Mammals Flashcards
Cenozoic Radiation of Mammals
- Shrew-like, nocturnal
Radiated after the K-T extinction - 66 mya (end of Cretaceous)
- Massive asteroid
- Only <25 kg tetrapods survived (except sea turtles and crocodiles)
- New habitats
>5400 living spp. - Many characteristics made them so successful!
Mammal Evolution
- Evolution of mammals from earliest amniote ancestors is well documented
Over last 150 my: - From small, ectothermic, hairless ancestors… to..
- endothermic, furry mammals
Skull structures, especially teeth, provide abundant evidence of evolutionary descent
Cynodont
The reptile-mammal connection (260 mya)
- Laid eggs
- More mammal-like skull
- Secondary Plate
Characteristics of Mammals
- Hair/fur/bristles/wool
- Integumentary glands: Sweat glands, sebaceous glands, scent glands, mammary glands
- Skull and Teeth
Hair/Fur/Bristles/Wool
Hair Follicle:
- epidermal structure
- found in dermis of skin
A hair grows continuously by rapid proliferation of cell in the follicle
Cells in hair shaft are shifted upward away from their source of nourishment, accumulate keratin, and die
Numerous modifications for diverse uses
- Primarily thermal insulation
Eccrine Sweat Glands
- Controlled by sympathetic nervous system
- Regulates body temperature by secreting watery fluid that draws heat away from the skin surface
- Water with some electrolytes (e.g., salt) derived from blood plasma
Apocrine Sweat Glands
- Open into hair follicles
- Secrete a fatty sweat
- Do not function in temperature regulation and are correlated with reproductive & chemical communication functions
- Emotional stress, hormonal changes (e.g., human puberty)
Sebaceous Glands
- Most associated with hair follicles
- Glandular cells produce an oily secretion, sebum
- lubricates skin and hair
Scent Glands
- Modified from sebaceous or apocrine glands
- Allow for communication: mark territory, warning and defense signals
Mammary Glands
- Modified apocrine glands
- Rudimentary in males and occur on all females
- Females lactate/produce milk to feed young
- Epidermis thickens to form a milk line along each side of the abdomen -> nipples/teats
- Monotremes (echidnas and platypuses) lack nipples; lap milk from hairs
- Skull and Teeth
Diphyodonts
- Teeth replaced only once from milk teeth to permanent teeth
Bony secondary palate
Single lower jaw bone (dentary bone)
Heterodont teeth
- differentiation of teeth
- vs homodont (uniform) teeth in reptiles
Structure of teeth reveal mammal lifestyle
Reproduction
- Mating seasons: often coincide with most favourable time to give birth and rear young
- Females usually restrict mating to a fertile period during the estrus (also commonly called heat) cycle
- Mostly viviparous, some oviparous (e.g., platypuses, echidnas)
Human Importance of Mammals
- Domestication
- Biomedical research
- 783 spp. endangered or critically endangered (IUCN Red List, Dec 2023)
- 27% of all assessed endangered spp.
Divergence of Mammals
- Monotremes
- Marsupials
- Placentals
-All 3 groups of living mammals derived from the same lineage
- early synapsid (“pelycosaur”) -> early therapsid -> early cynodont -> early therian
- Divergence most obvious in their reproductive differences
Class Mammalia -> Subclass Prototheria -> Order Monotremata (Monotremes)
Monotremes
- “one + hole” (cloaca)
- 4 spp. of Echidna
- 1 sp. of duck-billed Platypus
Oviparous
- lay eggs with one breeding season per year
- After hatching, young nourished by milk lapped off mother’s skin/hair near mammary glands (no nipplets/treats)
- Australia and New Guinea
- Lineage mostly extinct!
Class Mammalia -> Subclass Theria -> Clade Metatheria (Marsupials)
- “resembling pouch”
>330 spp. - Pouched, viviparous
- Two major divisions:
1. Americas
2. Australia, New Guinea
Marsupials Development
- Embryo is first encapsulated by shell membrane & floats free in uterus for several days bf attaching
- Yolk sac placenta (yolk + chorion) to nourish embryo less complex than Placentals
- Brief gestation & give birth to tiny embryonic (altricial) young
- Front pouch contains multiple nipples
Class Mammalia -> Subclass Theria -> Clade Eutheria (Placentals)
- “true + beast”
- 21 orders
Examples:
1. Rodentia >2200+ spp.
2. Chiroptera; >1400 spp.
3. Primates; ~500 spp.
4. Carnivora; > 290 spp.
5. Artiodactyla; > 280 spp.
Placental Pelvis
- Wider pelvic opening
- Loss of epipubic bones
The Eutherian Placenta
Fetal membranes:
- Yolk sac
- Chorion -> Chorioallentoic
- Allantois -> Placenta
- Amnion
Limbs - Walking Types
Plantigrade: Walking with feet flat on the ground (primates, carnivores (bears, racoons..), rodents..)
Digitigrade: Walking on toes (raised heel/wrist) (cats, dogs)
Unguligrade: Walking on nail (digits and remainder of foot raised) (Horse, deer, antelope..)
Marine Mammals
Return to the ocean were separate events
- Order Sirenia (Manatees and dugongs)
- Order Artiodactyla (Whales, dolphins)
- Order Carnivora (Walruses, seals, sea lions)
Primate Evolution
- From small, tree-shrew-like nocturnal animal
Mostly arboreal
- long arms w. large range of motion
- Acute binocular vision
- Large cerebral cortex
Synapomorphies: - Opposable thumbs/big toes
- Nails instead of claws
The First Humans (Hominins) & Bipedalism
- Foreman magnum under braincase = bipedalism
- less robust jaws = omnivores
- At least 22 spp.
- Homo sapiens (us)
~ 200 000 - 300 000 yrs - only extant humans (hominins)
