Placental Mammals

Young are born in a developed state
- able to move about
- able to feed on their own with less support from the parents
Longer period of gestation

Placental vs. Marsupial

Offspring are the result of combined genetic information from both parents
Normally, foreign tissue is rejected by the mother's immune system
In placental mammals, an embryonic tissue (the placenta) forms a barrier between fetus and mother
This may explain why marsupials give birth so quickly after conception
Marsupials do not complete well with placentals when living in the same environment
The reason for this may be due to offspring rearing
In terms of metabolic energy used to raise an offspring, placental mammals use less energy
- i.e. they are more energy efficient

Placental Mammals

Placental mammals first appeared in the Cretaceous
90% of all fossil and living mammals are placentals
First placental mammal was a shrew-like animal

Paleocene Diversification

Major adaptive radiation of mammals
13 new Orders of mammals first appeared at this time
All tended to be small animals
Large mammals did not appear until the late Paleocene

Eocene Diversification

9 new Orders evolve, 8 of which are still present today (22 Orders at this time)
Extinction of 3 Orders in the late Eocene - Condylarths, Pantodonts, Uintathers

Oligocene Diversification

All present living Orders of mammals had evolved by this time
Extinction of Pyrotheres
Diversification of genera and species continues
Forms familiar today are starting to evolve

Miocene - Pliocene Diversification

Major modifications due to the appearance and spread of grass
Extinction of Astrapotheres

Pleistocene and Holocene Diversification

Modern forms evolve

Carnivores

First evolved during the Paleocene
Small with short, heavy limbs
Specialized teeth for eating meat
- carnassials - "shearing teeth" - canines - "fangs"
Miacids - ancestor to all present member of Order Carnivora
Felines, canines, bears, weasels, pandas, seals, sea lions, and walruses

Saber-tooth carnivores

Canines developed to lengths of 15 cm
Modification to jaw structure
Saber-tooth evolved independently 4 times during the Cenozoic
- twice in felines
- once in an extinct order of mammals
- once in marsupials

Ungulates

Hoofed animals
Two groups - based on number of toes
Artiodactyls
- even-toed, 2 or 4 toes
- most common ungulates Perissodactyls
- odd-toed, 1, 3 or 5 toes
- only 16 living species of horses, rhinos and tapirs

Herbivores

Dental and jaw structure designed to grind vegetation
Premolars and molars - grinding teeth
Ungulates show a trend in the premolars called molarization
- enlargement of the premolars
- larger grinding surface
Ungulates can also be divided into two groups based on diet
Grazers
- grass eaters
- high-crowned teeth (these are long teeth as measured from root to crown)
Browsers
- tender shoots, twigs and leaf eaters

Artiodactyls - Even-Toed Hoofed Mammals

First appeared in the early Eocene
Initially small - rabbit-sized
Pigs, hippos, deer and giraffes

Artiodactyls - Camels

Two- and four-toed types evloved - at present only two-toed remain
Evolved in North America
Became extinct in N.A. during the Pleistocene

Artiodactyls - Bovids

Most diverse group of Artiodactyls
Cattle, bison, sheep, goats, antelopes and pronghorns
Evolved during the Miocene
- grass eaters

Perissodactyls - Odd-Toed Hoofed Mammals

Horses, rhinos and tapirs
Evolved during the Eocene
Increased in diversity through the Oligocene but have declined since that time
Decline may be related to diet - grazers
Digestive system is less efficient at digesting grass than the Artiodactyls

Pleistocene Mammals

Giant mammals species became common
Mastodons, mammoths, giant bison, huge ground sloths, giant camels, giant beavers, giant kangaroos and the Irish elk
Larger size may have been in response to cooler temperature during Pleistocene glaciation

Pleistocene Large Mammal Extinction

10,000 year ago nearly all large terrestrial mammals of North and South America and Australia became extinct
Unusual extinction in that it only affected large mammals
Two main theories for this extinction

Climatic Change

Rapid climatic change at the end of the most recent glacial maximum - warmer temperatures
Changing vegetation
Problems:
Why didn't the large mammals migrate to cooler climates?
Why isn't there similar extinctions during prior interglacial periods?

Prehistoric Overkill Theory

This mass extinction coincides with the arrival of humans in North America, South America and Australia
Hunting practices killed off the large mammals
Mammals in these areas were unfamiliar with humans and had not developed natural defenses against them
Problems:
Early human migrations were in small groups
Could they have killed off large populations? Hunters in primitive societies tend to hunt small, abundant and less dangerous animals
Large bones are scarce in archeological sites while small bones are common
Few human artifacts have been found with large mammal fossil remains

Holocene Extinction

Estimated 18,000 species lost every year
Humans and human activity is the cause
- destruction of natural habitat
- agriculture, mining, construction

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