EVOLUTIONARY TRANSITION FROM
THE PRECAMBRAIN TO CAMBRIAN
I. INTRODUCTION
Prior to discovery of Pre-Cambrian fossils of soft-bodied
organisms in Australia (1947), the pre-Paleozoic was known as the Azoic
(without life). All this changed with the discovery of a soft-bodied fauna
in the Ediacarin Hills of South Australia. Additional discoveries of similar
aged fossils elsewhere has initiated a debateon how to define the boundary
between the Cambrian and Late Proterozoic. Today we discuss the nature
of the three phase evolutionary transition between the Precambrian and
Cambrian.
II. PHASE I. THE EDICARIAN
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Soft bodied fauna from Adelaide Basin, Australia
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most fossil casts and molds in sandstones
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very rare earliest shelly fossils of problematic affinity
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1500 specimens from type locality fossilized as molds and casts
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30 species, 20 genera, 2/3 of taxa have a close affinity to coelenterates
("jelly-fish forms"), 1/4 of taxa are ~annelid worms "sea-pens", primitive
arthropods.
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Arguments persist as to whether these fossils are
ancestors of the abundant invertebrates of the Cambrian.
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simple two dimensonal trace fossils
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younger than Proterozoic tillites (glacials) - other Ediacarian assemblages
in Africa, Scandanavia, N.A., China & elsewhere but rare.
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Exposures in N. America appear on the edge of the craton (e.g. Great Basin
and Newfoundland)
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P. Cloud: proposed Ediacarian Period ~670-570 Ma beginning with earliest
appearance of soft-bodied metazoans & ending with 1st appearance of
skeletonized metazoans (base of Cambrian Period)
III. PHASE II: FIRST APPEARANCE OF LOW DIVERSITY SHELLY FAUNAS
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the appearance of shelly faunas not as sudden as once thought
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between ~570 and ~550 Ma a low diversity assemblage of shelly faunas have
been found
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initial complex three-dimensional trace fossils
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small, chitinous, calcareous, and phosphatic metazoans of problematic affinity
IV. PHASE III. THE TOMMOTIAN STAGE
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base represents the earliest abundant and diverse small skeletonized assemblage
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stage stratotype in Siberia
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first archeocyathids, primitive molluscs, possible gastropods, inarticulate
brachiopods, sponges.
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Advantages of skeleton:
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adaptation provides an advantage for bottom dwelling organisms
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leverage,
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articulation,
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protection,
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size increase
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time of changing conditions
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continental breakup and separation
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marine transgression
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rising oxygen levels
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appearance of predation
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appearance of grazers (e.g. gastropods)
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changing ocean chemistry, promoting biomineralization
V. THE PRECAMBRIAN-CAMBRIAN BOUNDARY
Class discussion of how to define the Precambrian/Cambrian boundary.
Topics:
1. What makes a good stratotype
2. How to define the boundary
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first trace fossils?
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first shelly fauna?
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first complex trace fossils?
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first abundant and diverse shelly fauna
VI. POST-TOMMOTIAN CAMBRIAN
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rise of herbevore grazers (e.g. gastropods), decline of algal stromatolites
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dominance of trilobites (75% of species)
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brachiopods (15-20%) phosphatic shells
by the end of the Cambrian, all living phyla had appeared (except perhaps
bryozoans) that are now well skeletonized - all arose from ancestral worm
groups.
VII. Links