Lecture 9  The Cambrian Explosion and early Paleozoic Life

1.  The beginning of the Cambrian Period was marked by big changes:

a.        First appearance of shelled organisms (small shelly fauna - the Tommotian Fauna)

b.      Change in ocean conditions-more oxygen, less acidic, more alkaline (basic)

2.       Why?   Precambrian oceans-acidic, lots of metals, low in Calcium.  As Cyanobacteria produced oxygen (photosynthesis), pH shifted to higher values (more alkaline).  Eukaryotic multi-cellular organisms pushed the pH back towards acidic (respiration-exhaling CO2), balanced at about pH of 8 in Early Cambrian time. 

a.        Alkaline pH (above 7) is a stable pH for seashells (will not dissolve calcium carbonate)

b.      Explosion of different forms of shelled organisms strongly influenced by change in chemistry

3.        This balance between the photosynthesis and respiration creates the biological pump, which keeps the oceans balanced at about pH 8.  This has been going on, with a few minor hiccups here and there, since Early Cambrian time.

4.       Early Cambrian Reefs-yes, reefs, and no corals!  Built with  now extinct sponge-like organisms called Archaeocyathids, and cyanobacteria.  White Mountains, California

5.       Two major fossil assemblages of the Cambrian

a.        ChengJiang Fauna-China-Lower Cambrian

b.      Burgess Shale Fauna, British Columbia, Middle Cambrian

c.       Diverse group of organisms, many still around, included Chordates

6.    Phylum Chordata includes the vertebrates.

7.  The shallow tropical seas (rather like the Bahamas today) of late Ordovician Ohio are still visible !  The fossil rich carbonate layers are exposed at Caesar’s Creek State Park, just south of Wilmington, Ohio, off I-71.  Go wading in those tropical seas of long ago, and collect 450 million year old seashells!  These fossils are typical of shallow marine environments with lots of carbonate muds.

a.       Top predator-the Nautiloid-a large cephalopod with a long straight, chambered shell.

b.      Common coral-the horn coral (a Rugosa coral)

c.       Other common marine organisms:  bryozoans, brachiopods, other mollusks, and trilobites.

8.        The Devonian Period is very important in the Paleozoic for several reasons-1)  Fish are extremely diverse from this point onward

a.       One group of fish, the Placoderms, were top predators.  Example: Dunkleosteus from the Cleveland Shale

b.      Another group, the Lobe-Fin Fish, gave rise to the first Tetrapods (four legged vertebrates) that emerged onto land in the Devonian.  (A lobe-fin fish is around today-a Coelacanth living in the Indian Ocean).  A famous fossil of one of these animals is Tiktaalak )

c.       Another group, the sharks and rays, were very abundant during the Devonian, and are still around today.

9.        Another important feature of the Devonian is the invasion of the continents by plants

a.        First plants are tiny, spore bearing plants called Cooksonia, living in damp sediments near rivers and estuaries

b.      Rapidly through Devonian time, plants move on to the continents, and by late Devonian time, trees (still spore bearing) are on the continents, developing soils and changing the continents forever.

c.       Soils add complexity to the ecosystem.  Bacteria, lichens, algae, higher plants all have a role in developing soils.

d.      By the close of the Devonian, the biosphere is fully engaged-on land, as well as in the sea.  This was an entirely new situation, and as we will see in the next couple of lectures, had big consequences.

Next Lecture: 

Paleozoic Mountain Building