Mammals – Part I

VERTEBRATE ZOOLOGY (VZ Lecture29 – Spring 2012 Althoff - reference PJH Chapter 18)

Bill Horn

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) 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)

123

21

Fig. 18-2, p489 PJH

Mammal-like reptilest Mammals

1

2

3

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

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 & reptiles60-144 mya

• Significant extinctions for mammals:37-58 mya24-37 mya100,000 – 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)

“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 openingvs.

• Diapsid = 2 temporal openings, separated by post orbital process

vs.

• __________ = 1 opening MAMMALS

SYNAPSID

ANAPSID

DIAPSID

MAMMALS arose from… Lizards & snake arose from..

Lineage lead to turtles

“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

Anapsid(primitive amniotes

& turtles)

Synapsid(mammals)

More room for “bulging” muscles

Cross-sectional view

Lateral view

Adductormuscle

Adductormuscle

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)

Fig. 18-6, p495 PJH

Pelycosaur

Noncynodonttherapsid

Cynodonttherapsid

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 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

“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

Classification & Diversity of Mammals

CLASS Mammalia

ORDERS 29

153 FAMILIES

~ 1,200 GENERA

> SPECIES

Fig. 18-2, p489 PJH

Mammal-like reptilestEXTANT…mammals

321

• 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”)

1

2

3

TIME

EarlyMammalgroups

Monotremes

(egg-layers)Marsupials

(pouched) Eutherians

(placentals)