TWENTY-THREE

Emergent Mammals II

"The mammals arose within the therapsid radiation in the Late Triassic (252 - 201 mya), initially small and shrewlike. These Mesozoic (252 - 66 mya) animals [co-existed] with a terrestrial fauna then dominated by dinosaurs, especially the sauropsids" (Vertebrates, 2012, p. 122).

DEVONIAN PERIOD

(419 - 359 mya)

JURASSIC PERIOD

(201 - 145 mya)

JURASSIC PERIOD

(201 - 145 mya)

TRIASSIC PERIOD

(252 - 201 mya)

FOSSIL MAMMALIAN SKELETAL CHARACTERISTICS

(adapted from Vertebrates, 2012, p. 123)

1) The mammal possesses a chain of 3 bones in the middle ear, that conduct sound from the eardrum to the inner ear (reptiles have only one primary middle ear bone);

2) Whereas several bones comprise the reptilian lower jaw, mammals posses only a single, dentary, bone; and

3) Whereas reptiles have additional bones to complete the jaw joint, mammals have a joint between the dentary and squamosal  bones of the jaw.