2. Mammary Glands - modified sweat glands that provide nutrients for young. Most mammals are viviparous (give birth to live young), but the monotremes are oviparous, and most, if not all, non-mammalian synapsids were probably oviparous.
3. Diaphragm - to increase the inspiration of oxygen and expiration of carbon dioxide (necessary for a high metabolic rate).
4. A four chambered heart that separates oxygenated from unoxygenated blood:
5. Expansion of the neocortex of the brain - resulting in greater intelligence.
6. An ear more sensitive to a range of vibrations because the bone connected to the ear drum has detached from the rigid jaw, joined three new bones and become the bones of the middle ear where they are free to vibrate more easily.
8. A bony secondary palate separating the nasal passage from the mouth.
9. Greater differentiation of the teeth characterized by multi-rooted teeth with multicusped crowns.
Major Changes in the Transition
Locomotion
Mammalian locomotion involved a change in limb posture from sprawling (in
Pelycosaurs) to erect, with elbows pointing out and backward, and knees pointing out
and facing forward. As discussed earlier, erect body posture places the hands and feet
closer to the body's center of gravity, resulting in less need for large ventral limb
muscles just to hold the body up off the ground. All of the movement of the limb goes
into rotation back and forward (i.e., moving forward, not into lifting the body off the
ground). The long bones of mammalian limbs became more slender in conjunction
with the shift to more erect body posture.
Another feature that goes along with the erect body posture: the vertebral column becomes more rigid. To compensate for the loss of flexibility, more neck bones are added and a more rounded occipital condyle (joint at the base of the cranium) develops which allow more flexible movement of the head.
Feeding
Major changes in the teeth, jaws, and jaw musculature modified the feeding
system.
Teeth - the mammalian dentition differentiated into small incisors, large canines, and increasingly complex premolars and molars (called the cheek teeth). Increased complexity in the cheek teeth included the growth of additional cusps on the crowns with precise interlocking of cusps and ridges between upper and lower teeth.
The more elaborate cheek crowns provided mammals with shearing and crushing capability during chewing rather than the simple puncturing of single cusped reptile teeth.
As crown complexity increased, the cheek teeth developed multiple roots for firm anchorage.
One of the features of all mammals is the presence of interlocking teeth which results in
improved chewing (the upper and lower teeth in the jaw work together to cut or crush
and grind food). BUT, the interlocking cusps of upper and lower teeth cannot be
maintained if the teeth are continuously lost and replaced. This led to evolution of just
two sets of teeth one set replacing the other in the juvenile (milk teeth and adult teeth --
I think the tooth fairy must have also evolved at about this time). This altered design
means the early egg-laying mammals must have evolved milk glands and lactation.
Once milk is available, the young can be born with few or no teeth that can later appear
when the jaw is larger and closer to the adult size. Without milk, a newborn mammal
would need a full set of teeth to eat and survive.
(1) reptiles have at least four bones in the lower jaw (e.g. the dentary, articular, angular, surangular, and coronoid), while mammals have only one (the dentary), and
(2) reptiles have only one middle ear bone (the stapes), while mammals have three (the hammer, anvil, and stapes)
The first mammals appear in the Jurassic. These first mammals were small shrew-sized animals (about 25 gms or 1 oz in size). This small mammal may have been successful because warm-bloodedness allowed it to live as a nocturnal insectivore.
By the late Cretaceous, 3 main types of mammals had evolved:
Multituberculates are the only major branch of mammals to have become completely extinct, and have no living descendants. Multituberculates get their name from their teeth, which have many cusps, or tubercles arranged in rows.
Although not known to many people, they have a 100 million-year fossil history
(the longest of any mammalian lineage) and were distributed throughout the world.
The platypus incubates its eggs in a nest, but the anteater egg is incubated in a temporary pouch that forms on the abdomen of the female.
Monotremes today produce milk from modified seat glands - but do not have nipples. The young feed by licking the milk off the mother's fur.
The echidnas and platypus have a sixth sense which all other mammals lack. In
their snouts are electro-receptors, allowing echidnas to detect small electric currents.
