulls behind the eyes. The synapsids are considered to have eventually evolved into mammals, through descent via the cynodont therapsids. Although synapsids have traditionally been referred to as reptiles, when the term is used cladistically, the taxon also includes the mammals because of descent through the therapsids.

Taxonomically, therapsids are an order of synapsids, the Therapsida.

Classification

There are different classification schemes that have been developed involving the Order Therapsid. The following is one scenario.

  • Series Amniota
    • CLASS SYNAPSIDA
      • Order Pelycosauria
        • Suborder Caseasauria
        • Suborder Eupelycosauria
          • Family Varanopseidae
          • Family Ophiacodontidae
          • Family Edaphosauridae
          • Family Sphenacodontidae
      • Order Therapsida
        • Suborder Biarmosuchia
        • Suborder Dinocephalia
        • Suborder Anomodontia
        • Suborder Gorgonopsia
        • Suborder Therocephalia
        • Suborder Cynodontia

or:

  • Suborder Cynodontia
    • Family Probainognathidae
      • Superfamily Chiniquodontoidea

CLASS MAMMALIA

Evolutionary history

The first true "reptiles" or amniotes are categorized as anapsids. Anapsids (Anapsida) are vertebrates characterized by solid skulls without openings near the temples, but with holes in the skull only for nose, eyes, spinal cord, and so forth. Turtles are believed by some to be surviving anapsids.

Shortly after the appearance in the fossil record of the first reptiles, two branches appeared in the fossil record. One led to the Anapsida, which did not develop holes in their skulls, and the other to the Diapsida (diapsids), which possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. Diapsids ("two arches") are a group of tetrapod animals that appeared in the fossil record about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and are considered to include all birds, crocodiles, lizards, snakes, and tuataras (and possibly even turtles). While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their assumed ancestry.

The earliest, solid-skulled amniotes are also considered to have given rise to a separate line, the Synapsida (synapsids). Synapsids have a pair of holes in their skulls behind the eyes (similar to the diapsids); this feature has the advantage of lightening the skull and increasing the space for jaw muscles. The synapsids eventually evolved into mammals and thus are often referred to as mammal-like reptiles.

Archaeothyris and Clepsydrops were the earliest known synapsids. They belonged to a group called pelycosaurs and they lived in the Carboniferous Period. The pelycosaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals, in the latest Carboniferous and Early Permian Periods.

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.

Therapsids' evolutionary track began in the Early Permian, when a group of pelycosaurs, the Sphenacodontia, a lineage that gave rise to Dimetrodon and its family, gave rise to therapsids. The evidence was their anatomical features such as the skull, and the vertebrae. Therapsids became the dominant land animals by the Middle Permian, replacing the pelycosaurs. Therapsid temporal fenestrae were larger than those of the pelycosaurs. Endothermy (warm-bloodedness) in therapsids probably evolved by the Middle or Late Permian (Dinocephalians and anomodonts were probably warm-blooded). Therapsids probably had naked skin, like that of mammals, rather than scales as in reptiles and pelycosaurs. Early therapsids did not have fur, which developed in the Middle or Late Permian, in the theriodonts. Therapsida consists of three major clades, the dinocephalians, the herbivorous anomodonts and the mostly carnivorous theriodonts, with the carnivorous biarmosuchians as a paraphyletic assemblage of primitive forms. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian (Guadalupian) but the anomodont dicynodonts and the theriodont gorgonopsians and therocephalians flourished, being joined at the very end of the Permian by the first cynodonts.

Therapsids 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: 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 mass extinction about 250 Mya, the largest extinction in Earth's history, which may have been related to the Siberian Traps volcanic event.

Only a few therapsids and no pelycosaurs, survived the Permian extinction and went on to be successful in the new early Triassic landscape. The very successful gorgonopsians died out altogether and the remaining groups were represented by only one or two families of a few species, each surviving into the Triassic. Of these, the dicynodonts, now represented by a single family of large stocky herbivores, the Kannemeyeridae, and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide, throughout the Early and Middle Triassic. They died out across much of Pangea at the end of the Carnian (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.

Some exceptions were the still further derived eucynodonts. At least three groups of them survived.

  1. The extremely mammal-like family, Tritylodontidae, survived into the Early Cretaceous.
  2. An extremely mammal-like family, Tritheledontidae, are unknown later than the Early Jurassic.
  3. The third group, Morganucodon and similar animals, were stem-mammals.

Dicynodonts are thought to have become extinct before the end of the Triassic, but there is evidence that they survived the extinction. Their fossils have been found in Gondwana. Other animals that were common in the Triassic also took refuge here, such as the Temnospondyls. This is an example of Lazarus taxon.

See also

  • Mammal-like reptile
  • Dinosaurs - therapsids' evolutionary counterparts

External links

References
ISBN links support NWE through referral fees

  • Kemp, T.S. (2005): The origin and evolution of mammals. Oxford University Press
  • Benton, M.J. (2004): Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd
  • Carroll, R.L. (1988): Vertebrate Paleontology & Evolution. W.H. Freeman & Company, NY
  • Romer, A.S. (1966): Vertebrate Paleontology. University of Chicago Press, 1933; 3rd ed.

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