684 Mammals, Biodiversity of of tooth replacement and development is a derived character and suggests that the trait is ancestral to marsupials A further primitive character common to the Monotremata and Metatheria, but thought to be lost in the eutherian mammals, is the epipubic bones, which extend forward from the pubic bone in both sexes Hypotheses about the function of the epipubic bones have been various Initial speculation was that the structure provided support for the attached young found in marsupials or support for the marsupium, in which they are often contained Others have suggested it was involved with locomotion, although more recently it has been proposed that epipubic bones provided a place for the attachment of abdominal muscles, thus allowing for greater expansion of the abdominal cavity, increasing the function of the diaphragm and improving efficiency of respiration Recent paleontological finds show that epipubic bones were present in Cretaceous eutherian mammals Because the loss of epipubic bones is associated with prolonged gestation, these findings have been interpreted as evidence that the complex suite of adaptations that typify placental mammals were derived later in the evolution of eutherians Reproduction Whereas placental mammals tend to have long gestation periods and relatively short periods of lactation, gestation in the marsupials is always relatively brief and is followed by a much longer period of lactation Typically, neonatal marsupials are extremely small, only a few grams, and show very incomplete development The exception to this generality is seen in the jaw and forearm structure of the neonate: the nearly embryonic young need strong arms to navigate up the mother’s midline to the teat and well-developed jaws with which to form an immovable attachment to the teat once arriving A greater proportion of development occurs outside the mother’s uterus after the neonate has attached to a teat In approximately half the marsupial species, the teats are found inside a marsupium By the time the neonate leaves the pouch, or detaches from the teat, it weighs more or less the same as a placental mammal of an equivalent adult size Diagnostic differences in the structure of the reproductive tract of female marsupials and in the development of the embryo may explain many of the life history differences between eutherian and metatherian mammals While eutherian mammals form a complex placenta that nourishes the embryo, marsupials retain an eggshell membrane with a simple yolk sac to nourish the young This arrangement limits the nutrients that a mother can transfer to her embryos The size of a neonatal marsupial is also limited by the structure of the female reproductive tract In eutherian mammals, the reproductive tract is arranged linearly (Figure 2): the vagina leads directly into the uterus, with two ureters transporting urine from the bladder to the urethra Growth of the fetus is essentially constrained only by the size and elasticity of the vaginal canal Given that the spotted hyena gives birth through a vagina that has evolved to look like a pseudopenis, this constraint does not appear to be too great In contrast, in marsupials, the structure of the vagina and uterus is like a double jug handle (Figure 2) Just above the urethra, two lateral vaginas form loops on either side of a Metatheria Eutheria Uterus Ovary Ureter Lateral vagina Pseudovaginal canal Vagina Urethra Figure Comparison of eutharian and metatherian reproductive tracts Reprinted from Sharman GB (1970) Reproductive physiology of marsupials Science 167: 1221–1228, with permission from American Association for the Advancement of Science pseudovaginal canal The two vaginal loops rejoin and feed into a bipartite uterus The ureters pass inside these vaginal loops Because birth takes place through the lateral vagina, placement of the ureter inside this loop may limit the size of the neonate Mammalian Phylogeny Early History Mammals have their origins deep in geological history Just more than 200 million years ago, in the late Triassic period, primitive cynodont mammals evolved from their mammallike reptile progenitors, the Therapsids The Therapsids, whose members dominated the terrestrial landscape during the Triassic, faded into insignificance by the end of the Triassic/ Jurassic boundary, leaving the stage of the late Mesozoic era open to the radiation of great sauropods, the dinosaurs For 140 million years, dinosaurs filled most terrestrial (and many aerial) ecological roles, with mammals for the most part relegated to small, terrestrial rodent-like forms The earliest evidence of true mammals occurs in the late Triassic, although by this time, mammals were found worldwide, suggesting a somewhat earlier divergence from the mammal-like cynodonts Poor fossil records from many areas leave gaps, which slowly are being filled Although the Mesozoic mammals were once thought to be lacking diversity, recent finds suggest this was not the case Fossil evidence suggests they were never abundant and rarely showed any great size – the first mammal weighing more than kg does not appear in the fossil record until the early Cretaceous Yet a variety of unusual forms evolved in the Mesozoic, including Symetrodonta, which are characterized by well-developed, triangular molars, the predatory Triconadonta, and the omnivorous and herbivorous members of the Multituberculata The great radiation of modern mammals began in the midCenozoic era, in the late Cretaceous period, approximately 100 million years ago More rapid evolution of larger body sized mammals did not begin until the mid-Cenozoic, beginning about 65 million years ago during the Paleocene epoch of the Tertiary Following this, mammalian radiation accelerated