Precambrian-Paleozoic Boundary

INTRODUCTION

• INITIALLY EASILY RECOGNIZED BY THE FIRST OCCURRENCE OF FOSSILS (SHELLED, MULTICELLULAR ANIMALS [TRILOBITES, BRACHIOPODS])
  • Evolutionary Problem - Where are their ancestors?
  • Hiatus between Precambrian & Cambrian in continental interiors
  • Later workers found continental margin sedimentary sections that continued more-or-less continuously beneath the Cambrian
• ISSUE REVERSED NOW (see below)

LATE PROTEROZOIC TECTONICS & CLIMATE

• WIDESPREAD CONTINENTAL RIFTING
• EARLY PART OF LATE PROTEROZOIC - WIDESPREAD GLACIATION
  • Low atmospheric CO2?
• LATER PART OF LATE PROTEROZOIC - WARMER
  • Continents distributed along the Tropics
  • Increased atmospheric CO2?

LATE PROTEROZOIC-EARLY CAMBRIAN LIFE EVENTS & FOSSIL RECORD

• MULTICELLULAR ANIMALS (METAZOANS) EVOLVED AT THE END OF THE PROTEROZOIC
  • Pound Quartzite - <600 my old (Ediacara Hills of Southern Australia) - first discovery of SOFT-BODIED METAZOANS - as impressions
    • CALLED EDIACARA ASSEMBLAGE, OR FAUNA
    • EDIACARA FAUNA OCCURS ON ALL CONTINENTS EXCEPT ANTARCTICA (Namibia, England, Scandinavia, Russia, Siberia, China, & N. America)
    • INCLUDES WHAT ARE THOUGHT TO BE JELLYFISH, & SOFT CORALS, WORMS, & PRIMITIVE ARTHROPODS & ECHINODERMS (see Fig. 9-18, p. 225)
      • Some workers consider Ediacara Fauna as unrelated to modern metazoans
      • Adapted to nutrient-poor environments and low atmospheric oxygen level (5-10% present) - flat with large surface area to volume ratio
  • Small, calcareous, TUBE-SHAPE SHELLED FOSSILS & SIMPLE TRACE FOSSILS are associated with the Ediacara Fauna
• LOW-DIVERSITY ASSEMBLAGES OF SMALL SHELLY FOSSILS REPLACE EDIACARA FAUNA - ~550 my ago
  • Geographically-widespread (Australia, India, China, Mongolia, Siberia, Kazakhstan, Iran, & N. America)
  • Chitinous, Calcareous, or Phosphatic Shells
  • Many of the small tubular & conical shells were sclerites (embedded in or on surface of animal for protection)
  • Small shelly fauna are accompanied by COMPLEX TRACE FOSSILS
• MODERATE-DIVERSITY TOMMOTIAN FAUNA EVOLVE DURING THE EARLIEST CAMBRIAN - ~543-538 my ago
  • Also geographically-widespread (Australia, China, Mongolia, Siberia, Kazakhstan, Russia, Poland, Scandinavia, England, & N. America)
    • TOMMOTIAN STAGE STRATOTYPE IN SW SIBERIA
  • Also Chitinous, Calcareous, or Phosphatic Shells
  • Again, shells were sclerites
  • Includes Inarticulate Brachiopods, Archaeocyathids, & primitive Mollusks (including Hyolithids)
• HIGH-DIVERSITY CAMBRIAN FAUNA EVOLVE FOLLOWING THE TOMMOTIAN STAGE - ~538 my ago
  • Trilobites evolve, and Inarticulate Brachiopods & Archaeocyathids diversify
• SHELLY FAUNAS OF INCREASING DIVERSITY REPRESENT AN ADAPTIVE RADIATION
  • New phylla & classes evolved & diversified rapidly
  • Probably resulted from:
    • 1. INCREASING O2 & CO2 LEVELS
    • 2. CONTINENTAL FRAGMENTATION
    • 3. END OF GLACIATION
    • 4. FLOODING OF CONTINENTS
    • 5. EVOLUTION OF HERBIVORES TO FEED ON CYANOBACTERIAL MATS
    • 6. EVOLUTION OF PREDATORS
• SEVERAL GEOCHEMICAL FLUCTUATIONS ASSOCIATED WITH PRECAMBRIAN-CAMBRIAN BOUNDARY
  • Sulfur & carbon isotope
    • REFLECT CHANGES IN PRODUCTIVITY & BURIAL OF SULFIDES IN RESTRICTED ENVIRONMENTS
  • Currently being used for stratigraphic correlation

VENDIAN SYSTEM/PERIOD

• VENDIAN PROPOSED NEW SYSTEM ENCOMPASSING PERIOD FROM <600 TO 550 MY & INCLUDED AS PART OF THE PHANEROZOIC
  • lies between youngest Late Proterozoic tillites & Tommotian stage
• PRECAMBRIAN-CAMBRIAN BOUNDARY IS NOT AN EVOLUTIONARY PROBLEM NOW, BUT RATHER A STRATIGRAPHIC PROBLEM - WHERE TO PUT IT?
  • Current favorite for Precambrian-Cambrian boundary: first appearance of COMPLEX TRACE FOSSILS & ASSOCIATED LOW-DIVERSITY SMALL SHELLY FOSSILS, instead of at the base of the Tommotian stage

Study Questions

1. What were the general characteristics of Late Proterozoic tectonics & climate?

2. What are the general characteristics of the Late Proterozoic fossil record?

3. What are the characteristics of the Ediacarian Fauna & the rock unit in which it was discovered?

4. Summarize the characteristics of the small shelled fossils & trace fossils associated with the Ediacarian Fauna.

5. Summarize the characteristics of the low-diversity assemblages of small shelly fossils that replace the Ediacara Fauna & the trace fossils associated with these small shelly fossils.

6. What are the general characteristics of the Early Cambrian fossil record?

7. What are the characteristics of the Tommotian Fauna?

8. What are the characteristics of the classical Cambrian Fauna?

9. Summarize the probable causes of this Ediacarian-Cambrian adaptive radiation.

10. Summarize the problems with defining the Precambrian-Cambrian boundary.

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