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MESOZOIC ERA

CRETACEOUS PERIOD

146-65 million years ago

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Ancestral mammals evolved modifications of other systems which are shared among modern placental mammals.

THE CARDIOVASCULAR SYSTEM
Early in the evolution of eutherian mammals, the proto-eta globin locus duplicated to produce e, ?, and ? globins and the proto-ß locus duplicated to form ß and d globins. In the heart, the sinus venosus is retained only as the SA node after embryonic development. The embryonic lateral abdominal veins become umbilical veins and, in many mammals, the left common cardinal vein degenerates in embryonic development producing only one resultant anterior/superior vena cava. The ductus arteriosus is functional in placental embryo but becomes the ligamentum arteriosum in adults.

THE URINARY SYSTEM
In placentals, the mesonephros does not persist after birth and the ureters empty into the bladder more superiorly (as opposed to near base as in monotremes).


GENETIC CHANGES
A number of genetic modifications are shared among placental mammals. Placental mammals use Mox2 in the formation of the placenta, possess 3 opioid receptors

THE SKIN
Some aspects of hair gene expression are shared among placentals given that mutations in the zinc finger gene hairless can cause the absence of hair (alopecia universalis) in both humans and mice. Mice and humans possess equivalent keratin clusters with virtually all of the same genes in the same order.