ƒ Meert & Liebermann (2007): reconstruction of the
scenario where the Ediacaran- Cambrian transition
happened;
MAS 603 Geological Oceanography
Proterozoic Evolution
Isabella D’Ambra
18 February 2008
ƒ Stanley (1976): factors affecting the transition
Reconstruction of the scenario
Proterozoic eon
Paleoproterozoic era
Mesoproterozoic era
Neoproterozoic era
2500-1600 Ma
1600-1000 Ma
1000-542 Ma
Siderian
Rhyacian
Orosirian
Statherian
Calymmian
Ectasian
Stenian
Tonian
Cryogenian
Ediacaran
…but there are several hypotheses
about the formation of this supercontinent!!!
From very simple ones (just the collision of pre-assembled
pieces) to very complex ones, based on paleomagnetic
studies (reconstruction of the latitude) and remains of the
orogenetic events.
Unfortunatelly, often just data from a single region are
taken into account, giving a partial vision of the
reconstruction.
The existence of a previous super-continent (Rodinia or
Paleopangea) seems well supported.
1
Important points:
1) The role played by tectonics and re-organization of the land
masses in affecting:
• climate (as for the Snow-ball events);
• geochemical processes in both ocean and atmospheric
reservoirs;
2) The rise in oxygen concentration (feedback mechanism
associated with the melting of glaciers);
3) This period of Earth’s history is highly unknown; reconstructions
of the assemblage of land masses are based on the presence of
similar Ediacaran fossils in areas that are now widely dispersed
and far away the one from the other
2
( the Burgess Shale)
(from Goy, 1997)
What did trigger the Proterozoic Evolution?
1) External triggers:
- sea level rise, resulting in flooding of continental margins;
- Snow-ball events;
- oxygenation of deep oceans;
- weathering of rocks followed by increase of nutrients in the
oceans;
- rise in oxygen in both ocean and atmosphere;
but also “disasters” (the rectangular model proposed by
Stanley(1973))such as
the Acraman impact;
the output of the Laurentinian plume
the three glaciation ((Sturtian, Marinoan, Glaskiers)
(the Ediacaran explosion may have followed these disastrous
events, as the explosion of Mammals followed the extinction of
Reptilians in the Cenozoic)
the increase in complexity in the environment, with the establishment
of interactions among organisms:
heterotrophy
predation
competition
parasitism
the “arms race” model (Stanley, 1973): the necessity to develop
offensive and defensive skills (like a shell, which is also well
preserved in fossils (!) or the increase in body size)
2) Internal triggers (Stanley, 1976):
the establishment of multicellularity;
the advent of sexuality with the following advantages:
• higher rate and dispersal of mutations,
• recombination of genetic characters;
• possibility of creating clades by speciation that can colonize new
environments better than the ones formed asexually,
• higher chance to survive to extinction events;
Uhmm…mumble mumble…
• Our knowledge is based on fossils ….so how
reliable are our reconstructions?
• Is there a leading factor or a co-occurrence
of factors triggering evolution?
• Can we adequately adapt the mechanisms
that we observe in present ecosystems to
past ecosystems that we know so scarcely?
3