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