Biology 4429 - Chapter 4

DOWNLOAD an Adobe Acrobat version of the Chapter Outline.

MAMMALIAN EVOLUTION

Geologic Time Scale (Table 4.1):

 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

• For more information see
  1. Geologic Time from the USGS
  2. Geologic Time Scale from the UCMP

MORPHOLOGICAL FEATURES OF SYNAPSIDS

1. Overview of mammalian evolution
   1. Phylogeny of Tetrapods  
   2. temporal fenestrae [Fig. 4.1]
2. Traditional classification of the amniotes
   1. Class Reptilia
      1. Subclass Synapsida
         1. Order Pelycosauria
         2. Order Therapsida
      2. Subclass Diapsida
   2. Class Aves
   3. Class Mammalia
3. The "reptile"-mammal transition [Fig. 4.2]
   • pelycosaurs --> primitive therapsids --> advanced therapsids (cynodonts) --> mammals
4. A cladistic classification of the Synapsida [Fig. 4.12]
   1. Synapsida [Fig. 4.2]: a monophyletic group that includes the mammals
      1. characters
         1. Single temporal opening below postorbital and squamosal (Figure 4.1A)
         2. Dominant terrestrial vertebrate group Pennsylvanian-early Triassic; Cenozoic
   2. Order Pelycosauria (lower Pennsylvanian-upper Permian)
      1. a paraphyletic group of the most primitive synapsids
      2. reptile-like posture, skull, lower jaw, dentition (Fig. 4.3A)
      3. some with sail-like fin
      4. dominant Pennsylvanian-middle Permian
   3. Order Therapsida (middle Permian - early Jurassic) [Fig. 4.3B, 4.16]
      1. A paraphyletic grade between pelycosaurs and mammals
      2. dominant late Permian-early Triassic
      3. Suborder Cynodontia
         1. Thrinaxodon; click on Tim Rowe's CAT scan images of a Thrinaxodon skull
         2. Eucynodonts and the origin of mammals from Toby White's "Vertebrate Notes"
   4. Class Mammalia (late Triassic - Recent)
5. Mammals originated 210 Ma in Late Triassic [Fig. 4.8]
   1. continents were single land mass Pangea
   2. first 2/3 of mammalian history coincides with tenure of dinosaurs (Mesozoic)
      1. Mesozoic mammals; mostly mouse-sized or smaller; the largest multituberculates were cat-sized, the up to 1.2 meter long Repenomamus fed on baby dinosaurs
      2. most were arthropophagous
      3. competition with lizards may have restricted mammals to nocturnal niches
      4. predation by small dinosaurs may have restricted mammals to mainly small size
      5. shortly before the time dinosaurs go extinct, mammals start to increase in diversity
      6. shortly after the time dinosaurs go extinct, mammals start to increase in body size; explosive increase in diversity

TRENDS IN ORGANIZATION FROM "MAMMAL-LIKE REPTILES" TO MAMMALS

• more information on this transition from
  1. Clifford Cuffey
  2. Paisley University

1. Enlargement of the temporal fossa (Fig. 4.13)
   1. movement of jaw muscle origins from temporal region to the braincase and the zygomatic arch (masseters)
   2. enabled "rotary" jaw action
2. Gradual increase in size of dentary at expense of other jaw bones
3. trend to dentary-squamosal jaw articulation [Fig. 4.4, 4.5]
   1. reptiles and primitive synapsids have an articular-quadrate jaw articulation
      • evolution of the mammalian jaw from Steven Carr, Memorial University of Newfoundland
   2. some cynodonts had both, i.e. double articulation
      1. quadrate-articular AND squamosal-dentary
   3. eventual loss of surangular then depression in squamosal (glenoid fossa)
   4. eventual transformation of articular and quadrate
4. Evolution of middle ear [Fig. 4.5, 4.6]
   1. stapes present in most tetrapods
   2. quadrate of skull becomes incus
   3. articular of lower jaw becomes the malleus
   4. developmental evidence: malleus formed from proximal end of Meckel's cartilage
   5. angular of lower jaw becomes the tympanic
5. development of a bony secondary palate [Fig. 4.14]
6. change from homodont to heterodont dentition
7. change from polyphyodont to diphyodont dentition
8. change from single to double occipital condyles
9. evolution of mammalian posture [Fig. 4.15]
10. loss of cervical and lumbar ribs
11. evolution of endothermy

Early Mammals

1. Major taxonomic groups (Wible, 1991; McKenna and Bell, 1997) [Fig. 4.8, Table 4.2]
   1. Class Mammalia
      1. Family Morganucodontidae
      2. Order Triconodonta
      3. Order Multituberculata
      4. Order Symmetrodonta
         1. I.C. Ornithodelphia
         2. Theria
            1. I. C. Marsupialia
            2. I. C. Placentalia

• note: crown cladists restrict Mammalia to the most recent common ancestor of the extant Mammalia and all of its descendants.  More primitive taxa with mammalian characteristics (e.g., the Morganucodontidae) are included in a the clade Mammaliformes, along with the Mammalia.

2. A more detailed look
   1. Family Morganucodontidae (late Triassic - early Jurassic) 
      1. linear tricuspid molars
      2. precise occlusion
      3. quadrate and articular still involved in jaw articulation
   2. Order Triconodonta (middle Jurassic - late Cretaceous)
      1. linear tricuspid molars [Fig. 4.9A]
      2. post dentary bones fully incorporated in middle ear
   3. O. Multituberculata (late Jurassic - Oligocene) (Fig. 4.9C, D)
      1. the most diverse and numerous of the Mesozoic mammals
      2. first mammalian herbivore
      3. Premolars differentiated from molars
      4. I^2-3 / I_1-2: rodent-like niche [Fig. 17.5]
      5. coexisted with placental mammals for over 70 million year
      6. longest lived (120 million years) of any mammalian order, extinct in Oligocene
      7. probably appearance of rodents in the late Paleocene led to extinction
   4. Family Kuehneotheriidae (early Jurassic)
      1. tooth-cusps in a triangular pattern (a reverse "triangle molar pattern")
      2. group that gave rise to Theria
   5. O. Symmetrodonta (late Jurassic - early Cretaceous)
      1. three cheek-tooth cusps in a "V"
   6. Infraclass Ornithodelphia
      1. egg-laying mammals (name means "bird womb")
      2. Order Monotremata [early Cretaceous - Recent]
   7. Subclass Theria
      1. characters
         1. tribosphenic dentition (Fig. 4.10)
         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.
      2. Infraclasses
         1. Metatheria--marsupial (pouched) mammals
         2. Eutheria--placental mammals
            • an up to date classification of the mammals based on molecular systematics is presented by Biodiversity Explorer, Iziko Museums of Cape Town 
            • The earliest placental mammal is Eomaia from the early Cretaceous of China (Nature, 2002)

Return to: