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PLACENTAL MAMMALS

PLACENTAL MAMMALS

Modern placental mammals have not always existed, nor have they existed since the first appearance of fossil placental mammals. The first placental mammals were preceded by primitive therian mammals which resembled them more than do any modern mammals. Modern placental mammals form a clade which shares a number of features of the skeletal system (sinuses in skull bones, an auditory bulla, other aspects of the skull and limbs), nervous system (increased manual dexterity, enlarged cerebrum which expanded over the midbrain, gyri and sulci, a corpus callosum, new brain nuclei, and folding patterns of the cerebral cortex), cardiovascular system (new globins, a reduced sinus venosus, modification of embryonic blood vessels), and reproductive system (more advanced state of live birth, a chorioallantoic placenta, prostate glands, seminal vesicles, a scrotum, ventral portion of the cloaca is retained in females as the vestibule).

BISON SKUNK

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