Mammals - are these your grandparents?

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Mammals - at these your grandparents? - Cynodonts

About 250 million years ago the most lethal mass extinction of all time, the end-Permian extinction, claimed 90 percent of all ocean life and 70 percent of all land species.
Cynodonts were one of the few groups of creatures to survive the mass extinction. They are reptiles that pre-date dinosaurs and would appear to be the grandparents of marsupials (like kangaroos), placental mammals (like humans) and monotremes—mammals that lay eggs instead of giving birth to live young (like the platypus and echidna).

By Nobu Tamura ( - Own work, CC BY 2.5, Link

Cynodonts lived millions of years before the appearance of the first known mammal: a shrew-like creature that lived about 160 million years ago in what is now China, yet they appear to be an important intermediate between mammals and reptiles, as they have many characteristics similar to each group

Cynodonts have nearly all the characteristics of mammals.
  • they changed to more "dog-like teeth" from being designed for catching and holding prey and then swallowing whole, to adding specialized teeth, including molars, designed for better mastication of food allowing for quicker digestion, unlike the cone shaped teeth of other reptiles
  • the braincase bulged at the back of the head
  • unlike most reptiles, many of them walked in an upright manner 
  • some large "mammal-like" holes in the skull allowed for more robust jaw made of fewer bones with the superfluous bones to evolve into an entirely new function, becoming parts of the mammal's inner ear. Improved hearing gave these creatures a better awareness of their environment and, in turn, this increasing sensitivity called for a greater capacity for processing the auditory information in the brain
  • they also have a secondary palate that other primitive other than close relatives, therapsids lacked - this allowed air to flow to the lungs through the back of the mouth, allowing cynodonts to chew and breathe at the same time - this characteristic is present in all mammals
  • their dentary was the largest bone in their lower jaw, as other smaller bones moved into the ears.
  • they were possibly partially if not completely warm-blooded making them active hunters at night , an advantage when other reptiles were sluggish
  • they were possibly covered in hair allowing them to expend less energy to keep warm and active

  • But unlike most mammals - cynodonts still laid eggs.
    Though the last cynodonts lived into the Jurassic, most became extinct at the end of the Triassic - but not before giving rise to the first true mammals. Mammals evolved after this mass extinction in small ways that did not stand out much from their cynodonts for many millions of years.


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