Mammalogy
(Spring 2015 Althoff - reference FDVM Chapter 4)
Mammals Origins
Bill Horn
LEC
02
Mammals are…
space-age reptiles.
THEY _______ & _______
FASTER!!!!
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)
1 pelycosaurs (~300-250 MYA…now extinct)
2)
2 therapsids (~250-140 MYA…now extinct)
3)
3 mammals (~_____ MYA to today)
• 1 and 2 considered “mammal-like reptiles
• Mammal major radiations peaked in the
Cenozoic (starting ~ _______ MYA)
Mammal-like
2
1
reptilest
3
Mammals
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….
Reptiles “out”, Mammals “in”??
• Competition from other reptile groups
• Changing climatic conditions
Which is most likely???
Back to Mammalian Dominance
• Specifically, late Cretaceous period to
Paleocene epoch
• Adaptative _________ led to dominance
during Cenozoic era
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)
225 MYA
Present
135 MYA
65 MYA
“Modern” mammals
• Geologic time division when most modern
families (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
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
MAMMALS arose from… Lizards & snake arose from..
SYNAPSID
DIAPSID
ANAPSID
Lineage lead to turtles
“Better” skull design…advantages
• Mammalian skull design (originating from reptilian
lineage subclass Synapsida)
a) increased freedom for expansion
of ______________________
or
b) selective advantage gained by
________________________
(i.e., less bone, less wt. = bone
replaced by opening)
Anapsid
Synapsid
(primitive amniotes
& turtles)
(mammals)
Lateral view
Adductor
muscle
Adductor
muscle
Cross-sectional view
More room for “bulging” muscles
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)
Pelycosaur
Noncynodont
therapsid
Cynodont
therapsid
Early
Mammal
Megazostrodon
“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
“Mods” …con’t
TEETH—specialization of the dentition.
Went from
homodont to ____________ 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
“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 pushoff from the gastrocnemius (calf) muscle)
 FORM OF VERTEBRAL COLUMN— loss of
lumbar ribs , reduction of cervical ribs suggest
presence of _____________________
Classification & Diversity of Mammals
Mammalia
CLASS
29
ORDERS
FAMILIES
153
GENERA
SPECIES
~ 1,200
>
??
EXTANT…
mammals
Mammal-like reptilest
1
Fig. 18-2, p489 PJH
2
3
1 • MONOTREMATA order
(3 species)
“egg-laying” mammals)
2 • MARSUPIALIA infraclass
(242 species)
(7 orders within “pouched mammals”)
• All others-“placental mammals”
3
Infraclass EUTHERIA (eutherians)
(4,500-plus species)
(21 orders within “placentals”)
Early
Mammal
groups
TIME