See

Monotremes Eutherians Metatherians

Node - Divergence Event Branch - Common Ancestor

Captorhinomorphs- 350 MYBP (Carboniferous) Synapsids- 320 MYBP

anapsid j: jugal p : parietal po : postorbital sq : squamosal synapsid Synapsida: ‘Together Arch’

Dimetrodon Pelycosaurs

Lycaenops Early Therapsids

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

Reptilia Archosauria Reptiles- a grade based definition 1.Scales 2.Lack of feathers 3.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 = 355 Size/volume = 0.88 Size: an animals squared dimension Volume: an animal’s cubed dimension 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 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.

***

Evolutionary consequences of endothermy

Behavioral Implications Exploitation of marginal environments Loris tardigradus