MAMMALIAN EVOLUTION

CHARACTERISTICS OF MODERN MAMMALS

1. mandible consists only of dentary bone
2. dentary-squamosal jaw articulation
3. three ear ossicles: malleus, incus, stapes
4. two occipital condyles
5. determinate growth (long bones with epiphyses)
6. tympanic bone
7. hair
8. four chambered heart (also in crocodilians, Aves)
9. enucleate erythrocytes (red blood cells)
10. skin glands (including mammary glands)
11. muscular diaphragm
12. complex, differentiated teeth (heterodont) that occlude - incisors/canines/premolars/molars
13. two tooth generations (diphyodonty)
14. highly developed neopallium (roof of the forebrain)
15. secondary palate (also in crocodilians)

The Geologic Time Scale:

 Era              Period                    Epoch
______________________________________________________________

                                          Recent
                Quaternary
                                          Pleistocene
                ----------------------------------------------
 C                                        Pliocene
 E 
 N		                          Miocene
 O
 Z              Tertiary                  Oligocene
 O 
 I                                        Eocene
 C 
                                          Paleocene 
_________________________________________________________________

 M
 E              Cretaceous
 S
 O              Jurassic
 Z
 O              Triassic
 I
 C
_________________________________________________________________

                Permian
 P
 A              Pennsylvanian
 L                            } Carboniferous
 E              Mississippian
 O
 Z              Devonian
 O
 I              Silurian
 C  
                Ordovician
  
                Cambrian

MAMMALIAN EVOLUTION

The "reptile"-mammal transition (Fig. 19-1)

pelycosaurs 6 primitive therapsids 6 advanced therapsids (cynodonts) 6 mammals

 

Overview of mammalian evolution

Traditional classification of the amniotes

Class Reptilia

Subclass Synapsida

Order Pelycosauria

Order Therapsida

Subclass Diapsida

Class Aves

Class Mammalia

Better to think of reptiles as a (parpaphyletic) grade

Synapsids

Single temporal opening below postorbital and squamosal

Dominant terrestrial vertebrate group Permian-Triassic

Order Pelycosauria (lower Pennsylvanian-upper Permian)

1. reptile-like posture, skull, lower jaw, dentition (Figs. 19-2, 19-3)
2. some with sail-like fin (Fig. 19.4)
3. dominant Pennsylvanian-middle Permian

Order Therapsida (middle Permian - early Jurassic)

1. dominant late Permian-early Triassic (Fig. 19-5)
2. Suborder Cynodontia had many of advanced characters found in mammals

Early Mammals

1. Mammals originated 210 Ma in Late Triassic
2. Mesozoic mammals all small; mostly mouse-sized or smaller; largest (multituberculates) were cat-sized

Infraclass Ornithodelphia

1. egg-laying mammals
2. Order Monotremata [early Cretaceous - Recent]

Theria

1. tribosphenic dentition (Fig. 19-17)
2. alisphenoid forms a major part of the lateral wall of the braincase
3. anal and urogenital openings separate
4. cochlea spiral
5. eggshell absent.
6. Metatheria--marsupial (pouched) mammals
7. Eutheria--placental mammals

TRENDS IN ORGANIZATION FROM MAMMAL-LIKE REPTILES TO MAMMALS

1. Enlargement of the temporal fossa
2. movement of jaw muscle origins from temporal region to the braincase and the zygomatic arch
3. Gradual increase in size of dentary at expense of other jaw bones
4. trend to dentary-squamosal jaw articulation
5. Evolution of middle ear with three ossicles
6. development of secondary bony palate  - backward extension of the maxillary and palatine bones
7. change from homodont to to heterodont dentition
8. change from single to double occipital condyles
9. evolution of mammalian posture
10. change in ilium to forward pointing
11. loss of bones in pectoral girdle
12. loss of cervical and lumbar ribs
13. evolution of endothermy

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