The Mammals
Most influenced by humans
- domestication
- biomedical research
- introductions into new habitats
- most harmed by destruction of habitat
Synapsids
Pelycosaurs - 1st synapsids
Therapsids - only group to survive beyond Paleozoic - mammal like reptiles.
- erect gait
- herbivores and carnivores
- most disappeared during great extinction at end of Permian except
Cynodonts - increased jaw musculature
- secondary palate (bony), so young could suckle and breathe at same time
- long bones more slender and number of ribs reduced
The earliest mammals
- mouse like
- diphyodont teeth -
1st set of teeth =
2nd set of teeth =
- endothermic but with lower body temperature than modern mammals
- young hatched from eggs - immature and used mother's milk for food.
(Like monotremes)
Class Mammalia - 21 orders
1 order =
1 order =
19 orders =
Adaptations of Mammals:
I. Skin and derivatives
A. Hair
skin with outer epidermis and inner dermis
hair is epidermal structure that sinks into dermis
1. underhair -
2. guard hair -
Can be modified for many purposes:
- spines
- bristles
- vibrissae or whiskers
B. Horns and Antlers
1. true horns -
2. antlers -
3. rhinoceros horns -
C. Glands
1. sweat glands
a. eccrine glands
b. apocrine glands
2. scent glands
3. sebaceous glands -
4. mammary glands (modified apocrine glands)
II. Food and Feeding
A. Teeth - all have teeth except monotremes, anteaters, and certain whales.
1. homodont teeth -
2. heterdont teeth -
a. incisors
b. canines
c. premolars
d. molars
Tooth formula = number of tooth types in one half of the upper and lower jaw.
3/3, 1/1, 4/3, 3/3
B. Specializations
1. insectivores -
2. herbivores -
- canines reduced
- molars broad for grinding
- long digestive tract
- harbor anaerobic bacteria and protozoa in fermentation chambers in gut to break down cellulose
Cecum -
Ruminants - four chambered stomach; 1st three chambers from esophagus, only last chamber a true stomach.
Four chambered stomach of ruminants:
a. rumen -
b. reticulum
c. omasum -
d. abomasum -
3. Carnivores -
- biting and piercing teeth
- shorter digestive tract, cecum small or absent
- discrete meals
4. Omnivores -
III. Reproduction
estrous cycle
1. proestrus - period of preparation, new ovarian follicles grow
2. estrus - mating occurs and ovarian follicles burst releasing egg
eggs fertilized, implants
3. If not fertilized, metestrus - period of repair
4. diestrus - reduction in size of uterus.
Cycle repeats itself
Humans and old world monkeys - postovulation = menstruation
A. Monotremes
- 3 genera each with one species: echidnas (spiny anteaters), duck-billed platypus Australia, Tasmania and New Guinea.
- possess some features more related to reptiles than mammals
- skull is bird like and toothless.
- limb structure similar to reptiles - limited running ability; girdles are similar to the therapsids (mammal-like reptiles)
- lay eggs and incubate them, female echidnas temporarily develops a pouch-like structure but not the platypus. Suckle young with mammae (but no nipples)
- long period of parental care
B. Marsupials
- opossum, wombat, wallaby, Tasmanian "wolf" and devil, kangaroo, bandicoot, koala bear and others
- specialize feet, aboreal heritage - opposable 1st digit and enlarged and powerful 4th digit.
- ~ 50% have a marsupium (abdominal pouch). Uterus is bifid (two vagina and two uteri.
- pouched mammals, viviparous
- primitive type of placenta - choriovitelline or yolk sac placenta
- embryo in shell membrane free in uterine fluid
- hatches from shell and erodes shallow depression in uterine wall
- gestation period short - 8-43 days (33 days -red kangaroo), joey borne tiny
- young crawl to pouch, attach to nipple
- mother pregnant again
- while 1st embryo suckles, 2nd embryo is arrested in diapause for about 235 days
- joey leave pouch, 2nd embryo resumes development and born 1 month later
- mother pregnant again. Embryo arrested while 2nd joey in pouch
So, mother has three young: one out of pouch, one in pouch, and one in uterus in diapause.
Marsupials and eutherian mammals diverged from common ancestor in early Cretaceous: Course of evolution suggests the two groups are not of equal adaptive ability:
1. marsupials not equaled funtional adaptive radiation of mammals.
2. marsupials more conservative in structural plan
3. marsupials not exploited great body size
4. marsupials no high social behaviors
5. marsupials not developed systematic diversity of mammals
WHY?
Style of reproduction and general biology - mammals developed territoriality, antipredator behaviors, greater reproductive output...
C. Placental Mammals
eutherians or true mammals. Viviparous
- Prolonged gestation, chorioallantoic placenta
- gestation related to body size. Mice = 21 days, elephants = 22 months
- some born fully furred, some naked
- number young produced in a season depends on mortality rate. Usually smaller the mammal the more young produced.
Human Evolution:
- Darwin stated that humans shared a common ancestor with apes. No fossil evidence just anatomy
- The "missing link" search began - Homo erectus 1891.
- Comparative cytology - chromosomes of humans and apes are homologous.
Humans are Primates:
- grasping fingers on all four limbs
- flat fingernails instead of claws
- forward-pointing eyes
Prosimians (lemurs, tarsiers)
Ancestral primate stock split
Simians (monkeys, apes)
- many of both groups are arboreal
- arboreality - increased intelligence. active movement through trees. Grasping digits - hang from branches, seize food
- highly developed sense organs, good coordination
~ 8 million years ago - grasslands predominated, standing upright was adaptive - search for predators and use hands for feeding.
Large fossil gap
About 4.4 million years ago we see first "near humans"
Australopithecus afarensis - bipedal hominid "Lucy" 40% near complete skeleton.
Between 3 and 4 million years ago - two hominid lines
One lineage was Homo, the other Australopithecine line.
~ 2 mil yrs ago - Homo habilis - fully erect and larger brain than Australopithecines Used bone and stone tools.
~1.5 mil yrs ago - Homo erectus - large (5-5.5 ft). Larger brain than habilis but not the size of modern humans. Social species that lived in tribes; complex culture
~ 300,000 yrs ago - Homo sapiens - Neanderthals (130,000 yrs ago). Good hunters and tool users. Replaced about 30,000 yrs ago by modern humans.