Significance of the Cambrian Explosion - Instructional Web Server -

Document Sample
Significance of the Cambrian Explosion - Instructional Web Server - Powered By Docstoc
					Significance of the Cambrian Explosion
          Base of Cambrian/Paleozoic/Phanerozoic

The base of the Phanerozoic Eon is defined by the
first appearance of burrows (presumably made by
worm-like organisms). This indicates the evolution of a coelom that
could be manipulated for locomotion and feeding.

Burrowing also added an important tier to community structure (no
organisms burrowed prior to this time).
               Oldest Skeletons

       Tommotian Fauna (small shelly fossils):
             first skeletonized organisms
                     Early Cambrian
                    (scale bar: 1 mm)
Typically phosphatic (but calcified forms now known) !
  Why Do So Many Metazoans Have Skeletons ?:

1. Receptacles for excess mineral matter
    - note Kidney Stones and Gallstones in humans
    - warm seawater commonly saturated with calcium carbonate

2. Storehouses for scarce minerals (or means of slow release)
   -bones and teeth, some shells composed of calcium phosphate
   - phosphate rather scarce in nature, but essential for
   metabolism (e.g. Adenosine triphosphate- ATP)
    and is also key component of genetic material
   -calcium essential for heart, nerve, muscle functions,
   enzyme activation

3. Support and muscle attachment areas for locomotory organs

4. Serves as protective cage for soft internal organs

5. In shelled organisms, serves as a box to ensure controlled
   environment for metabolic functions

6. Protection from predators
                    The Pitfalls of Preservation

It is obvious that a major change
occurred in the Earth’s metazoan
biota by Cambrian time
(particularly in the development of
skeletal tissue).

But while skeletal remains give us
some indication of the magnitude
of change that occurred in the
earliest Phanerozoic, the perils of
fossil preservation prevent us from
seeing the entire biota (both
skeletonized and soft-bodied).

But… we do have a window shortly
after this (Middle Cambrian). This    Trilobites from Middle Cambrian
window is the Burgess Shale.          Burgess Shale showing soft part
         Charles Walcott

Discovered Middle Cambrian Burgess Shale
              Burgess Shale, Yoho National Park, B.C.

Exceptionally preserved soft-bodied organisms
Middle Cambrian age (shortly after Cambrian Explosion of Early Cambrian)
About 505 ma
A snapshot of life assemblages (in general, fossil record
only preserves hard parts of organisms)
Surprisingly diverse assemblage with very unusual forms
Vauxia -sponge   Vauxia “forest” with Leanchoilia

A worm, a mollusc, or something completely different ?

           An early arthropod (presumably related to trilobites)

                                                    Marella caught in the
Marella with squished-out guts
                                                       act of molting
Anomalocaris: A Case of Mistaken Identity

                        An unusual shrimp-like arthropod ?

                          A jellyfish ?
            A composite of components previously
              Believed to be separate organisms


                                          (Original Anomalocaris)
Anomalocaris – oblique view
      A “Lobopod”
Anomalocaris - reconstruction
(Lobopod ?)
The great Hallucigenia flip-out
              Presently classified as onychophoran


Modern “velvet worms” (onychophorans): in tropical rainforests
                       An aside:
are arthropods just onychophorans with exoskeletons ?


earliest known Chordate
             (Santa Claws)

           An undoubted arthropod
      (A Chelicerate without chelicerae ?)

        Subphylum Chelicerata includes:
spiders, mites, ticks scorpions,horseshoe crabs
all having feeding appendages called chelicerae

Makes one wonder what really defines this group
Chengjiang Fauna

       A similar Burgess Shale- type
       biota has since been found in the
       Chengjiang County, Yunnan
       Province, China.

       Yu'anshan Member of the
       Heilinpu Formation.

       These fossils are about 525
       ma (Early Cambrian), and
       therefore slightly older than
       the Burgess Shale fossils.
Haikouella lanceolata

             Of major importance in the
             Chengjiang biota is the
             occurrence of a chordate
             named Haikouella (this is older
             than Pikaia, which was
             previously declared to be the
             oldest chordate).

             It is decidedly lamprey-like,
             indicating the appearance of
             jawless, fish-like chordates by
             the early Cambrian (even
             though the oldest definite
             remains of jawless fish date to
             the Early Ordovician)
             Note differences in community structure

 Ediacaran marine community        Cambrian marine community

                                       Suspension feeders
All suspension feeders (or              Deposit feeders
at least passive food gatherers)
                                       The first arms race !
                        Catastrophic Burial

Burgess Shale organisms living on foot of escarpment (and possibly
on edge and top of escarpment as well) smothered by due to slumping,
Killed and buried instantly

Rapid burial + low oxygen (+ possible mineralization in vicinity of cold seeps ?)
= exceptional preservation
                 Stephen Jay Gould – Wonderful Life
Could some Burgess Shale organisms belong to extinct phyla ?
Is it possible that a phylum could be represented by few or
single species ?

If so, suggests that the Cambrian Explosion produced more
phyla than are present today. This view has been “softened” a bit since Gould’s
publication of Wonderful Life (perhaps more Classes than today)


       Conventional view:                Gould’s View:
       Gradual increase in number        Sudden appearance of phyla,
       of phyla through time             removal of many by mass extinction

Shared By: