Mammal-like reptile

Chiniquodon, an Upper Triassic cynodont, close to the ancestry of mammals. Museum of Paleontology, Tübingen

Bienotherium, a Lower Jurassic tritylodont from China

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

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 reptiles

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.

Mammal-like Reptile Media

Comparison of "pelycosaurian" skulls: 1 sphenacodont, 2 ophiacodont, 3 caseid. The quadratojugale is green, squamosale is red, jugale is blue.
Cotylorhynchus (background), Ophiacodon and Varanops
Eothyris parkeyi, pencil drawing, digital coloring
Dimetrodon – a sphenacodontid

References

↑ 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)
↑ Benton, Michael J. 2005. Vertebrate paleontology, 3rd ed. Blackwell, Oxford: chapter 10, The mammals. ISBN 0632056371
↑ 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
↑ Goodrich E.S. 1930. Studies on the structure and development of Vertebrates. Macmillan, London.
↑ ^5.0 ^5.1 Kermack D.M. & K.A. 1984. The evolution of mamalian characters. Croom Helm, London. p6/7 ISBN 0709915349

[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

[Good-4] Goodrich E.S. 1930. Studies on the structure and development of Vertebrates. Macmillan, London.

[KK-5] 5.0 ^5.1 Kermack D.M. & K.A. 1984. The evolution of mamalian characters. Croom Helm, London. p6/7 ISBN 0709915349

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