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Mesozoic Life   article on "Sue" 

Learning Objectives


After the Permian extinction life rebounds, diversifies and expands.

Invertebrates:

Large numbers of Brachiopods went extinct at the end of the Permian.  During the Mesozoic Bivalves expand to replace them.  Oysters are very successful.

Scleractinian Coral evolves.

Gastropods, Shrimp, Crabs, Starfish and Sea Urchins are all very successful.

Cephalopods: ammonites were present in the Paleozoic but they become abundant during the Mesozoic.
Octopoid organisms appear in the Late Cretaceous.

Plants:
 
Marine plankton evolve and expand. Climate connection?

Cycads and gymnosperms are abundant during the Mesozoic.

Angiosperms (flowering plants) appear in Middle Cretaceous.
Angiosperms affect evolution of insects and animals by providing new food source.

Vertebrates:
 
Reptiles are the dominant group in the Mesozoic.  There are 4 groups based on skull morphology.

Two orders of dinosaurs:

Saurischia (lizard hipped): the hip structure resembles that of a modern lizard.

Ornithischian (bird hipped): the hip structure resembles that of a bird.



Saurischia:

Started out as bipedal, some evolved to be quadrupeds.

Bipedalism improves speed and agility.

    Theropods: carnivorous saurischians.  All are bipedal  with short forelimbs that are powerful and used to grab prey.  The middle toes had a claw on them to rip prey.  They possessed a large head with double serrated teeth.
    Theropods started out small in the early Mesozoic becoming increasingly large throughout the Mesozoic.  Some famous theropods:
    Coelophysis: seen in the opening scene of Jurassic Park II
    Allosaurus: Jurassic, 20 - 25 ft long
    Tyrannosaurus Rex: Cretaceous, 42 ft long, >4tons
    The links that follow discuss the story of "SUE" a T-Rex skeleton that was found and the legalities surrounding this find           Peter Larson jailed 

    Sauropods: herbaceous saurischians.  They start as bipedal in the Early Mesozoic, eventually becoming quadrupeds. They get larger throughout the Mesozoic.
    Apatosaurus: Jurassic.
    During the Cretaceous many large herbivores:

      Seismosaurus: 115 ft long
      Diplodicus: very long neck and tail
      Supersaurus and Ultrasaurus
    Large size is believed to be either a defense mechanism OR a method for regulating body temperature.
     

Ornithischia:

Evolved in the Triassic this group of dinosaurs were all herbivores.  The back teeth were arranged into grinding plates and the front teeth were in a beak like arrangement for cropping vegetation.  Both bipeds and quadrupeds existed.

    1) Ornithopods: all bipedal and included the Trachodons (Hadrosaur) or  "duck bill dinosaurs."  These were very successful.  Some possessed elaborate skull crest with tubular nasal passages. (vocal resonators?)
     2) Stegasaurs: 2 pairs of horny spikes on the tail for defense.  This group had one or two bony plates along the back.  May have been for protection of thermal regulation.  Died out in the Cretaceous and replaced by the Anklyosaurs.

    3) Anklyosaurs: squat and heavily armored with a club tail.

    4) Ceratopsians:  evolved in the Cretaceous, horned dinosaurs.  Possessed a horn over the nostril and a neck frill of bone.  Very common along the epicontinental seas of North America.
     


Warm vs Cold Blooded:

Dinosaurs were always thought to be cold blooded. There are some paleontologists that believe that dinosaurs were actually warm blooded.
 
Argument for warm bloodedness:   see class notes.


Flying Reptiles:

Started as gliders and first appeared in the Permian.

Pterosaur: Triassic: skin between limbs.  Some fossils from the Jurassic seem to show hair.

Pteranodon: size of a goose with a 23 ft wingspan.

Quetzalcoatlus: 50 ft wingspan.



 
Marine Reptiles:

Adaptations of marine reptiles:

    Paddle shaped limbs
    Streamlined bodies
    More efficient lungs (did NOT have gills)
    Modified reproduction for birth at sea
Plesiosaurs: Jurassic and Cretaceous, short broad body with boned flippers, some had long necks and sharp teeth (Nessie)

Placodonts: bulky with large teeth and jaws.  Extinct in Triassic.

Icthyosaurs: fish like body, similar to a toothed whale.

Mosasaurs: large, porpoise like.

Sea Turtles: evolve in Triassic, by Cretaceous some are 12 ft long.


Birds:

Believed to evolve from small theropods.  The first undisputed bird fossil is
Archeopteryx: very small the size of a pigeon.  The skeleton is reptile like but has a fossil imprint of feathers.

Birds do not fossilized well because the bones are hollow and delicate so there is an incomplete record of bird evolution.


Mammals:

Mammals lived among dinosaurs during the Mesozoic.  The first mammals appeared in the Triassic and were small rodent like creatures.

Mammal characteristics:

    Warm blooded
    Possess mammary glands to suckle young
    Have hair
The above characteristics are not preserved in the fossil record.  Mammals are recognized in the fossil record by:
    One bone in the lower jaw (reptiles have several)
    Several ear bones
    Large skull capacity for body size
    Differentiated dentition
Throughout the Mesozoic mammals remained small.  It is believed evolutionary development took place that improved nervous and circulatory systems and reproductive systems.



Mass Extinction:

The end of the Mesozoic is marked by the K/T boundary or Cretaceous extinction.

Marine extinctions: most marine reptiles, Many invertebrates and plankton
Land extinctions: dinosaurs, flying reptiles

Survivors: turtles, snakes, lizards, crocs and mammals

Reasons for extinction:

1) Climate change: The climate changed drastically at the end of the Mesozoic.  It became colder with more seasonality.

2) Sea level: Sea level dropped at the end of the Mesozoic destroying habitats and killing off some plankton.

3) Arctic Ocean: see class notes about influx of fresh water into oceans.

4) World wide volcanism: produced large quantities of carbon dioxide and sulfur dioxide in the atmosphere.

5) Asteroid impact: see class notes