GY 112 Lecture Notes
D. Haywick (2006)
1
GY 112 Lecture Notes
Proterozoic Life forms
Lecture Goals:
A) Eukaryote Life forms (Acritarchs)
B) Metazoans (Ediacaran Fauna)
Textbook reference: Levin 7th edition (2003) Chapter 7; Levin 8th edition (2006), Chapter 9
A) Eukaryotes
The first life forms on this planet were rather
primitive. This is not to say that they were
unimportant, rather that they were simple in
their makeup. The prokaryotes resembled the
bacteria pictured to the left (from
http://micro.magnet.fsu.edu/cells/bacteriacell.html),
were small (less than 0.020 mm), and lacked
nuclei and organelles. In other words, DNA was
spread throughout the cell rather than localized
to the nucleus.
Eukaryotes probably first evolved in the Archean.
They differed from the prokaryotes in several ways
(see image to right from http://www.earthlife.net)
1. they had nuclei
2. they has organelles
3. they were larger (>0.060 mm)
4. they had sex (yahoo!)
I have told you several times that I am not a
biologist, so my understanding of the origin of
eukaryotes is a bit weak. However, it seems likely
that the earliest eukaryotes formed from the
assimilation of 2 prokaryotic cells. It is tempting to
envision that one cell simply ate the other, but for reasons
unknown, just didn’t digest it. My stomach sometimes behaves the same way, but back in
the Archean, these two cells developed a unique relationship. The digested cell resided
within the other and mutated into what biologists call a mitochondrion. This is the
structure that helps cells to produce energy by producing ATP through respiration. A
similar assimilation may also have occurred between a eukaryote and a cyanobacteria cell
to form chloroplasts which regulate photosynthesis in plants. Both of these events were
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critical to all future evolutionary changes on this planet, because they allowed
multicellular organisms to develop.
Before we get to those beasties, it is first necessary to discuss the acritarchs. These were
small (perhaps up to 0.1 mm across) unicellular eukaryotic organisms that first appeared
some 2 billion years ago and peaked in abundance and diversity at around 750 MA (see
image below from http://www.nap.edu/openbook/0309051916/html/71.html). They were pretty
much gone by the end of the Proterozoic, but some persisted until the Ordovician. They
resembled in many ways the cysts of modern planktonic algae called dinoflagellates, and
it is generally felt that the acritarchs may have been Proterozoic equivalents to them.
Maybe the acritarchs didn’t die off in the Paleozoic after all. Acritarchs were mostly
spherical, but developed highly ornamental cysts over time. Their decline at the end of
the Proterozoic may have been caused in part by the onset of the Varangian Glaciation.
B) Metazoans
Metazoans are multicellular organisms that are composed of different types of cells
arranged into tissues and organs. Most plants and animals that you are familiar with fall
into this grouping. The transition from unicellular to multicellular beasties was a big, big,
BIG advance in evolution. When this occurred is a subject of great debate in both
geology and biology.
The first fossilized metazoans were called the Ediacarans and
they were found in Neoproterozoic sedimentary rocks near
Adelaide, Australia (in the Ediacara Hills to be exact). Since
their original discovery in the middle part of the 1900’s,
Ediacaran fauna have been found in Newfoundland, China,
Ireland, parts of Africa, Russia and England.
The Ediacaran beasties first appeared in rocks about 750
million years old and persisted into the lowest part of the
Phanerozoic (to about 533 MA; the Early Cambrian). This is
noteworthy for two reasons: 1) Ediacarans seem to have been
unaffected by the event that initiated the decline in the
acritarch population and 2) the ediacaran time span overlapped the Cambrian explosion
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that marks the boundary between the Proterozoic and Phanerozoic Eras.
The dominant mode of preservation of the ediacaran fauna is impressions, but amazingly,
those impressions are in sandstones. Fine grained rocks like shales are usually the hosts
of fossil impressions because they are much more likely to preserve the fine detail of
most deceased beasties. This is particularly true for beasties that lacked hard body parts
such as the ediacarans.
The ediacarans are divided up into 3 different groups based upon morphology:
It was once thought that the discoidal forms were primitive jelly fish, that the frond-like
forms were primitive sea fans and that the elongate forms were possibly primitive sea
cucumbers, but that is now not the case. In the first place, these beasties left impressions
in sand. Jelly fish are too gooey to do this. The discoidal ediacarans must have had a
pretty competent (or at least firm) membrane to distort soft sand. The second and much
more important difference is that the ediacaran fauna display tantalizing features
suggesting very advanced features. Kimberella sp., an elongate ediacaran, appears to
have had a central cavity that was possibly used for digestion (e.g., a stomach) and other
internal organs. It may have also had a mantle very similar to those used by gastropods.
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Dickinsonia sp., another elongate form, had a central linear feature and segmentations
that looks an awful lot like a notochord (a pre-back bone). On closer examination, the
frond-like forms look more like bryozoan colonies than they do sea fans.
So what in God’s name were these things? There are two schools of thought about the
ediacaran beasties.
1) they were the early ancestors of several living phyla including molluscs (e.g.,
gastropods), Cnidaria (e.g., corals) and annelids (e.g., worms), that appeared before the
development of hard skeletons, shells and tests etc.
2) they are completely different animals (wholly different phyla?) that are now extinct.
Their resemblance to living organisms is superficial and was purely coincidental.
Which is correct? Well text books
frequently have pretty lively discussions
about the origin and ecology of the
ediacarans (see image to right from
http://www.emc.maricopa.edu/faculty/farabee)
and they are well worth reading to help
you make up you mind. It is also worth
mentioning that new research into DNA
evolution is lending support to one of
these ideas over the other. Unless you
have been clinically dead for the past
couple of years, you know that significant
GY 112 Lecture Notes
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inroads have been made by biochemists into genome mapping and DNA characterization.
I seem to remember that humans and chimpanzees are almost identical as far as DNA is
concerned (Darwin could have used this info tidbit during the Monkey Trials), and that
even mice are more like us than we probably want to admit. If DNA in all of the living
beasties ultimately evolved from a common stock which evolutionary theory suggests, it
should be possible to determine when a lot of the phyla split off from common stock. The
last date I heard of was 1GA. This was almost 500 million years before the Cambrian
Explosion. If this is correct, representatives of most phyla must have been around during
the Proterozoic. They just didn’t fossilize well because they lacked anything hard that
was capable of easy preservation. The Cambrian Explosion might better be termed the
Carbonate Explosion in honor of the relatively sudden appearance of calcitic shells and
other skeletal elements.
A an outcrop photo of Proterozoic rocks in Newfoundland containing ediacaran beasties
(From Canadian Geographic Magazine)
Important terms/concepts from today’s lecture
(Google any terms that you are not familiar with)
DNA
Eukaryote
Mitochondrion
Chloroplasts
Acritarchs
Dinoflagellates
Nuclei
Organelles
Metazoans
Ediacaran fauna (elongate, frond-like,
discoidal forms)
Mantle (gastropods)
Notochord
Cambrian Explosion