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Evolution of Mammals: Key Characters and Divergence Events

Explore the evolution of mammals through key characters like jaw joints and unique traits. Learn about the divergence events and common ancestors of Monotremes, Metatherians, and Eutherians. Understand the shift from 'Key Character' to 'Suite of Characters' approach in mammalian classification. Discover the Size-Refugium hypothesis and the evolutionary consequences of endothermy in mammals.

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Evolution of Mammals: Key Characters and Divergence Events

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

  2. Monotremes Metatherians Eutherians

  3. Node - Divergence Event Branch - Common Ancestor

  4. Captorhinomorphs- 350 MYBP (Carboniferous) Synapsids- 320 MYBP

  5. Synapsida: ‘Together Arch’ anapsid synapsid j: jugalp : parietalpo : postorbitalsq : squamosal

  6. Pelycosaurs Dimetrodon

  7. Early Therapsids Lycaenops

  8. Cynodonts*: Advanced Theraspids (*’dog teeth’) Cyognathus

  9. ~250 MYA

  10. 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

  11. Using a ‘Key Character approach’…Morganucodontids as first mammals? Q-A D-S ~250 MYA

  12. Probainognathus Diarthrognathus

  13. Shift to a ‘Suite of Characters’ approach… (Feldhammer et al.) 1) D-S jaw joint 2) Strongly heterodont dentition

  14. Shift to a ‘Suite of Characters’ approach… (Feldhammer et al.) 3) Molars with occlusion, complex surface, wear facets 4) Alternate side chewing

  15. Shift to a ‘Suite of Characters’ approach… (Feldhammer et al.) 5) Well developed inner ear (Petrosal)

  16. Shift to a ‘Suite of Characters’ approach… (Feldhammer et al.) 6) Small 7) Axial skeleton (dorso-ventral flexion, thoracic rib placement)

  17. And… based on a ‘suite of characters’ approach, where does the mammal/non-mammal division lie? ~250 MYA

  18. 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

  19. Reptilia Archosauria • Reptiles- a grade based definition • Scales • Lack of feathers • Lack of hair Possible common traits of members of Archosauria?

  20. Our definition… clade-based to the Morganucodontid node • ***Mammal-like fossils (e.g. Sinocondon) can be referred to as Mammaliaformes ~250 MYA

  21. Reasons behind evolution of Mammalian characters? • Unknowable- can only be inferred, hypothesized • Many hypotheses exist… we’ll look at one • The Size-Refugium hypothesis

  22. Size-Refugium hypothesis, cont. Size: an animals squared dimension Volume: an animal’s 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

  23. 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

  24. *** Size-Refugium hypothesis, cont.

  25. Evolutionary consequences of endothermy

  26. Evolutionary consequences of endothermy Behavioral Implications Loris tardigradus Exploitation of marginal environments

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