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.
7. A mandible composed of only one bone (the dentary) that articulates with the
squamosal bone of the skull.
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 can not 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.
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.
Platypus:
The duckbill platypus is one of the oddest looking mammals alive today. It lives
in burrows on the banks of streams and has webbed feet to aid in paddling through the
water. Its fleshy nose is used to search out earthworms in the bottom of stream beds.
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.
1. Pholidota (the pangolins)
Pangolins feed on burrowing social insects, such as ants and termites. Theys have stout,
strong, clawed limbs, used for digging into ant and termite mounds. The tongue of a
pangolin can be extended about 25 cm, and has muscular roots that attach to the
animal's pelvis! Pangolins completely lack teeth, and the lower jaw is reduced to a small
bladelike bone. Perhaps the most obvious feature of pangolins is their scaly epidermal
armor, making them look a bit like pine cones with legs.
The relationship of pangolins to other mammals is uncertain.
2. Edentata
Edentata, an order of mammals, includes 31 living species of armadillos, anteaters, and
tree sloths
The entire evolutionary history of the edentates is restricted to the Western
Hemisphere and the majority of the living species occur today in South America.
Edentata means "lacking teeth", though in reality only the true anteaters are toothless. The majority of edentates have simple, peglike cheek teeth that lack enamel. Edentates possess specialized traits, such as reduced dentition, a long sticky tongue and powerful, clawed forefeet. They also have small, uncomplicated brains, suggests they may be relatively primitive mammals.
3. Insectivores (moles, hedgehogs, shrews)
All insectivores are relatively small animals with long narrow snouts. Most species in
the order are nocturnal, solitary and feed mainly on invertebrates, especially insects.
[NOTE: Insects are marvelously diverse in habit and morphology. They provide a rich
resource for anything that eats them, and their diversity alone suggests that their
predators might exhibit similar variety. And so it is not surprising that their are many
different species of insectivores.]
4. Chiroptera (bats)
Bats, the only true flying mammal, are thought to have evolved more than 50
million years ago during the Eocene period, from an insectivorous ancestor. The
scientific name for this mammalian order is Chiroptera meaning "hand-wing". This
name describes the fact that a bat's hand is modified into a wing. Each wing is
composed of an elongated forearm and, except for the thumb, extremely long fingers
sandwiched between two thin sheets of skin. The diminuative thumb is left free and is
used to help the animal climb about on its perch. Elastic webbing connects the finges to
one another and then to the body, form a broad wing surface. A similar membrane
spreads between the legs and tail, completeing an air foil that surounds the entire body.
5. Dermoptera (colugos)
The order Dermoptera includes only two living species commonly known as the colugo.
The colugo, was once called the flying lemur, but it is not a lemur and does not fly but
glides on flaps of skin stretched from the side of the animals neck to the tip of the fingers
an and toes and continues to the tip of the tail. Colugos can make controlled glides of 70
meters or more using this kite-like feature.
Colugos are herbivores; they have teeth unlike any other mammal - ll of the incisors
are comb-like, with as many as 20 comb tines coming from one root. The exact function
of the combs is not known. Colugo's diet consists mostly of leaves , shoots, buds, and
some soft fruits.
6. Primates (lemurs, monkeys, apes)
The Primates are an ancient and diverse group, currently with around 233 living species.
Most dwell in tropical forests. The smallest living primate is the pygmy marmoset,
which weighs around 70 g; the largest is the gorilla, weighing up to around 175 kg.
Primates radiated in arboreal habitats, and many of the characteristics by which we recognize them today (shortened face - no snout - and forwardly directed eyes, associated with stereoscopic vision; opposable thumb and big toe; unfused and highly mobile radius and ulna in the forelimb and tibia and fibula in the hind, nails instead of claws) probably arose as adaptations for life in the trees. Several species, including our own, have left the trees for life on the ground; nevertheless, we retain many of these features.
7. Carnivores
The most primitive group of true carnivores are the dogs (wolves, dogs, coyotyes, etc):
(2) The racoons, weasels (includes otters, minks, skunks, etc), pandas and bears - which
have flat feet:
8. Hoofed Mammals (Ungulates)
Hoofed mammals they walk on the terminal bones of the toes and have enlarged
toenails forming hoofs. This feature appeasrs to have evolved as an adaptation for life
on open grasslands. There are two major types--
| Artiodactyla, or even-toed | Perissodactyla, or odd-toed |
(b). Unlike artiodactyls, perissodactyls either walk on three toes (like rhinos, tapirs -see picture below, many extinct horses, and other extinct groups) or on a single toes (like recent horses).
This order flourished in the early Cenozoic period, with over 11 different families spread over the globe. However, the diversity of odd-toed ungulates has gradually been lost up to the present, with the living species mere remnants of a bygone evolutionary marvel. Most perissodactyl lineages (including some of the very large forms) went extinct in the late Eocene or Oligocene.
