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Mammals – Part I

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  1. VERTEBRATE ZOOLOGY (VZ Lecture29 – Spring 2012 Althoff - reference PJH Chapter 18) Mammals – Part I Bill Horn

  2. Mammals are…space-age reptiles.THEY ______ & _______ FASTER!!!!

  3. Origins of Mammals • Have to think “synapsids” first…. • Synapsids thought to have originated at the end of the Paleozoic ~300-290 MYA • Three major radiations: 1) pelycosaurs (~300-250 MYA…now extinct) 2) therapsids (~250-140 MYA…now extinct) 3) mammals (~206 MYA to today) • and considered “mammal-like reptiles • Mammal _________________ peaked in the Cenozoic (starting ~ 90-65 MYA) 1 2 3 1 2

  4. 3 Mammal-like reptilest Mammals 2 1 Fig. 18-2, p489 PJH

  5. Dominance of Mammals • Dominance by mammals occurred during late Cretaceous period…after decline of ruling reptiles…90-65 MYA • Why did Synapid reptiles decline…and mammals flourish? Two explanations have been offered….

  6. Reptiles “out”, Mammals “in”?? • Competition from other reptile groups • Changing climatic conditions

  7. Back to Mammalian Dominance • Specifically, late Cretaceous period to Paleocene epoch • Adaptative __________led to dominance during Cenozoic era

  8. Periods of Extinction & Radiation • MASS Extinction: dinosaurs & reptiles 60-144 mya • Significant extinctions for mammals: 37-58 mya 24-37 mya 100,000 – 2 mya • Significant radiations associated with break-up of Pangaea & continental drift as well as increased faunal and floral diversity…most pronounced during Paleocene(next slide)

  9. “Modern” mammals • Geologic time division when most modernfamilies (aka family diversity) appeared is the Oligocene (37-24 MYA) & Miocene (24-5 MYA) • Oligocene and Miocene may have also represented the period of greatest species diversity as well

  10. Skull Characteristics • Differ from reptiles with development of perforations of temporal portion • Anapsid = no temporal opening vs. • Diapsid = 2 temporal openings, separated by post orbital process vs. • __________ = 1 opening  MAMMALS

  11. Lizards & snake arose from.. MAMMALS arose from… SYNAPSID DIAPSID ANAPSID Lineage lead to turtles

  12. “Better” skull design…advantages • Mammalian skull design (originating from reptilian lineage subclass Synapsida) a) increased freedom for expansion of ____________________ b) selective advantage gained by _______________________ (i.e., less bone, less wt. = bone replaced by opening) or

  13. Synapsid (mammals) Anapsid (primitive amniotes & turtles) Lateral view Adductor muscle Adductor muscle Cross-sectional view More room for “bulging” muscles

  14. Evolutionary Trends in Synapsids • Key change from during evolution of synapsid lineage was crossing of physiological boundary from ectothermy to ________________ • “Changes” from a physiological perspective—which would influence metabolic rate—obviously not fossilized. But changes in skulls and skeletons can offer clues how higher metabolic rates could have been achieved: 1) greater food intake or rate of feeding (FR = food-related) 2) greater respiration rate (AR = activity-related)

  15. Pelycosaur Noncynodont therapsid Cynodont therapsid Early Mammal Megazostrodon Fig. 18-6, p495 PJH

  16. “Mods” • SIZE OF THE TEMPORAL FENESTRA—larger fenestra indicates a greater volume of jaw musculature…also larger temporal fossa • CONDITION OF THE LOWER TEMPORAL BAR—bar of bone owed out from skull behind the orbit indicates presence of masseter muscle. For mammals, zygomatic arch “bows” out • LOWER JAW AND JAW JOINT—change from only about half of lower jaw with teeth to greatly expanded dentary. For mammals, dentary now forms a new jaw joint with skull—coronoid process prominent

  17. “Mods” …con’t • TEETH—specialization of the dentition. Went from homodont to heterodont condition (i.e., differentiated teeth) resulting in change in size, form, and function. Impacts chewing motion as well in combo with #2 • DEVELOPMENT OF SECONDARY PALATE—separates nasal passage from mouth allowing animal to eat and breath at the same time • POSITION OF THE LIMBS—placed more underneath the body

  18. “Mods” …con’t • SHAPE OF THE LIMB GIRDLES—more lightly built girdles than the “mammal-like” reptiles. Mammals have reduced pubis and a rod-shaped ilium….allows more dorsoventral flexion (see 10, too) • SHAPE OF FEET—Shorter toes. Calcaneal heal provides a lever arm for a greater degree of push-off from the gastrocnemius (calf) muscle) • FORM OF VERTEBRAL COLUMN— loss of lumbar ribs , reduction of cervical ribs suggest presence of muscular diaphragm

  19. Classification & Diversity of Mammals CLASS Mammalia ORDERS 29 FAMILIES 153 GENERA ~ 1,200 SPECIES >

  20. EXTANT… mammals Mammal-like reptilest 1 2 3 Fig. 18-2, p489 PJH

  21. 1 • MONOTREMATA order (3 species) “egg-laying” mammals) • MARSUPIALIA infraclass (242 species)(7 orders within “pouched mammals”) • All others-“placental mammals” Infraclass EUTHERIA (eutherians) (4,500-plus species) (21 orders within “placentals”) 2 3

  22. Early Mammal groups Monotremes (egg-layers) Eutherians (placentals) Marsupials (pouched) TIME