The Mesozoic Era
  Pamela J. W. Gore, 1997, 2010
Georgia Perimeter College

Triassic

Jurassic

Cretaceous

245-208 Ma

208-144 Ma

144-65 Ma

  1. Early Mesozoic: Triassic and Jurassic

    1. Global setting
      All continents have assembled into Pangaea (in the Permian).
      Much land far from sea and as a result became arid (evaporites).

      Pangaea began to rift apart in the late Triassic.

      Tethys Seaway existed as an embayment between Africa and Europe. (see map on p. )
      Located in equatorial Gondwanaland (between India & Asia).

      As N. America rifted from Africa, the Tethys Seaway expanded westward.

      (Gondwanaland continents remained attached until the Cretaceous).

      Evaporite deposits, accumulated as dry areas, became intermittently flooded by the sea during rifting.

      Examples:

      1. Morocco, Nova Scotia/Newfoundland (on opposite sides of the widening Atlantic Ocean.
      2. Gulf of Mexico "Louann Salt" - salt domes there are now petroleum traps.

      Sea Level:
      Gradual sea level rise from early Triassic to late Jurassic. Some fluctuations.
      Late Jurassic: epicontinental seas flooded large areas of N. America and Europe.

    2. Organisms
      1. Marine
        Two biogeographic provinces in Europe.
        1. Tethyan realm - warm, tropical, coral reefs, limestones (equatorial).
        2. Boreal realm - cooler, more northern. No reefs. (Siliciclastic seds).

        Recovery from the extinctions at the end of the Permian was slow for many groups.

        Molluscs re-expanded to become more diverse than in Paleozoic.

        Adaptive radiation of ammonoids from 2 to 100 genera.

        Bivalves and gastropods also increase in diversity; sea urchins diversified.


        Mold of a Triassic bivalve in red shale from a freshwater lake.

        Modern reef - building corals appeared. (Hexacorals or scleractinian corals).
        (Some Triassic coral reefs in deep water.
        No symbiotic algae?)

        - symbiotic relation between corals and algae may not have appeared until the late Triassic or early Jurassic.

      2. Pelagic Marine Life
        1. acritarchs - meager record.
        2. dinoflagellates - mid-Jurassic diversification.
        3. coccolithophores - appeared in early Jurassic.
        4. conodonts - extinct at end of Triassic.
        5. swimming predators:
          1. Ammonoids & Belemnoids - many types evolved.

          2. Fishes (ray finned, bony fishes)
            • scales with little or no overlap.
            • skeletons partly cartilagineous.
            • simple primitive jaws.
            • asymmetrical tails.
            • many changes in Mesozoic; swim bladder appeared (evolved from lungs) and sac of gas (buoyancy regulation).


              Scales and fins of a Triassic freshwater fish in black shale.

          3. Marine Reptiles (amphibious or aquatic reptiles)
            1. Placodonts (Triassic only)
              - broad armored bodies.
              - blunt teeth crushed shells.

            2. Nothosaurs (Triassic only)
              - first reptiles to invade the sea.
              - paddle-like limbs (sea-like).

            3. Plesiosaurs
              - evolved from Nothosaurs.
              - fed on fish.
              - up to 40 feet long (12 m).
              - paddle-like limbs.


        Plesiosaur. Elasmosaurus platyurus, Pierre Shale, NW Kansas.

            1. Ichthyosaurs ("fish-lizards")
              - fish-like reptiles (resemble dolphins)
              - top predators.
              - large eyes to pursue prey.
              - had live young, not eggs.

            2. Crocodiles
              - evolved in Triassic as terrestrial animals.
              - some adapted to marine environment by earliest Jurassic.
              - rapid swimmers.
              - evolved from Thecodonts (p. ). See below.


      3. Terrestrial Organisms:

        - mass extinction of many mammal-like reptiles.
        recall the pelycosaurs (Dimetrodon) and therapsids of the Permian.
        Therapsids probably arose from the pelycosaurs.

