Anapsida Diapsida

SynapsidaSaurapsida

Turtles

Lizards & snakesCrocodilesDinosaurs & birds

Mammals &reptile-like mammals

Amniota -evolution of cleidoic (shelled) egg; ca. 350 mya (late Paleozoic)

Amphibians

-ca. 320 mya

-apsid conditions

Anapsid Synapsid Diapsid

PelycosaurModern mammal

Double occipitalcondyle:

greater range of motion

Pelycosaur

Therapsid

s

Single bone in mandible: dentary

post-dentary bones

Dentary-squamosal articulation

Reptile:

Mammal:

stapes

stapes

stapes

(incus)

incusmalleus

(malleus)

(stapes)(incus)

(malleus)

3 inner ear ossicles

Tympanic bone (="annulus" or "tympanic ring")

tenrec skull; no auditory bulla

Heterodont dentition

Diphyodonty (vs. polyphyodont)

Matched shear surface on upper & lower teeth

Complex occlusal surface on cheeckteeth

Secondary palate

Modified limbs and pelvic girdle

Reptile-like Mammal-like

Sprawling posture Upright postureLateral gait Fore-aft gaitPoorly developed calcaneus Well-developed calcaneus

calcaneus

fibula

femur

tibia

pelvicgirdle

calcaneus

fibulatibia

femur

astragalusastragalus

Indeterminite growth vs. Determinite growth

Long-bone epiphyses

Stabilized and regionally specialized vertebral column

AtlasAtlas Axis

Dorsal-ventralflexion Rotation

Simplified skeleton

Hair (=fur or pelage)

Mammal hair:invagination Bird feather: evagination

cuticle

cortex: pigment

medulla: dead cells containing air; variably present.

Hair structure

Hair function: tactile perception (vibrissae)

Hair function: locomotory specializations

Mammary glands (=mammae), structural variation

Monotreme Nipples(most mammals)

Teat(e.g., cows)

Skin glands

4-chambered heart

Mammal

Crocodyle

Enucleated blood cells

Muscular diaphragm

Improved olfaction

Endothermy---Body temperature regulated internally.

Homeothermy---Body temperature maintained within a narrow range.

An integrated package?

•Natural selection favored increasingly efficient use of energy

•Endothermy facilitated nocturnal activity

•Endothermy is especially difficult for young

•Diphyodonty, lactation, social behavior

Occipital condylesDentary, ear ossiclesDentitionUpright postureDeterminite growth/ephiphysesSimplification of skeletonVertebral columnMammary glands4-chambered heartEnucleated red blood cellsDiaphragmEndothermy/homeothermyOlfactionHair

Fossil evidence?