During the evolution of the synapsid reptiles and
the early mammals (a period of more than 150 million years), a number
of changes evolved in other systems of the body including a diaphragm
and a modified kidney.
THE RESPIRATORY SYSTEM
Mammals evolved thyroid cartilages which fuse in the larynx. The diaphragm
evolved to increase the amount of air exchanged with each breath.
THE DIGESTIVE SYSTEM
Infant mammals lost horny egg teeth and sucrase became an enzyme of the
mammalian digestive system. The cloaca began to subdivide.
THE URINARY SYSTEM
Mammalian nephrons developed a loop of Henle and increased the number
of kidney tubules. The mammalian kidney (typically bean-shaped) was composed
of a renal cortex and medulla with a prominent renal pelvis. The mesonephros
was no loner functional after birth. Ureters entered the bladder (rather
than to the cloaca). The mutation in ancestral vasotocin which gave rise
to vasopressin seems to have occurred only in the ancestral mammals; all
non-mammalian vertebrates possess vasotocin.
THE REPRODUCTIVE SYSTEM
Female mammals developed a corpus luteum. In all mammals, the uterus produces
nutrients for developing embryos (which is absorbed through the thin shells
of monotremes). Even in monotremes, the embryos do develop inside the
uterus to some degree. Male mammals developed bulbourethral glands, a
corpora spongiosum of penis, and a penis possessing glans and a foreskin.
The monotreme penis is transitional between that of reptiles and higher
mammals. Mammals in general express estrogen receptors in the limbic areas
of the brain.