Mammal-Like Reptiles: Transitional Forms?

Hall of Life—Reptile and Amphibian Exhibits

on May 9, 2016

Some creatures appear to have features found in both mammals and reptiles. These “mammal-like reptiles” are alleged to be the transitional forms between reptiles and mammals. As with all alleged evolutionary transitions, evolutionists themselves are unclear about what exactly evolved into what. They realize that this group of animals does not show an obvious transition from reptile to mammal.

It is not known which cynodont [a group of mammal-like reptiles] family was ancestral to mammals, or whether all the mammals originated from the same group (family) of cynodonts. In the vast literature concerning mammalian origins, it is easier to find suggestions that one or the other therapsid or cynodont family cannot be ancestral to the Mammalia, rather than to find a positive answer. (Z. Kielan-Jaworowska, “Interrelationships of Mesozoic mammals,” Historical Biology 6:3, 1992, p. 195.)

Additionally, there are tremendous differences between mammals and reptiles, which cannot be accounted for by information-losing mutations or natural selection.

  • They have different circulatory systems, and breathe differently (mammals have a diaphragm).
  • They hear differently (mammals have an organ of corti, which reptiles lack).
  • They feed their young differently (female mammals produce milk, reptiles don’t).
  • They have different skin structures (mammals have hair and sweat glands, which reptiles lack).
  • Mammals are warm-blooded; reptiles are cold-blooded.

By Eduard Solà, via Wikimedia Commons

Fossil of Tiktaalik, a type of fish that evolutionists mistakenly supposed was a transitional form between reptiles and mammals.1

Futher, John Woodmorappe points out that “rather than a progression to ‘mammalness,’ we observe an assortment of unmistakable reptilian traits and unmistakable mammalian traits.” (John Woodmorappe, “Walking whales, nested hierarchies, and chimeras: do they exist?,” TJ 16:1, 2002, pp. 111–119, available online at He adds elsewhere:

…the traits usually considered unique to mammals are distributed variously throughout the mammal-like reptiles. While this distribution is not haphazard or random, it does not form lineages. … Just because some “mammalian” traits are present in mammal-like reptiles, this does not entail evolution in the slightest. It simply means that some traits now considered mammalian (by virtue of the fact that they are found only in extinct mammals) once existed in some extinct non-mammals. (J. Woodmorappe, “Mammal-like reptiles: major trait reversals and discontinuities,” TJ 15:1, 2001, pp. 44–52, available online at

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  1. David Menton, “Tiktaalik and the Fishy Story of Walking Fish” Answers in Depth, Volume 2, 2007,