EVOLUTIONARY TRANSITION FROM THE PRECAMBRIAN 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 Ediacarian Hills of South Australia. Additional discoveries of similar aged fossils elsewhere has initiated a debate on 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 VENDIAN (EDICARIAN, 575-543 Ma)

Soft bodied fauna from Adelaide Basin, Australia (~560 Ma)
most fossil casts and molds in sandstones
very rare earliest shelly fossils of problematic affinity
1500 specimens from type locality fossilized as molds and casts
30 species, 30 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.
Arguments persist as to whether these fossils are ancestors of the abundant invertebrates of the Cambrian.
simple two dimensional trace fossils
younger than Proterozoic tillites (of Varangian glaciation 610-590 Ma) - other Ediacarian assemblages in Africa, Scandanavia, N.A., China & elsewhere but rare.
Exposures in N. America appear on the edge of the craton (e.g. Great Basin and Newfoundland)
P. Cloud: proposed Ediacarian Period beginning with earliest appearance of soft-bodied metazoans & ending with 1st appearance of skeletonized metazoans (base of Cambrian Period)

until 1995 a gap between Ediacarian assemblages and the base of the Cambrian
this gap now filled by new finds in Namibia where Ediacarian type assemblages found up to the Cambrian boundary
Ediacarian assemblages diversify through time
small shelly fossils present by the end of the Vendian

III. PHASE II: FIRST APPEARANCE OF LOW DIVERSITY SHELLY FAUNAS

THE NEMAKIT-DALDYNIAN STAGES (543-530Ma)

the appearance of small shelly faunas (SSFs)

SSFs appear sequentially and diversify through a period of some 3-6 m.y.

initial complex three-dimensional trace fossils

small, chitinous, calcareous, and phosphatic metazoans of problematic affinity

IV. PHASE III: THE RISE OF DIVERSIFIED METAZOANS WITH SKELETONS

THE TOMMOTIAN STAGE

Tommotian- Adtabanian Stages (530-525 Ma)
base represents the earliest abundant and diverse small skeletonized assemblage
stage stratotype in Siberia
great diversity of SSFs
first archeocyathids, primitive molluscs, possible gastropods, inarticulate brachiopods, sponges.
Advantages of skeleton:

adaptation provides an advantage for bottom dwelling organisms
leverage,
articulation,
protection,
size increase

time of changing conditions

continental breakup and separation
marine transgression
rising oxygen levels
falling carbon dioxide levels
upwelling, high phosphorous levels (large phosphorite deposits)
skeletons of calcium phosphate
appearance of predation
appearance of grazers (e.g. gastropods)
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

first trace fossils?
first shelly fauna?
first complex trace fossils?
first abundant and diverse shelly fauna

VI. POST-TOMMOTIAN CAMBRIAN

rise of herbivore grazers (e.g. gastropods), decline of algal stromatolites
dominance of trilobites (75% of species)
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

VII. See and learn more about Vendian Animals