Synapsida (Extinct Mammal Relatives)

Abstract

In the phylogenetic system, the Synapsida include Mammalia and all other amniotes more closely related to Mammalia than to Reptilia. The most characteristic feature is the presence of a single large opening behind the orbit on either side of the skull. This opening is small in basal synapsids but increases in size in later, more derived forms including mammals. Nonmammalian synapsids form an extensive series of taxa that are successively more closely related to mammals and thus are now often referred to as stem‐mammals or ancient mammal relatives. They range from the Late Carboniferous to the Early Cretaceous. Crown‐group mammals (which include all present‐day mammals) are first known from the Middle Jurassic. Synapsida and Reptilia are now considered two entirely distinct evolutionary lineages of Amniota with a common ancestor in the Carboniferous.

Key Concepts

  • Synapsida is one of the two principal groups of Amniota.
  • Synapsida comprises mammals and all extinct taxa more closely related to mammals than to reptiles including birds.
  • Synapsids form a distinct evolutionary lineage from reptiles.
  • There was a gradual evolutionary acquisition of mammal‐like features during synapsid evolution.
  • The ear bones of mammals developed from the bones at the back of the lower jaw in their antecedents.

Keywords: evolution; origin of mammals; fossils; Carboniferous; Permian; Triassic

Figure 1. Evolution of the key feature of Synapsida – the temporal opening (highlighted in light blue) –from small temporal opening to fully developed temporal fossa (which is confluent with the orbit in many mammals). (a) sphenacodontid ‘pelycosaur’ Dimetrodon; (b) gorgonopsian therapsid Sauroctonus; (c) opossum (Didelphis). Not to scale. Modified from Bystrov .
close

References

Angielczyk KD and Kammerer CF (2018) Non‐mammalian synapsids: the deep roots of the mammalian family tree. In: Zachos FE and Asher RJ (eds) Mammalia: Mammalian Evolution, Diversity and Systematics, pp 117–198. de Gruyter: Berlin.

Bonaparte JF (2012) Evolution of the Brasilodontidae. Historical Biology 25: 643–653.

Bystrov AP (1957) Past, Present, and Future of Man (In Russian). Medgiz: Leningrad.

Fröbisch J (2007) The cranial anatomy of Kombuisia frerensis Hotton (Synapsida, Dicynodontia) and a new phylogeny of anomodont therapsids. Zoological Journal of the Linnean Society 150: 117–144.

Kielan‐Jaworowska Z, Cifelli RL and Luo Z‐X (2004) Mammals from the Age of the Dinosaurs: Origins, Evolution, and Structure. Columbia University Press: New York.

Lautenschlager S, Gill P, Luo Z‐X, et al. (2017) Morphological evolution of the mammalian jaw adductor complex. Biological Reviews 92: 1910–1940.

Luo Z‐X, Yuan C‐X, Meng Q‐J, et al. (2011) A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445.

Rowe TB (1988) Definition, diagnosis, and origin of Mammalia. Journal of Vertebrate Paleontology 8: 241–264.

Rubidge B and Sidor CA (2001) Evolutionary patterns among Permo‐Triassic therapsids. Annual Review of Ecology and Systematics 32: 449–480.

Further Reading

Benton MJ (2014) Vertebrate Palaeontology, 4th edn. Wiley Blackwell: Chichester.

Kammerer CF, Angielczyk KD and Fröbisch J (eds) (2014) Early Evolutionary History of the Synapsida. Springer: Dordrecht.

Kemp TS (2005) The Origin and Evolution of Mammals. Oxford University Press: Oxford.

Kielan‐Jaworowska Z, Cifelli RL and Luo Z‐X (2004) Mammals from the Age of the Dinosaurs: Origins, Evolution, and Structure. Columbia University Press: New York.

Zachos FE and Asher RJ (eds) (2018) Mammalia: Mammalian Evolution, Diversity and Systematics. de Gruyter: Berlin.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Sues, Hans‐Dieter(Sep 2020) Synapsida (Extinct Mammal Relatives). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029199]