The Origin and Early Evolution of the Mammals Class Mammalia: ~4800 extant species (extremely diverse in size, form,

and lifestyle); exhibit hair, mammary and sweat glands, three middle-ear bones and fleshy external ears, skull with two occipital condyles, secondary palate (allows breathing during suckling), turbinate bones in nose (hold mucus membranes that retain heat), fused pelvic bones, muscular diaphragm, teeth diphyodont and heterodont; enlarged cerebral cortex

Early Evolution of Mammals: evolved from therapsids (Paleozoic synapsids) Cynodonts: the only therapsids to survive the Permian Extinction; exhibited erect

gait with ventral versus lateral limbs, increased jaw musculature, heterodont teeth, turbinate bones, secondary palate, loss of lumbar ribs (related to the evolution of the diaphragm)

Earliest true mammals (Triassic fossils) mouse- or shrew-sized, with diphyodont teeth, three middle-ear bones (stapes homologous to columella,

malleus and incus derived from articular and quadrate, respectively mammalian jaw joint between dentary and squamosal bones); likely laid amniotic eggs and nursed immature young (like extant monotremes)

Early mammals existed 150 million years with little diversification (likely nocturnal); greatest radiation of the mammals not until early Cenozoic (after the extinction of the dinosaurs; likely related to newly available niches)

Fig. 28.2

Fig. 28.3

Fig. 28.4 and

Fig. 28.5

Class Mammalia – Diversity and Taxonomy Subclass Prototheria, Order Monotremata (Monotremes): lay an amniotic egg,

examples include platypus (with electroreceptive bill and venomous spine) and echidnas of Australia and New Guinea

Subclass Theria Infraclass Metatheria (Marsupials): immature young nursed in abdominal

pouch (marsupium); include American opossums, possums, koalas, wombats, wallabies, kangaroos, and Tasmanian devils (mostly Australian region)

Infraclass Eutheria (Placental Mammals): 18 orders (12 shown below)Insectivora: shrews, hedgehogs, moles; 440 species

Rodentia: squirrels, woodchucks, rats, mice, beavers, porcupines; 2052 species

Chiroptera: bats; 977 species Sirenia: sea cows and manatees; 4 species

Primates: prosimians, monkeys, apes, humans; 279 species

Perrisodactyla: odd-toed hoofs (horses, asses, zebras, tapirs, rhinos); 17 species

Xenarthra: anteaters, armadillos, sloths; 29 species

Artiodactyla: even-toed hoofs (swine, camels, deer, hippos, giraffes, cattle, sheep); 221 species

Proboscidea: elephants; 2 species Carnivora: dogs, wolves, cats, bears, weasels, pinnipeds (seals and sea lions); 280 species

Lagomorpha: rabbits, hares, pikas; 81 species

Cetacea: whales, dolphins, porpoises; 78 species

Fig. 28.41

Mammals – Integument, Teeth, and Feeding Integument and Derivatives

Hair: composed of dead, keratin-packed epidermal cells; follicles of epidermal origin, but extend into dermis; underhair for insulation, guard hairs protect against wear and provide coloration of pelage (fur coat); hair continuously replaced, but most mammals molt entire coat periodically (summer coats often brown, thin vs. white, thick winter coats); vibrissae: sensory hairs on snout (whiskers); quills modified hairs for defense (ex. porcupines)

Horns and Antlers: true horns (keratin) with bony core, not shed, grow continuously (ex. sheep and cattle); antlers (deer family) branched and composed of bone covered by soft skin (velvet), shed after breeding season; rhino horns not attached to skull, composed of cemented filaments of keratin

Glands: sweat glands cool body (eccrine glands) and signal reproductive stage (apocrine glands); scent glands secrete pheromones used to

attract opposite sex and mark territories; sebaceous glands secrete sebum, which lubricates hair; mammary glands secrete milk via nipples (excl. monotremes)

Teeth and Feeding: heterodont teeth (incisors, canines, premolars, molars) Specializations: insectivores; herbivores (grazers, gnawers) feed continuously,

with long digestive tracts and fermentation chambers (endosymbiotic bacteria digest cellulose); carnivores with shorter GI tract, piercing and slicing teeth (incl. carnassial teeth); omnivores include bears, pigs, raccoons, primates; rodents store food in caches; rabbits coprophagous

Metabolic rates decline with increasing body size

Fig. 28.6

Fig. 28.10

Fig. 28.11

Fig. 28.12

Fig. 28.15

Mammals – Reproduction, Behavioral Ecology, Populations, and Endangered Species

Reproduction: female fertility limited by estrous cycles; monotremes lay eggs in a burrow, young drink milk after hatching; marsupials with short gestation, embryonic diapause, and development of young in external pouch (marsupium); placentals with long gestation, fecundity highly variable (usually higher for small mammals)

Territory and Home Range: territories (defended home ranges) commonly marked by scents; home ranges typically foraging areas (can overlap); social mammals include prairie dogs

Migration: more rare than in birds and fishes; caribou winter in taiga, calve in tundra; cetaceans and pinnipeds with longest migrations (ex. gray whales migrate between Alaska and calving lagoons in Baja California)

Flight and Echolocation: bats only true flying mammals (gliding in some rodents and marsupials); bats and toothed whales use

echolocation to find food and navigate; bats collect echoes with ears, toothed whales create sound in sinuses, collect echoes with lower jaws

Mammalian Populations and Endangered Species: small mammals often with highly fluctuating populations; top predators with naturally low populations; in 2006, 510 mammals listed as endangered (imminent risk of extinction), including most cetaceans (baiji recently declared extinct), wild cats, and non-human primates, plus many more considered threatened (forseeable risk of extinction)

Fig. 28.22

Fig. 28.23

Fig. 28.25

Fig. 28.17

Fig. 28.18

Fig. 28.20

Fig. 28.28