Biology 4429 - Chapter 2 Mammalian Origins

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Wikipedia provides an overview of the origin and evolution of the mammals

Geologic Time Scale for the Phanerozoic (Table 2-1):

 Era              Period                    Epoch

 C                                        Pliocene
 N		                     Miocene
 Z              Tertiary                  Oligocene
 I                                        Eocene
66 Million years ago

 E              Cretaceous
 Z              Jurassic
 C             Triassic

250 Million years ago

 A              Pennsylvanian
 L                            } Carboniferous
 E              Mississippian
 Z              Devonian
 I              Silurian
542 Million years ago
USGS Geolologic Time Scale

USGS image from Wikipedia


  1. temporal fenestrae [Fig. 2-2]
    1. Openings in the skull roofing bones (not the braincase)
  2. Amniote Phylogeny (Fig. 2-1)
    1. Phylogeny of Tetrapods  
    1. Sauropsida
      1. Anapsida: Turtles
      2. Diapsida: lizards and snakes, crocodilians, birds
    2. Synapsida
      1. Pelycosaurs, therapsids, mammals



Image Use -- Creative Commons License -->  This media file is licensed under the Creative Commons Attribution-NonCommercial License - Version 3.0. Copyright 1996 Michel Laurin

  2. The "reptile"-mammal transition
    • pelycosaurs --> primitive therapsids --> advanced therapsids (cynodonts) --> mammals
  3. A cladistic classification of the Synapsida [Table 2-2]
    1. Synapsida [Fig. 2-3]: a monophyletic group that includes the mammals
      1. characters
        1. Single temporal opening below postorbital [po] and squamosal  [sq](Figure 2-2B)
        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
      3. some with sail-like fin (Fig. 2-5A): Dimetrodon, and others
      4. dominant Pennsylvanian-middle Permian
    3. Order Therapsida (middle Permian - early Jurassic) [Figs. 2-5B, C]
      1. Wikipedia article
      2. A paraphyletic grade between pelycosaurs and mammals
      3. dominant late Permian-early Triassic
      4. 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)

primitive synapsid skull from wikipedia
Single temporal opening below postorbital [po] and squamosal  [sq]

Dimetrodon mileri, Harvard Museum of Natural History

Thrinaxodon liorhinus
  1. Mammals originated 210 Ma in Late Triassic [
    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 (Fig. 2-14) 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


  1. Enlargement of the temporal fossa (Fig. 2-2)
    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 [Figs. 2-6, 2-8]
    1. reptiles and primitive synapsids have an articular-quadrate jaw articulation
    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 - Jaws to ears in the ancestor of mammals [Figs. 3-11, 2-8]
    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. 2-4]
  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. 2-5]
  10. loss of cervical and lumbar ribs
  11. evolution of endothermy

Early Mammals

     =============== †Morganucodontidae 
     |  ============ Monotremata 
     |  |
     |  |=========== †Multituberculata 
 --==|  |
     ===|     ====== Metatheria (Marsupials) 
        |     |
              ====== Eutheria (Placentals)
  1. Major taxonomic groups [Fig. 21.12] (Luo, Nature, 2007, 450:1011-1019)
  2. Abbreviated classification of McKenna and Bell (1997)
    1. Unnamed rank MAMMALIAFORMES
      1. Family Morganucodontidae
      2. Class MAMMALIA
        1. Subclass Prototheria
          1. Order Monotremata
        2. Subclass Theriiformes
          1. Order Multituberculata
          2. Theria
            1. Infraclass Marsupialia (Metatheria)
            2. IInfraclass Placentalia (Eutheria)


  1. Some of the major groups of Mammaliaformes
    1. Family Morganucodontidae (late Triassic - early Jurassic) [Fig. 2-10]
      1. linear tricuspid molars [Fig. 2-15]
      2. precise occlusion
      3. quadrate and articular still involved in jaw articulation along with squamosal and dentary

    2. Order Triconodonta (middle Jurassic - late Cretaceous)
      1. linear tricuspid molars
      2. post dentary bones fully incorporated in middle ear

    3. Order Multituberculata (late Jurassic - Oligocene) (Fig. 2-16)
      1. the most diverse and numerous of the Mesozoic mammals
      2. first mammalian herbivore
      3. Premolars differentiated from molars
      4. I2-3 / I1-2: rodent-like niche
      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") [Fig. 3-15]
      2. group that gave rise to Theria
    5. O. Symmetrodonta (late Jurassic - early Cretaceous)
      1. three cheek-tooth cusps in a "V"
    6. Infraclass Prototheria
      1. egg-laying mammals
      2. Order Monotremata [early Cretaceous - Recent]
    7. Subclass Theria
      1. characters
        1. tribosphenic dentition (Fig. 3-20 - 3-26
        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


Morcanucadont with double jaw articulation

Diagram showing characteristic double jaw joint of a morganucodont.  Image from Wikipedia

Skull of Ptilodus, a Paleocene multituberculate,
  after Vaughan, 1986, pencil drawing by ArthurWeasleyThis file is licensed under the Creative Commons Attribution 3.0 Unported License.












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