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

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See http//geology.com/time.htm
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• Monotremes

Metatherians
Eutherians
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Node - Divergence Event Branch - Common Ancestor
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• Captorhinomorphs- 350 MYBP (Carboniferous)

Synapsids- 320 MYBP
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Synapsida Together Arch
anapsid
synapsid
j jugalp  parietalpo  postorbitalsq 
squamosal
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Pelycosaurs
Dimetrodon
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Early Therapsids
Lycaenops
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Cynodonts Advanced Theraspids (dog teeth)
Cyognathus
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250 MYA
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Mammalian classification (especially
fossil-based) The key character approach
Ovis aries
Dixeya (a late therapsid)
Q-A vs. D-S jaw joint- the defining key
character for 80 years
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Using a Key Character approachMorganucodontids
as first mammals?
Q-A
D-S
250 MYA
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Probainognathus
Diarthrognathus
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Shift to a Suite of Characters
approach (Feldhammer et al.)
1) D-S jaw joint
2) Strongly heterodont dentition
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Shift to a Suite of Characters
approach (Feldhammer et al.)
3) Molars with occlusion, complex surface, wear
facets
4) Alternate side chewing
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Shift to a Suite of Characters
approach (Feldhammer et al.)
5) Well developed inner ear (Petrosal)
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Shift to a Suite of Characters
approach (Feldhammer et al.)
6) Small
7) Axial skeleton (dorso-ventral flexion,
thoracic rib placement)
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And based on a suite of characters approach,
where does the mammal/non-mammal division lie?
250 MYA
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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

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Reptilia
Archosauria

• Reptiles- a grade based definition
• Scales
• Lack of feathers
• Lack of hair

Possible common traits of members of Archosauria?
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• Our definition clade-based to the
  Morganucodontid node
• Mammal-like fossils (e.g. Sinocondon) can be
  referred to as Mammaliaformes

250 MYA
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• Reasons behind evolution of Mammalian characters?
• Unknowable- can only be inferred, hypothesized
• Many hypotheses exist well look at one
• The Size-Refugium hypothesis

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Size-Refugium hypothesis, cont.
Size an animals squared dimension Volume an
animals cubed dimension

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

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

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

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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
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Size-Refugium hypothesis, cont.
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Evolutionary consequences of endothermy
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Evolutionary consequences of endothermy
Behavioral Implications
Loris tardigradus
Exploitation of marginal environments