Historical Geology 102
Life in the Mesozoic I

Mesozoic Life

Mass extinction at the end of the Permian Period resulted in many ecological niches being left open
Diversification of the surviving organisms quickly filled those niches
Mostly immobile organisms before the mass extinction.
More mobile organisms after.
Scleractinian corals evolved in the Triassic


Mesozoic is often referred to as:
- Age of Reptiles - Age of Dinosaurs
Dinosaurs and reptiles are closely related but are not the same thing
One line of reptiles, thecodonts, did give rise to the dinosaurs

Marine Reptiles

Marine reptiles evolved during the Triassic
Ichthyosaurs, Mosasaurs, Plesiosaurs, etc.
Modifications included:
- paddle-like flippers
- stream-lining of the body
- greater lung capacity
Ichthyosaurs were fish-like in appearance, similar ecological niche as dolphins and sharks today.
Mosasaurs (Cretaceous only)
- larger than Ichthyosaurs
- 2.5 - 9 meters
- flat tail as opposed to a tail fin
- carnivores - large sharp teeth
- long neck & short neck species
- 3-15 meters in length.
Sea Turtles
- grew to large proportions

Flying Reptiles

- first flying vertebrates
Believed to have been gliders
- capable of powered flight
- thin membrane of skin stretched between forelimbs and hind limbs/body.


First mammals evolved during early Triassic
Sparse fossil evidence
Evolved from Therapsids - mammal-like reptiles
Initially small, rodent-like animals
Remained small throughout the Mesozoic


Two groups became dominant during the Triassic and Jurassic
Cycads and Ginkgos
- evolved from seed ferns in Carboniferous
Pines, cedars and fir trees also abundant
During the Jurassic, the most important evolutionary event was the rise of angiosperms.


Ammonites began to diversify in the early Mesozoic but were hit hard by the mass extinction at the end of the Triassic.
During the Jurassic ammonites flourished
Good index fossils during this period
Two evolutionary trends
- increased coiling
- increased complexity of suture structure


Birds evolved during the Mesozoic
Believed to have evolved from Triassic theropods (dinosaur group) or thecodonts (small reptiles which gave rise to dinosaurs).
The first discovered bird-like fossil was Archaeopteryx


Feathers, teeth, lizard-like tail
Late Jurassic (150 M.a.)
Shows some similarities to modern birds and dinosaurs


Triassic fossil bird (200 M.a.)
Crow-sized, hollow bones, breast bone structure similar to birds
Argument as to whether Protoavis is a bird.

Bird or Dinosaur?

Are birds really dinosaurs?