Cranial (skull) features
Mammalian characteristics:

Lizards & snakes
Crocodiles
Dinosaurs & birds
Mammals & reptile-like mammals
Turtles
Anapsida Diapsida
Amphibians
Saurapsida Synapsida -ca. 320 mya
Amniota -evolution of cleidoic (shelled) egg; ca. 350 mya (late Paleozoic)

Anapsid
-apsid conditions
Synapsid Diapsid
Pelycosaur
Modern mammal

Double occipital condyle: greater range of motion

Pelycosaur
Single bone in mandible: dentary

Dentary-squamosal articulation post-dentary bones

Reptile:
(incus)
(malleus) stapes
Mammal: stapes incus malleus
3 inner ear ossicles stapes
(incus)
(stapes)
(malleus)

Tympanic bone (="annulus" or "tympanic ring") tenrec skull; no auditory bulla

Mammalian characteristics:
Dental features

Diphyodonty (vs. polyphyodonty)

Matched shear surface on upper & lower teeth

Complex occlusal surface on cheeckteeth

Mammalian characteristics:
Postcranial (skeletal) features

Modified limbs and pelvic girdle
Reptile-like femur pelvic girdle fibula calcaneus tibia astragalus
Sprawling posture
Lateral gait
Poorly developed calcaneus femur
Mammal-like fibula tibia astragalus calcaneus
Upright posture
Fore-aft gait
Well-developed calcaneus

Indeterminite growth vs. Determinite growth
Long-bone epiphyses

Stabilized and regionally specialized vertebral column

Dorsal-ventral flexion
Atlas
Rotation
Axis
Atlas

Simplified skeleton

Mammalian characteristics:
Soft anatomy, sensory, and physiological features

Hair (=fur or pelage)

Mammal hair: invagination Bird feather: evagination

cuticle
Hair structure cortex: pigment medulla: dead cells containing air; variably present.

New PNAS article on origin of hair: PNAS November 25, 2008 vol. 105 no. 47 18419

Hair function: tactile perception (vibrissae)

Hair function: locomotory specializations

Hair function: defense

Hair function: aggression and display

Hair function: communication

Skin glands
Mammary glands (=mammae), structural variation
Monotreme Nipples
(most mammals)
Teat
(e.g., cows)

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

Did these all appear simultaneously????
How do we know???

Fossil evidence?
Occipital condyles
Dentary, ear ossicles
Dentition
Upright posture
Determinite growth/ephiphyses
Simplification of skeleton
Vertebral column
Mammary glands
4-chambered heart
Enucleated red blood cells
Diaphragm
Endothermy/homeothermy
Olfaction
Hair

Fossil evidence?
Occipital condyles
Dentary, ear ossicles
Dentition
Upright posture
Determinite growth/ephiphyses
Simplification of skeleton
Vertebral column
Mammary glands
4-chambered heart
Enucleated red blood cells
Diaphragm
Endothermy/homeothermy(??)
Olfaction (??)
Hair