There can be no doubt that the earliest mammals made their appearance in Jurassic times as descendants of some of the true mammal-like reptiles, the Therapsida.
However, scientists put forwarded two important ideas- Ancestry through Amphibia and Ancestry through Reptilia about the origin and ancestry of mammals.
Ancestry through Amphibia:
It was T.H. Huxley (1880) who advocated amphibian ancestry of mammals.
The points on which Huxley based his arguments were:
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(a) Presence of two occipital condyles in the skull of both Amphibia and Mammalia,
(b) Presence of left aortic arch in mammals.
However, Huxley’s theory is not tenable because occipital condyles are derived from exoccipitals in Amphibia but from basioccipitals in Mammalia. Apart from this, the two classes have different modes of life and exhibit many fundamental differences.
Ancestry through Reptilia:
Palaentological evidences establish the fact that mammals arose from reptiles. This view also gets strong support from the fact that the monotremes (primitive mammals) and living reptiles have close resemblances in their anatomical features including soft as well as hard parts.
Ancestral Mammal-Like Reptiles:
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Fossils of Synapsida that have been discovered in the Carboniferous strata indicate many characters leading to mammalian line. They lived throughout the Permian and Triassic periods, dating back 280 million years or more. The more mammal-like synapsids belonged to the order Therapsida.
One of the more advanced carnivorous therapsids (suborder Theriodontia) was called Cynognathus (dog jaw). It lived during the early Triassic period.
It was wolf-sized and showed the following mammalian characters:
(i) The presence of two occipital condyles which develop from two exoccipitals, and enlarged lateral temporal fossa.
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(ii) A well-developed secondary or false palate which separates the nasal passages from the m