Trends in Mammalian Evolution and Hypotheses of Reptilian

Reptilian Ancestry of Mammals

Mammals possess characteristic molar teeth. The origin of molar teeth in the phylogenetic history of mammal has given rise to a number of theories. All the theories indirectly support reptilian ancestry of mammals.

Tritubercular Theory

Cope and Osborn hold that mammalian molar teeth passed through a trituberculate stage found in Cynognathus. Multitubercular theory of Woodward and Goodrich advocates that mammalian molar teeth arose from a multituberculate stage found in Cynodontia (Diademodon) and the modern mammals are the evolutionary derivatives of the Diademodon.

Compromise theory of diphylectic origin of Mivart and Beddard advances the logical idea of mammalian evolution from Cynodontia having both tritubercular and multitubercular molar teeth.

Judging all the theories the inevitable conclusion that can be drawn is that reptilian ancestry is full of evolutionary significance. Cumulative evidences from palaeontology, comparative morphology and serology thus prove that the mammals sprang from reptiles. Very likely the source is best represented by either Cynodonts or Ictidosauria.

Probable Ancestry of Mammals

According to most evolutionists of today, the Theriodont reptiles hold the key of mammalian ancestry. From Cynodontia, the Ictidosauria and the mammalia arose and underwent parallel evolution. The probable line of ancestry is shown in Fig

TRENDS IN MAMMALIAN EVOLUTION 2

Biological Trends in Mammalian Evolution

A review of the fossil mammal like reptiles reveals that the evolution of mammals is preceded through the following biological trends.

  • The gradual growth of brain which is related to the increment of the cranial cavity.
  • Gradual acquisition of hard palate.
  • Loss and condensation of some bones of the skull and lower jaw.
  • Gradual reduction of elements of the pectoral girdle.
  • Gradual condensation, differentiation and compactness of the general body system to increase the efficiency.

Time of Origin

The mammal like reptiles were abundant in Permian and in early Triassic periods.

The probable time is assigned either to be late Permian or early Triassic.

Place of Origin

There are two views regarding the place of origin of mammals. First view is that the mammals arose in South Africa as evidenced from the abundance of Therapsid fossils. Based from the history of reptilian disappearance the second view holds that the place, of origin was Central Asia.

Cause of Origin

From Permian to Jurassic age, there was a great climatic fluctuation. There had been periodic aridity alternating with glacial periods. So nature has to provide a new mechanism of temperature control. The achievement of temperature control probably saved the insignificant mammals from the one stroke of geological changes.

At the present state of knowledge it will be better not to go too far regarding the origin of mammals. Further researches on palaeontology, comparative anatomy are necessary t6 picture out the completeness of mammalian ancestry.

However, with all certainties and probabilities, the insignificant Cynodonts whose existence was at the mercy of the mighty Dinosaurs, survived the perilous existence and gave origin to mammalian form of life in time and space.

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