See http://geology.com/time.htm

Monotremes

Eutherians

Metatherians

Monotremes Metatherians (Marsupials) Eutherians (Placentals)

Node - Divergence EventBranch - Common Ancestor

Amphibians Mammals Turtles Squamates Crocodylians Dino1 Birds Dino2

Transition to land

Amnion

Synapsida

Stem Reptiles - Captorhinomorphs

Captorhinomorphs- 350 MYBP (Carboniferous)

Synapsids- 320 MYBP

anapsid

j: jugalp : parietalpo : postorbitalsq : squamosal

synapsid

Synapsida: ‘Together Arch’

"Pelycosaurs"

Therapsids

Early Therapsids

Cynodonts

Dimetrodon

Pelycosaurs

"Pelycosaurs"

Therapsids

Early Therapsids

Cynodonts

Lycaenops

Early Therapsids

"Pelycosaurs"

Therapsids

Early Therapsids

Cynodonts

Cyognathus

Cynodonts*: Advanced Theraspids(*’dog teeth’)

~250 MYA

Mammalian classification (especially fossil-based): The ‘key character’ approach

Dixeya (a late therapsid) Ovis aries

Q-A vs. D-S jaw joint- the defining ‘key character’ for 80 years

~250 MYA

D-S

Q-A

Using a ‘Key Character approach’…Morganucodontids as first mammals?

Probainognathus

Diarthrognathus

Shift to a ‘Suite of Characters’ approach…(Feldhammer et al.)

1) D-S jaw joint

2) Strongly heterodont dentition

Shift to a ‘Suite of Characters’ approach…(Feldhammer et al.)

3) Molars with occlusion, complex surface, wear facets

4) Alternate side chewing

Shift to a ‘Suite of Characters’ approach…(Feldhammer et al.)

5) Well developed inner ear (Petrosal)

Shift to a ‘Suite of Characters’ approach…(Feldhammer et al.)

6) Small

7) Axial skeleton (dorso-ventral flexion, thoracic rib placement)

~250 MYA

And… based on a ‘suite of characters’ approach, where does the mammal/non-mammal division lie?

Both approaches (‘Key character’, ‘Suite of Characters’) are referred to as ‘Grade-based’ definitions

Problems?

•Evolution is a continuum (many transitional fossils)

•Traits evolve at multiple locations on phylogeny

Amphibians Mammals Turtles Squamates Crocodylians Dino1 Birds Dino2

Transition to land

Amnion

Synapsida

Stem Reptiles - Captorhinomorphs

Reptilia

Archosauria

Reptiles- a grade based definition1. Scales2. Lack of feathers3. Lack of hair

Possible common traits of members of Archosauria?

~250 MYA

Our definition… clade-based to the Morganucodontid node

***Mammal-like fossils (e.g. Sinocondon) can be referred to as Mammaliaformes

Reasons behind evolution of Mammalian characters?

•Unknowable- can only be inferred, hypothesized

•Many hypotheses exist… we’ll look at one

The Size-Refugium hypothesis

•Radius = 5 •Surface area = 314 •Volume = 355Size/volume = 0.88

Size: an animals squared dimensionVolume: an animal’s cubed dimension

•Radius = 10•Surface area = 1256•Volume = 2842Size/volume = 0.44

•S/V ratio decreases as organisms gain body size•Lower S/V ratio equates to lower thermal inertia

Size-Refugium hypothesis, cont.

Early therapsids were large, and therefore were ectothermic homeotherms (gigantothermy)

The Size-Refugium hypothesis posits a physiological adaptation over ~ 100 million years of large size

Size-Refugium hypothesis, cont.

Size-Refugium hypothesis, cont. ***

Evolutionary consequences of endothermy

Evolutionary consequences of endothermy

Behavioral Implications

Exploitation of marginal environments

Loris tardigradus