see . monotremes eutherians metatherians
Post on 20-Dec-2015
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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
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
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.