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Chapter 4 Reptile
Mammalian Origins
Early mammal
Carnivore
Amniota
Herbivore
Feldhamer Table 4.1 Savage and Long 1986
Fig. 3.2, Vaughn, Fig. 4.1, Feldhamer
Mammalian Origins
Overview Synapsids Pelycosaurs and Therapsids First Mammals Mesozoic Era appear Cenozoic Era radiate
Major groups of Synapsida
Dimetrodon
Feldhamer Fig. 4.2
Savage and Long 1986
Skull, jaw musculature, and teeth
Pelycosaur
Pelycosaur
Cynodontia -- Advanced, predaceous therapsids
Scymnognathus Cynognathus
Therapsid
Early Cynodont
Derived
Therapsid/Mammal Late Cynodont
Primitive
Fig. 3.2, Vaughn
Fig 4.3 & 4, Feldhamer
Thrinaxodon
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Skeletal transition
Pelycosaur Fig. 4.15
Extinction of Cynodonts
Possibly competition from dinosaurs Early Cynodonts were dog-size, last surviving were squirrel sized Mammals that survived while Cynodonts went extinct (contemporary) were mouse-sized.
Cynodont
Thrinaxodon
Fig. 3.5, Vaughn
Modern Mammal
Fig. 4.16c, Early Cynodont
Early mammals
200 - 250 million years ago Derived characters: Dentary/squamosal jaw articulation Diphyodont dentition Mammary glands Secondary palate Viviparity (loss of eggshell) When?
Feldhamer 4.11
Changes in land masses
180 MYA MidJurassic
200 MYA Early Jurassic
135 MYA Early Cretaceous
Feldhamer 4.5, 4.9
65 MYA Early Cenozoic
Skull and teeth of mammals
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Teeth and Dentition of Mammals
Teeth
One of the major keys to success of mammals Teeth of mammals are extremely variable with different diets -more than other taxa
Heterodont teeth with different functions Differentiated on the basis of function, resulting in increased efficiency acquiring and digesting food. Teeth occur in 3 bones of skull: premaxilla, maxilla, dentary
Feldhamer et al.
Additional Notes on Tooth Structure Not all teeth have enamel over entire surface of tooth, results in differential wear Open-rooted teeth grow continuously (e.g., rodent incisors)
Incisors: rooted in premaxilla (upper) and dentary (lower) Often reduced in number with a diastema in herbivores
Usually canines are lost
Canines: posterior to incisors, rooted in maxilla, dentary Function: piercing/tearing prey, holding, display, fighting Structure: moderately to very long (compared to other teeth), usually simple form (unicuspid), single rooted Mammals have single canine in each quadrant of jaw if present. Never present in modern rodents, often absent in herbivores Often used in social displays or fighting.
Lynx canadensis
Molars and Premolars
Carnivores vs. Herbivores
Generalizations and exceptions
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TOOTH CROWN HEIGHTS
Brachydont: Low crowned teeth, typical of omnivores Hypsodont: high crowned teeth, typical of many herbivores
Tooth Replacement
Most mammals are diphyodont which means they produce 2 sets of teeth during lifetime (milk teeth, permanent dentition) Milk teeth consist of incisors, canines, and premolars. Molars only grow in as permanent teeth Pattern of tooth replacement/eruption is well-ordered and speciesspecific; tooth eruption schedules can be used for aging mammals Tooth replacement is vertical except in a few species in which it is horizontal (elephants, manatees, dugongs)
High crown tooth
Low crown tooth
Vaughn Fig. 2.22
Proboscidea
Functional teeth -- consequences of long life -Single tooth active (may be adjacent one too) Tusks -- 2nd upper incisor
Cheekteeth Cusp Patterns
BUNODONT: Separate, rounded cusps for crushing, grinding, typical of omnivores LOPHODONT: Cusps forming continuous ridges, or lophs, seen in herbivores SELENODONT: Cusps as lophs which are crescent- shaped and longitudinal, also in herbivores
Deinotherium 40 million years ago
Examples of Selenodont Teeth
DENTAL FORMULAS Gazelle
1. Dental formulae are always given in the order: incisors (I), canines (C), premolars (P), molars (M) 2. Humans have 2 incisors, 1 canine, 2 premolars, and 3 molars = I2 C1 P2 M3, in each quadrant of their upper jaws 3. The numbers for each quadrant of the lower jaw are the same; general formula for humans is written I2/2 C1/1 P2/2 M3/3. White-tailed deer (Odocoileus) 4. To calculate total number of teeth from dental formula, sum these numbers (2+2+1+1+2+2+3+3 = 16) then multiply by 2 16 X 2 = 32 total teeth in adult human
