Lecture 4
Evolution & Dentition
1. Evolutionary timeline
2. Morphological changes
3. Mammal characteristics (hard
anatomy this time)
Sept. 20, 2010
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Once upon a time …
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Synapsids
Turtles
Anapsida
Lizards & snakes
Crocodiles
Dinosaurs & birds
Mammals &
reptile-like mammals
Diapsida
Amphibians
Saurapsida
Amniota
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Synapsida
-ca. 320 mya
-evolution of cleidoic (shelled) egg;
ca. 350 mya (late Paleozoic)
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Synapsids (about 320 mya)
• 1st group to
radiate widely in
terrestrial
habitats.
• Development of
the shelled egg.
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Synapsids diverge into Pelycosaurs
and Therapsids
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Single bone in mandible: dentary
Pelycosaur
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Therapsids = Mammal evolved from Reptiles?
FALSE!
Currently, it is thought that mammals and our “mammal-like
reptiles” did not evolve from reptiles; rather, reptiles and
mammals shared a common ancestor (the Amniota) from which
each group evolved in divergent ways.
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Most suborders of Therapsids go
extinct except Cynodontia.
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Cynodontia (almost mammal grade)
• Tricuspids
• Double-rooted cheekteeth
• Mammalian jaw structure and masseter
muscles
• Hearing
• More erect posture (better movement)
• Postcranial skeletal features
– Differentiated vertebrae
– Modified pectoral and pelvic girdle
–2010Thoracic ribs
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Class Mammalia (200-250 mil/yr ago)
• Defined by articulation of between the
squamosal and dentary bones.
– Changes increased auditory acuity and efficiency
of chewing. http://www.youtube.com/watch?v=AXcYXRFQ_DE
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http://www.youtube.com/watch?v=SIVs6lixFFQ&NR=1
http://www.youtube.com/watch?v=L-FCPC6IDzY\
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Lets Recap & Symplify!
Synapsids
325mya
Pelycosaurs
300mya
Therapsids
275mya
Cynodonts
250mya
Mammals
225mya
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First Mammals
http://www.youtube.com/watch?v=sCEeefdaRcw
• What advantages can you think of that would
have allowed the mammals to expand while
the dinosaurs collapsed?
• Mammals have been the dominant terrestrial
vertebrates for the last 65 million years.
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Linking Evolution and Mammal
Characteristics
• Changes in skeletal features occurred in
association with metabolism, physiology, and
reproduction.
• Mammals need ~10 times more food and
oxygen than a reptile of equal size.
• Most morphological changes directly assisted
endothermy and homeostasis.
• How do we know?????????????
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Mammal Characteristics
Occipital condyles
Dentary, ear ossicles
Dentition
Upright posture
Determinite growth/ephiphyses
Simplification of skeleton
Vertebral column
Mammary glands
4-chambered heart
Enucleated red blood cells
Diaphragm
Endothermy/homeothermy
Olfaction
Sept. 20,Hair
2010
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Fossil evidence?
Occipital condyles
Dentary, ear ossicles
Dentition
Upright posture
Determinite growth/ephiphyses
Simplification of skeleton
Vertebral column
Mammary glands
4-chambered heart
Enucleated red blood cells
Diaphragm
Endothermy/homeothermy(??)
Olfaction (??)
Sept. 20,Hair
2010
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Summary of Trends
Mammal-like Amniotes
Sept. 20, 2010
Mammals
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Dentary-squamosal articulation
post-dentary bones
1. Larger and heavier dentary bone. Process food better?
2. Allowing jaw muscles to attach from inner surface of
cranium to outer surface. Ossification of braincase. More
protection?
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Formation of secondary palate
• More efficient airflow
• Constant breathing while chewing; thus, more
efficient transfer of oxygen and enhanced
metabolism
• Also may have related to efficient suckling.
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Double occipital
condyle:
greater range of
motion
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Modified limbs and pelvic girdle
Reptile-like
Mammal-like
femur
pelvic
girdle
fibula
femur
calcaneus
tibia
astragalus
Sprawling posture
Lateral gait
Poorly developed calcaneus
http://www.youtube.com/watch?v=_ut5jENqBX8
Sept. 20, 2010
fibula
tibia
astragalus
calcaneus
Upright posture
Fore-aft gait
Well-developed calcaneus
http://www.youtube.com/watch?v=jMwefq1ZtNI
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Loss of cervical and thoracic ribs
• More flexible movement, especially in lower
spine
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Mammal Teeth & Dentition
(complicated subject)
• One of the most important aspects of living
mammals.
– Reflect trophic level and specialization
– Along with feeding, used for borrowing, grooming,
and protection
– One of the main tools for distinguishing mammals
from other classes and from each other.
• Little variation in skeletal system of mammals relative
to variation in dental patterns.
• They fossilize
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Heterodont or Dyphodont Dentition
Synapsids
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had homodont dentition (& some
mammals still do)
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Enamel: hardest
tissue in mammals.
3% organic...
“pre-fossilized”
Crown
gumline
Root
Alveolus (pl. “alveoli”)
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Dentine: same mineral composition,
but 30% organic; softer.
Pulp cavity: nerves & vessels.
Remains open in rootless, evergrowing teeth (rodent incisors,
some cheekteeth)
Cementum: bony material
surrounding root
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Teeth grow from one of 3 bones
Maxilla, Premaxilla, and Dentary
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Tooth replacement
•Living mammals DIPHYODONT (EXCEPT
molars!). Ancestral condition for mammals. Most
other vertebrates have POLYPHYODONT dentition.
•“Deciduous” teeth replaced by “permanent” teeth.
•Replacement timing varies (e.g., shrews)
•Morphology often different in deciduous teeth vs.
permanent teeth
•Sequence of replacement constant w/in species:
useful for AGING
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4 basic kinds of teeth
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Incisors
incisors
•Anteriormost teeth, grow out of premaxilla, usually single-rooted
•Used mainly for picking, grasping, grooming, but heavily modified in
some taxa...
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Incisors
incisors
Dog
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Rodent
Lemur
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Incisors
incisors
Elephant
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Vampire bat
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Incisors
incisors
I C P M
1
0
0
0
3
3
3
3
Tusk is modified 2nd incisor.
How do we know???
Elephant
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Canine
canine
•First tooth in maxilla or at maxillary/premaxillary suture.
•All mammals have AT MOST one canine per quadrant
•Usually single-cusped (unicuspid), single-rooted (but exceptions)
•Generally used for stabbing, holding prey in carnivores (but exceptions)
•Often absent (rodents, some artiodactyls) or heavily modified (elephants)
•Can be problematic to identify if absent and next tooth is caniniform
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Canine
canine
Cat
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Moschid deer
Rodent
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Canine
canine
Walrus
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Babirusa
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Premolars
premolars
•Posterior to canines, rooted entirely in maxilla
•Variable: tiny, peglike in some, massive crushing tool in others
•Often “molariform,” but generally smaller, less developed than molars
•Premolars are replaced, molars aren’t
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Premolars
premolars
Sept. 20, 2010
Dog carnassial
pair
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Molars
molars
•Posteriormost teeth in toothrow
•Variable, but late to erupt
•Erupt anterior-to-posterior
•NOT REPLACED!
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Molars
Capybara
molars
Shrew
Raccoon
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Elk
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Cheekteeth
premolars
molars
“cheekteeth”
Despite variation in I, C, much of the important functional
variation is found in CHEEKTEETH, particularly MOLARS.
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“Assembly line” for food processing
capture, pick up
puncture
puncture, shear
grind
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gulp!
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