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THERIAN MAMMALS

THERIANS

Modern therian mammals (marsupials and placentals) have not always existed, nor have they existed since the first appearance of fossil therian mammals. Modern therian mammals form a clade which shares a number of features of the skeletal system (modifications of occipital condyles, sphenoid bone, and teeth, cervical ribs which fuse to vertebrae, a spine of the scapula, a modified shoulder), nervous system (shorter spinal cord, larger cerebellar hemispheres), reproductive system (live birth, oviducts, descent of testes, reduction of the cloaca), and other features (a more compact spleen, umbilical arteries and the umbilical vein, a foramen ovale in the fetal heart, globin duplications, and the SRY gene on the Y chromosome to determine gender).

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