Mammal-like reptile

old term for synapsids

Mammal-like reptile is an old term for the therapsids: those synapsids which gave rise to the true mammals.

Chiniquodon, an Upper Triassic cynodont, close to the ancestry of mammals. Museum of Paleontology, Tübingen
Bienotherium, a Lower Jurassic tritylodont from China

The term is both outmoded and a mistake, because mammals did not descend from reptiles. Both groups descended from early amniotes (egg-laying tetrapods), probably in the Lower or Middle Carboniferous.[1] The precursors of reptiles are called sauropsids, and the precursors of mammals are called synapsids. The immediate ancestors of the mammals came from a group of therapsids called the cynodonts.[2][3]

Mammals and reptilesEdit

There are a number of characteristics which cannot be seen on fossils, but which are of great importance.

Mammals are distinguished from reptiles by fundamental differences in the development of the blood system.[4][5] These differences are such that it would be almost impossible for mammals to be derived from reptiles.

" It is clearly quite impossible for the condition found in birds and modern reptiles to have arisen from that found in mammals or vice versa". Kermack.[5]6

This supports the division of amniotes into sauropsida and synapsida.


  1. Clack, Jennifer 2012. Gaining ground: the origin and early evolution of tetrapods. 2nd ed, Bloominton, Indiana: Indiana University Press, Chapter 8, p295. ISBN 978-0-253-35675-8, (Mammal-like reptile at Google Books)
  2. Benton, Michael J. 2005. Vertebrate paleontology, 3rd ed. Blackwell, Oxford: chapter 10, The mammals. ISBN 0632056371
  3. Prothero, Donald R. 2007. Evolution: what the fossils say and why it matters. New York: Columbia University Press, Chapter 13, 271–280, especially p274/5. ISBN 978-0-231-13962-5
  4. Goodrich E.S. 1930. Studies on the structure and development of Vertebrates. Macmillan, London.
  5. 5.0 5.1 Kermack D.M. & K.A. 1984. The evolution of mamalian characters. Croom Helm, London. p6/7 ISBN 0709915349