The purpose of this sense is unclear but scientists have calculated that it is sensitive
enough to detect the natural electrical activity of underground grubs and worms,
though probably not ants and termites.
Therian Mammals - In the early Cretaceous, a new group of mammals appeared which
have an improved inner ear for detecting and analyzing sound. This was achieved by
the coiling and enlarging of the cochlea bone of the inner ear (see figure of ear above).
There are two types of therians: the marsupial (or pouched mammals) and the placental mammals. Both marsupials and placentals give birth to live young, but the marsupials have very immature newborns that they usually place in a pouch, while the placentals carry their young in their bodies until a later stage of growth and give birth to relatively more mature newborns.
(1) Marsupial Mammals - Though marsupials today do not have as many species as do
the placental mammals, they are quite structurally diverse. They are characterized by
premature birth and continued development of the newborn while attached to the
nipples on the lower belly of the mother.
In many marsupials, the hind legs are noticeably larger than the forelegs (this is most obvious in the kangaroos); and they have a unique bone associated with the pelvic girdle which helps support the pouch.
The largest and most varied assortment of marsupials--more than 100 species--is found in Australia alone: kangaroos, wallabies, wombats, the koala, and a bewildering assemblage of smaller rodent-like forms. They also include the marsupial wolf, which may have recently gone extinct. About 70 more species are distributed more widely, in Australia, New Guinea, and a cluster of nearby islands. The wide array of Australian marsupials is reflected in the extensive popular vocabulary of names, many of which are derived from descriptive Aboriginal words. Only a few marsupials are found in the Americas, the most "famous" of which is the oppossum.
As marsupials evolved into diverse forms, they came to occupy various habitats. The result is that several have converged on placental mammal body forms :
In general, marsupials have a lower metabolic rate and smaller brain size than
placentals. This led to a speculation that marsupials were biologically inferior to
placentals, and have only managed to survive in the isolation of Australia and New
Guinea because placentals were absent. There was some evidence for that position -
when placentals were introduced to Australia with aboriginal man (e.g., the dog or
dingo), native marsupials did not fair well and many carnivorous marsupials went
extinct except on the isolated island of Tasmania (e.g., the Tasmanian Devil). Then
again, when European settlers introduced other placentals (rabbits, mice, foxes, sheep,
cats, etc.) marsupials declined again. Furthermore, South America supported a huge
fauna of marsupials (some were spectacular like the saber-toothed "cat"). When South
America collided with North America, placental mammals from the North invaded and
wiped out the marsupials (except for the possum - which, of course, migrated the other
direction and invaded North America).
But recently, Mike Archer (U. of Sydney) has discovered fossil placental mammal teeth in Australia. So at least once in history, marsupials apparently outcompeted placentals and took over the continent of Australia.
Placental Mammals
Placental mammals (nearly 4000 species) include such diverse forms as whales,
bats, elephants, shrews, and armadillos. They are also some of the most familiar
organisms such as dogs and cats, as well as many farm and work animals, such as
sheep, cattle, and horses. And humans, of course, are also placental mammals.
Placental mammals all bear live young, which are nourished before birth in the
mother's uterus through a specialized embryonic organ attached to the uterus wall, the
placenta. The placenta is derived from the same membranes that surround the embryos
in the amniote eggs of reptiles, birds, and monotreme mammals. The term "placental
mammals" is somewhat of a misnomer because marsupials also have placentae. The
difference is that the placenta of marsupials is temporary and does not make as much of
a contribution to fetal nourishment as it does in placental mammals.
It seems that the placental mammals took several million years to evolve into even
moderately large body sizes. One reason may have been the dense forests - large
animals would have difficulty moving about, whereas small tree-dwelling animals
would have been favored.
Eocene
Began 55 million years ago and lasted for 16 million years. Climate grew
noticeably warmer - with the tropics reaching from Britain to the equator and
temperate climates further north. South America, Africa, North America, Greenland,
New Zealand and Eurasia were all distinct. Australia, and Antarctica remained
connected.
The most marked feature of the Eocene is the origin and radiation of the modern mammalian orders.
