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).
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
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
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.