Evolution of Mammals

Question 1

Synapsida

Answer 1

One pair of temporal openings

First amniotes lineage to radiate extensively into variety of terrestrial habitats

Question 2

“Pelycosauria”

Answer 2

Most basal synapsids

Adaptive radiation (carnivores/herbivores)

Heterodonty: Beginning of tooth type differentiation (i.e. caniniform teeth in derived carnivorous pelycosaurs)

Extinct by end of PALEOZOIC

Paraphyletic: Therapsids emerged from a carnivorous pelycosaur lineage

Question 3

Therapsida

Answer 3

Persisted into Mesozoic

Gait with limbs under body (SYNAPOMORPHY)

Adaptive radiation (carnivores/herbivores)

Cynodonts emerged from a therapsid lineage

Question 4

Cynodonta

Answer 4

Persisted deep into Mesozoic

Turbinate bones (SYNAPOMORPHY)
-Suggests endothermy (putative synapomorphy)

Greater degree of heterodonty

Changes in jaw musculoskeletal structure (SYNAPOMORPHY)

• Enlargemet of dentary and reduction of posterior jaw bones
• Evolution of masseter muscles

Mammal lineage emerged from a cynodont lineage

Question 5

Mammalia: SYNAPOMORPHIES

Answer 5

Mammary and other skin glands

hair (derived from reptile scales)

Diphyodont dentition: Teeth replaced only one time in life

New jaw joint between squamosal and dentary (quadrate and articular incorporated into middle ear)

Question 6

“Early mammals”

Answer 6

Arose >200 mya (Triassic)

Had hair: Likely endothermic

Secondary palate (shared with cynodonts)
-Separates feeding and breathing pathways to permit suckling young

Question 7

Mammals and dinosaurs coexisted for _____-

Answer 7

~140 mya

• Mammals smaller than most dinosaurs
• Few lineages of mammals

Question 8

Extinction of dinosaurs occurred _______

Answer 8

~65 mya (early Cenozoic)

Mammals diversified into many lineages and many achieved large body size

Question 9

Jaw joint and ear evolution

Answer 9

Reduction, loss, specialization, and new roles for old parts

Quadrate -> Incus
Articular -> Malleus

Question 10

Integument of mammals

Answer 10

Distinctive and tends to be thick compared to that of other vertebrates

Question 11

Derivatives of Integument

Answer 11

Horns and antlers

Question 12

Horns

Answer 12

Bovids (e.g. cattle, goats, antelope, etc.)

Bony core covered by keratinized sheath

Usually in both sexes

Unbranched

Not shed

Question 13

Antlers

Answer 13

Cervids (deer)

Solid bone

Usually only in males

Branched

Shed annually
-Big energetic investment for sexually selected trait

Question 14

Functions for horns and antlers

Answer 14

Intrasexual competition between males for access to females (original function) and protection against predators (secondary function)

Question 15

Irish elk

Answer 15

Actually a deer

2.1m (6.9ft) tall at shoulders

Largest antlers of any known cervid: Up to 3.65m (12ft) across and 40kg (88lb)
-Antler size result of sexual selection and positive allometry (antlers relatively larger in larger species)

Question 16

Hair

Answer 16

Unique mammalian structure

Dead cells impregnated with keratin

Epidermal structure (hair follicles producing hairs sunk into dermis)

Question 17

Derivatives of Hair

Answer 17

Vibrissae

• “Whiskers” in many mammals
• Tactile sensory structures (mechanoreception)
• Analogy in some birds? yes, used for catching insects

Spiny armor

• Echidnas, procupines, and hedgehogs evolved spines derived from hair (convergence/analogy)
• Protection against predators

Question 18

Mammal jaw function

Answer 18

Evolution of zygomatic arch and masseter muscle (cynodonts)

Masseter muscles insert on lateral aspect of dentary

Pterygoid muscles insert on medial aspect of dentary

Forms a muscle sling that allows side-to-side movements of lower jaw - more complex chewing and slicing motions

Question 19

Glands

Answer 19

Mammals have greatest variety of glands among vertebrates (humans have all major glands)

Sweat glands
-Evaporative cooling

Scent glands

• Communication (attracting mates), territorial marking, defense
• Various locations among species: eyes, cheeks, between digits, genitals, anus, and more
• Used for defense in skunks

Sebaceous glands
-Expel sebum to keep hair and skin pliable and glossy

Mammary glands (i.e. Mammalia)

• Possessed by females (rudiments in males)
• Derived from sweat glands (homologous)
• Mile secreted from glandular tissue to feed young
• Humans: adipose tissue around glandular tissue forms breasts (unique and plays role as visual signal of sexual maturity/fertility)

Question 20

Mammals: Prototheria

Answer 20

Monotremes

1 order

3 species in 1 order: echidna (2) and platypus (1)

Oviparous: lay and incubate eggs

Young fed with milk from mammary glands

Found in Australia:

• Platypus
• -Venomous spurs on hind limbs (males only): used in competition for females
• -Electroreception for detecting aquatic prey
• Echidnas
• -Protective spines derived from hair
• -4-headed penis: uses 2 heads on one side during mating and switched sides with each mating

Question 21

Mammals: Metatheria

Answer 21

Marsupials

334 species 7 order: Australia (>200), South/Central America (>100), and North America (1)
-Kangaroos, wallabies, opossums, marsupial mice, Tasmanian devils, Tasmanian wolves, bandicoots, wombats, and others

Viviparous with ALTRICIAL young

Evolved most ecological types observed in eutherians except powered flight (some glide) and marine habit

Females: Vagina with 3 canals (2 lateral and 1 medial)

Males: Forked penis fits into lateral vaginal canals

Sperm travel up lateral vaginal canals and young pass through medial vaginal canal

young born (~1g), climb to teat in pouch using strong forelimbs, and have extended development while nursing

Question 22

Mammals: Eutheria

Answer 22

Placentals

Viviparous with PRECOCIAL young

Tremendous variety of forms in almost every imaginable habitat

Huge range of body size: 2g (Etruscan shrew and bumblebee bat) to 190,000kg (blue whale)

Worldwide distribution

> 500 species in 22 orders
-Nearly 1/2 are rodents and nearly 1/4 are bats

Uterus/uteri join single vaginal canal

Connection between fertilized egg and maternal uterine tissue form placenta
-Site of gas and nutrient exchange

More energy towards in utero development and less towards lactation (compared to metatherians)

Question 23

Whale evolution

Answer 23

Extinct forms

Closest living relative to whales is Hippopotamus (same order as deer, pigs, camels, giraffes, buffalo, cows)