        1. Therapsids

          Therapsids were small to moderate in size, with several mammalian skeletal traits.
          • Differentiated teeth
          • Legs beneath body


          Cynognathus crateronotus, a therapsid from the Early Triassic (230-225 mya), Cape Province, South Africa. Note the differentiated teeth. This animal was obviously a predator. On display at the Smithsonian Institution, Museum of Natural History.

          Therapsids continued into the Triassic but became extinct in the Early Jurassic, after giving rise to the mammals.


          Kayentatherium, a reptilian ancestor of the mammals from the Late Triassic of Arizona (210-200 mya). The prominent front teeth and multicuspid cheek teeth suggest that this was a plant eater. It is an advanced mammal-like reptile. On display at the Smithsonian Institution Museum of Natural History.

        2. Basal Archosaurs or Thecodonts

          Thecodonts were small, agile reptiles with long tails and short fore-limbs.
          Many were bipedal (walked on 2 legs).
          Freed fore-limbs for other tasks -
          • catching prey
          • modified for flight

          The thecodonts gave rise to:

          • dinosaurs
            Click here to go to Dinosaurs

          • flying reptiles or pterosaurs
            Pteranodon, Rhamphorhynchus

          • armored carnivores
          • crocodile-like aquatic reptiles (phytosaurs)


            Phytosaur skull, Late Triassic of Arizona. Label shows external nostrils. On display at the Smithsonian Institution, Museum of Natural History.


            Phytosaur skull, Late Triassic of Texas. Label shows external nostrils. On display at the Smithsonian Institution, Museum of Natural History.


            Rutiodon, a Triassic phytosaur. On display at Dinosaur State Park, Rocky Hill, CT

          • Other Triassic reptiles

            Trilophosaurus, a Triassic euryapsid (skull similar to the aquatic reptiles) resembles a large lizard in body form, but has a turtle-like head with a toothless beak.


            Trilophosaurus buettneri, a Late Triassic plant-eating reptile. 210-200 mya, Texas. On display at the Smithsonian Institution Museum of Natural History.

          • Mammals

            Mammamls evolved from mammal-like reptiles in Late Triassic.

            Endotherms (warm-blooded)

            • have hair or fur
            • suckle their young.
            • remained small throughout Mesozoic (smaller than housecats).
            Early mammals were small furry animals. Rodent-like.


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        • Other animals

          - frogs appeared in Triassic (amphib).
          - turtles apppeared in Triassic (reptiles).

          - Pterosaurs (winged reptiles) in Triassic p.

          1. vertebrates invaded the air for the first time.
          2. long wings and tails.

          - birds appeared near the end of the Jurassic.

          Archaeopteryx ("first" bird?).

          • had feathers
          • dinosaur-like skeleton
          • teeth
          • large tail
          • forelimbs with claws
          • no breast bone (weak flier)
          • was bird-like with wings.

          - Protoavis is older (Triassic).
          - see Discover magazine April 1992.
          - not much to go on, and extremely controversial
          (Shankar Chattergee).

        • Land plants.

          1. Ferns dominated Triassic floras.
          2. Decline of lycopods, sphenopsids and cordaites trees began before end of Permian. Some lycopods and sphenopsids survived into the Mesozoic. Most were small.
          3. Gymnosperms (most trees) exposed seeds; pollination by wind.
            1. Cycads (dominant esp. in Jurassic).
            2. Conifers
            3. Ginkgos

              ginkgo
              Modern ginkgo leaf

          No flowering plants, grasses or hardwood trees until Cretaceous.


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    This page created by Pamela J. W. Gore
    Georgia Perimeter College, Clarkston, GA

    Last modified November 17, 1997
    Updated October 19, 1999
    Image added March 21, 2003
    Image added, typos corrected February 8, 2006
    Links updated August 15, 2009