Temporal range:
Synapsid diversity 3.jpg
Examples of synapsids: Cotylorhynchus, Dimetrodon, Inostrancevia, Moschops, Castorocauda, Adelobasileus, Tachyglossus, and Panthera tigris
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Osborn, 1903

Theropsida (Seeley, 1895)[5]

Synapsids[a] are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptiles and birds. The group includes mammals and every animal more closely related to mammals than to sauropsids.[6] Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name.[7] The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period,[1] when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

Traditionally, non-mammalian synapsids were believed to have evolved from reptiles, and therefore described as mammal-like reptiles in classical systematics, and primitive synapsids were also referred to as pelycosaurs, or pelycosaur-grade synapsids. These paraphyletic terms have now fallen into disfavor and are only used informally (if at all) in modern literature, because it is now known that synapsids are not reptiles, nor are they part of reptilian lineage in a cladistical sense.[8][9][10] They are now more correctly referred to as stem mammals, and sometimes as proto-mammals, or paramammals.[11][12]

Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago, equalled only by some large pareiasaurs at the end of the Permian. Most lineages of pelycosaur-grade synapsids were replaced at the end of Early Permian by the more advanced therapsids. Synapsid numbers and variety were severely reduced by the Permian–Triassic extinction. Only the therapsid dicynodonts and eutheriodonts (consisting of Therocephalia and Cynodontia) are known to have continued into the Triassic period. The cynodont group Probainognathia, which includes Mammaliaformes (mammals and their closer ancestors), were the only synapsids to survive beyond the Triassic.[13]

During the Triassic, the sauropsid archosaurs became the largest and most numerous land vertebrates, and gave rise to the dinosaurs. When all non-avian dinosaurs were wiped out by the Cretaceous–Paleogene extinction event, the mammalian synapsids diversified again to become the largest land and marine animals on Earth.

  1. ^ a b Steen, Margaret C. (1934). "The amphibian fauna from the South Joggins. Nova Scotia". Journal of Zoology. 104 (3): 465–504. doi:10.1111/j.1096-3642.1934.tb01644.x.
  2. ^ David S. Berman (2013). "Diadectomorphs, amniotes or not?". New Mexico Museum of Natural History and Science Bulletin. 60: 22–35.
  3. ^ Klembara, Jozef; Hain, Miroslav; Ruta, Marcello; Berman, David S.; Pierce, Stephanie E.; Henrici, Amy C. (2019). "Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group". Palaeontology. 63: 131–154. doi:10.1111/pala.12448.
  4. ^ Brocklehurst, N. (2021). "The first age of reptiles? Comparing reptile and synapsid diversity, and the influence of lagerstätten, during the Carboniferous and early Permian". Frontiers in Ecology and Evolution. 9: 669765. doi:10.3389/fevo.2021.669765.
  5. ^ Seeley, Harry Govier (1895). "Researches on the structure, organisation, and classification of the fossil reptilia. Part X. On the complete skeleton of an anomodont reptile (Aristodesmus rutimeyeri, Wiedersheim), from the Bunter sandstone of Reihen, near Basel, giving new evidence of the relation of the Anomodontia to the Monotremata". Proceedings of the Royal Society of London. 59: 167–169. doi:10.1098/rspl.1895.0070.
  6. ^ Laurin, Michel; Reisz, Robert R. (2011). "Synapsida: Mammals and their extinct relatives" (Version 14 ed.). The Tree of Life Web Project.
  7. ^ Romer, A.S; Parsons, T.S. (1985). The Vertebrate Body (6th ed.). Philadelphia, PA: Saunders.
  8. ^ Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York, NY: W.H. Freeman & Co. p. 397. ISBN 0-7167-1822-7.
  9. ^ Benton, Michael J. (2005). Vertebrate Palaeontology (3rd ed.). Oxford, UK: Blackwell Science Ltd. p. 122. ISBN 0-632-05637-1.
  10. ^ "Jaws to ears in the ancestors of mammals". Retrieved 2020-02-20.
  11. ^ "New proto-mammal fossil sheds light on evolution of earliest mammals". University of Chicago. August 7, 2013.
  12. ^ Naish, Darren. "The Stem-Mammals--a Brief Primer". Scientific American Blog Network. Retrieved 2022-02-27.
  13. ^ "Greatest mass extinction responsible for the making of modern mammals" (research publ. ann.). Bloemfontein, ZA: The National Museum [of South Africa]. 19 September 2013. Archived from the original on 2019-03-28. Retrieved 2015-08-22.

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