8. Whales, Dolphins, and Porpoises
Cetaceans, along with bats, are considered some of the most derived mammals on the
planet. They evolved from terrestrial animals (probably a hoofed mammal) to an
entirely aquatic life form that is completely separated from the land in all aspects of
biology. Cetaceans live, breed, rest, and carry out all of their life functions in the water.
All cetaceans share a number of similarities: they have a fusiform, or streamlined body shape; paddle shaped front limbs for steering; vestigial hind limbs (which are within the body wall); no external digits or claws; tail flattened laterally and bearing horizontal flukes constructed of cartilage; basically hairless body (a few hairs arround the blow hole and on the chin); thick subcutaneous blubber layer filled with fat and oil; nostrils (blowhole) on the top of the head; and an airway reinforced with cartilage down to the alveoli (small passageways in the lungs). Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment.
There are two main types of cetaceans: The toothed cetaceans (include dolphins, porpoises, killer whales, and narwhales) and the baleen cetaceans (grey whales, sperm whales, humpback whales).
9. Aardvaks
Aardvarks are pig-sized mammals (up to 82 kg) that specialize in insectivory, especially
in capturing and consuming termites. Their limbs are modified for digging into the very
hard termite mounds found in African savannahs. The nails are actually somewhere
between true nail and hoof in form. They are strongly constructed, shovel-like, and
obviously adapted for digging. Aardvark skin is thick and sparsely haired. The thickness
of the skin protects these animals from biting ants, and aardvarks may sleep in the ant
nests they have recently excavated for feeding.
10. Elephants and Relatives
There are only two species of elephants alive today: the Indian elephant (Elephas
maximus) and the African elephant (Loxodonta africana). In the past, however, a diversity
of unusual elephant relatives traversed areas around the world. The most famous of
these extinct relatives are the mammoth and mastodon. All have ighly modified
grinding teeth and large body sizes.
Proboscideans evolved and diversified in Africa but emigrated to most other continents, becoming a widespread and important group until the extinction of most species in the Pleistocene.
In addition to their unique morphology, modern elephants are unusual among
mammals in two key respects of their social structure. Firstly, they have a matriarchal
social structure, in which herds consist of related females led by the oldest among them.
Males tend to be solitary, but may come together in small herds.
Secondly, baby elephants are given care and guidance for several years -- longer than
most other mammals. During this time members of the herd may care for the babies,
regardless of which is the mother.
The hyraxes and manatees are the elephant's closest relatives.
Hyraxes, also known as dassies or conies, are jack-rabbit sized animals with short tails
and peculiar, 3-toed hind feet with almost hoof-like nails on two of the toes (the inner
toe has a claw). The forefeet have 5 toes. The soles of their fore- and hindfeet, which are
moistened by special sweat glands, are remarkably soft and elastic, which works to
increase their friction against the substrate. They have specialized muscles in the soles of
the feet that help them to work almost like a suction cup.
11. Manatees or Sirenians, which are sometimes called sea cows, are large mammals
that spend their entire lives in water. Their forelimbs are modified to form flippers, their
hindlimbs are reduced to nothing more than a vestigial pelvis, and their tail is enlarged
and flattended horizontally to form a fluke or paddle.
12. Rodents
Rodents are one of the largest groups of mammals. Most people are familiar
with mice, rats, squirrels, and guinea pigs, but there are many other interesting rodents
as well. Rodents are often quite small (in the mouse to rat size range), but others are
quite big. The capybarais the largest living rodent. It is about the size of a pig and lives
along rivers in the llanos of South America.
Despite their morphological and ecological diversity, all rodents share one characteristic: their dentition is highly specialized for gnawing. All rodents have a single pair of upper and a single pair of lower incisors which are rootless, growing continuously.
During gnawing, as the incisors grind against each other, they wear away the softer dentine, leaving the enamel edge as the blade of a chisel. This "self sharpening" system is very effective and is one of the keys to the enormous success of rodents.
13. Lagomorphs (hares, rabbits, and pikas)
Currently, we recognize 80 living species of lagomorphs, placed in 13 genera distributed among 2 families. Native populations are found on all continents except Australia, southern South America, and Antarctica; they are absent from most islands. Humans have introduced them, however, to many areas where they were originally not part of the fauna. They occupy a wide diversity of habitats, ranging from tropical forest to arctic tundra. All are herbivores that feed on grasses and other small plants.
Lagomorphs are small to medium-sized animals that appear to be closely related to
rodents. They have a rudimentary or short tail. Lagomorphs have a pair of
incisors in each quadrant of the upper jaw, one large and rodent-like, and the other a
small peg located immediately behind the larger tooth. These teeth grow throughout
the animal's life and have a layer of enamelthat extends around to the posterior surface
of the tooth (in contrast to rodent incisors, which have enamel on
one face only.
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.