Lecture 13 Mesozoic Ecology on Land and Sea: Rise of Dinosaurs and Marine Reptiles
Focus Question:What ecological
roles did these large vertebrates have in the Mesozoic world?
1.
Today,
we explore the lines of marine reptiles and dinosaurs. To do that, we need
to think about the kinds of environmental pressures on these groups in the
Triassic. This leads us to thinking about the Triassic volcanism again.
In mid to late Triassic time, extensive rifting and the formation of
large igneous provinces occurred (see previous lecture about this).
The Triassic ended in yet another major global extinction event, although
not as severe as the famous Permian-Triassic event.
a.
Volcanic
activity included the rifting of Pangaea, as North America and Eurasia pulled
apart
b.
This volcanic activity produced a lot of carbon
dioxide and other greenhouse gases.
c.
The increase in CO2 is recorded in the fossil
leaves (lots of these fossils available from the Triassic because of rise of
gymnosperms (conifers) and cycads).
1. Leaf stomates (tiny holes on the underside of the leaves that are the entry points for CO2 during the photosynthesis process) are the key
2.
Numbers of stomates drops when CO2 increases
and atmospheric temperature rises, and rises when CO2 levels drop and
atmospheric temperature cools.
3.
Numerous examples of fossilized leaves show the
decrease of stomata in leaves at the the close of the Triassic.
2.
Remember
those two major groups of tetrapod vertebrates, the Synapsids and Diapsids-(if
not, see previous lectures).
a.
The
ancestral line of mammals (Synapsids) did not do very well during the Triassic,
and the Cynodonts (small mammals ) stayed small in size and an insignificant
part of the population .
b.
The Diapsids on the other hand did very well, and
gave rise to the archosaur group.
The archosaurs included
1.
Dinosaurs
2.
Pterosaurs
3.
Flying Reptiles
4.
Birds (appearing in Jurassic)
5.
Crocodiles
c.
Two other
Diapsid groups were also very important in the Mesozoic: Ichthyosaurs and
Plesiosaurs.
3.
Plesiosaurs
and Ichthyosaurs first appeared in the Triassic, and were marine reptiles.
a.
Air
breathers, carnivores
b.
Streamlined bodies, open ocean swimming
c.
Forelimbs and hind limbs very short, digits fused
into paddles
d.
Very large sizes (Plesiosaurs-up to 30 meters in
length)
4.
Ichthyosaurs
have detailed fossil record showing rise from reptile-like body plan, living in
shallow, coastal water environment in Triassic, to open ocean “tuna-body shape”
by Jurassic time-all in about 20 million years.
a.
Huge
eyes, probably dove to depths at the edge of the photic zone, where last bit of
light penetrates
b.
Probably in part endothermic (warm blooded)
similar to modern tuna, which can control metabolic temperatures for periods of
time.
c.
Gave birth live (did not lay eggs), so, modern
analog for this would be like whales
5.
Diapsids
also gave rise to Thecodonts, which in turn gave rise to the Archosaur group,
including Dinosaurs.
a.
Changes
in skeleton show leg positions moved from lateral, sprawling arrangement (think
crocodiles here) to directly below the body (like most 4 legged animals you can
think of).
b.
This
solved an important problem, in that now it was possible for breathing
to occur while running (a significant
break-through in body plan!).
c.
Dinosaur group, then, had this new body design,
and also
1.
Bipedal
(early forms ran on two legs, in some groups this would be permanent)
2.
Changes in skull-lighter, more air pockets
3.
Specialized teeth
4.
Reduction in number of digits
6.
Dinosaurs
are divided into two groups: ornithischians (bird-hipped) and saurischians
(lizard hipped) . Ornithischians
are herbivores, and include Triceratops and Stegasaurus.
Saurischians include both herbivores and carnivores, and famous
Saurischians are the herbivore Apatosaurus
and the carnivore Tyrannosaurus.
a.
A group
of Saurischians we are going to follow is the Therapod group-these are
carnivores that have
1.
Hollow
bones
2.
Feathers for warmth, not flight
3.
Bipedalism
b.
Therapods
are the ancestral group that gave rise to birds.
(We will cover more on this in the next lecture)
7. How do we know so much about these Mesozoic animals? Big breakthroughs in the study of bone tissue (histology) have provided information about
a. metabolism
b. growth rates
c. body temperature
d. hormones
8. Collectively, this new information shows that these were complex, successful animals that dominated the Mesozoic for about 140 million years-a major success story! They fit into the ecosystems of the Mesozoic in familiar ways, as herbivores, carnivores, predators and prey, and radiated into many niches to establish a complex food web.
9. Next lecture: Diversity of Dinosaurs, and Birds and Flying Reptiles