Chapter 17

Chapter 17: Phanerozoic Eon – A Trip through Time

Introduction

  • Review of all major mass extinctions within the Phanerozoic Eon.

  • Timeline: Phanerozoic Eon extends from 542 million years ago (mya) to today.

  • Definition: The word 'Phanerozoic' means 'abundant life', indicating the presence of significant fossil evidence in rock records.

  • Note: The majority of literature focuses on five major mass extinctions, often disregarding extinctions in Precambrian time.

  • Key extinctions discussed include:
      - End-Permian extinction of 251 mya
      - End-Cretaceous extinction of 66.5 mya
      - Holocene extinction currently occurring.

  • The chapter will explore tectonics, the progression of life, and the timing and causes of mass extinctions, with preferences stated after each discussion.

  • Reference to Geological Time Scale provided in Unit 4 intro file.

  • Paleozoic Era is the first subdivision of the Phanerozoic Eon, encompassing the following periods:
      - Cambrian
      - Ordovician
      - Silurian
      - Devonian
      - Carboniferous
      - Permian

  • Ending the Paleozoic Era is known as "The Mother of all Catastrophes" (end of Permian), detailed in the next chapter.

Cambrian Period (542 – 488 mya)

Plate Tectonics and Life

  • Previous existence of supercontinent Rodinia from 1100 – 750 mya, which began to disintegrate around 750 mya.

  • Breakup of Rodinia may have triggered Snowball Earth events.

  • Evidence from this time is rare and interpretations of available evidence vary.

  • Rodinia was largely barren with only early life in seas; no terrestrial life present.

  • Cambrian seas advanced over Rodinia fragments, creating shallow, warm bodies of water conducive to life explosion.

  • Rapid emergence of diverse, complex organisms noted; Charles Darwin remarked on this biodiversity explosion.

  • Notable study region: Cambrian Burgess Shale of British Columbia.

  • Example organism: Anomalocaris (Fig. 2) - a predator drawing significant attention due to its features.

  • New studies show rapid evolution of complex life forms from Precambrian organisms rather than abrupt emergence.

  • Factors facilitating this explosion:
      - Availability of prerequisites for complex multicellular life developed in Precambrian.
      - Expansion of suitable habitats in newly created shallow and nutrient-rich seas.

Cambrian Extinctions

  • High background extinction rates observed as biodiversity surged; intense competition developed.

  • Two distinct periods of mass extinctions occurred during Cambrian:
      - One in Middle Cambrian
      - One in Late Cambrian

  • Each extinction accounted for a loss of approximately 40% of marine genera.

  • These extinctions were not classified among the 'big five.'

  • Proposed hypotheses for extinction causes:
      - Glacial cooling: Hypothesis by Miller suggests marine species evolved in warm waters intolerant of cold, supported by evidence of glaciation in South American Cambrian rocks.
      - Oxygen depletion: Linked to ocean water overturn introducing anoxic waters to the surface, though lacking observational support.

  • Personal preference stated for glacial cooling and lowering of sea levels.

Ordovician Period (488 – 444 mya)

Plate Tectonics

  • Formation of the supercontinent Gondwana, crucial to the assembly of future Pangaea.

  • Early Ordovician: Gondwana located in equatorial waters, gradually drifting south and becoming glaciated by late Ordovician (Fig. 4).

  • Subduction during assembly led to volcanic activity emitting carbon dioxide, creating a greenhouse effect, with marine waters possibly reaching around 45°C.

  • As Gondwana drifted and glaciers formed, volcanism decreased, causing climate cooling and ocean temperatures to stabilize.

  • Late Ordovician saw Laurentia and Baltica moving towards each other, provoking significant volcanic activities.

Ordovician Life and Extinctions

  • Despite climatic fluctuations, marine life thrived, but the end of Ordovician marked a series of mass extinctions accounting for the loss of about 50% of marine genera and 85% of all species.

  • Sedimentary rock evidence during 447-444 mya indicates sudden, intense glaciations ended greenhouse conditions.

  • Low sea levels due to glacier growth led to the elimination of shallow water niches. Extinctions were rapid and catastrophic.

  • Preference: lack of sufficient evidence to ascertain specific extinction causes.

Silurian Period (444 – 416 mya)

Life During the Silurian

  • Gondwana remained at the south pole, exhibiting less extensive ice sheets than in the Ordovician, leading to raised sea levels and warm shallow seas.

  • Geological activity decreased, resulting in a relatively stable environment.

  • Climate characterized as mild, creating abundant diversity in marine life, and the beginnings of land colonization with forests of moss appearing near shorelines.

Devonian Period (416 – 359 mya)

Tectonics

  • Continental nucleus representing modern Europe and North America unified. By late Devonian, forms the foundation of a potential supercontinent, Pangaea. (Fig. 7)

  • Increased seas flooding continental margins due to tectonic activity.

Climate

  • Generally, a temperate climate with limited glaciations; however, there was significant cooling during the Mid-Devonian.

Life During the Devonian

  • Known as the Age of Fish, experiencing dramatic biodiversity expansion.

  • Notable predatory species included Dunkleosteus measuring up to 10 m, weighing 3.6 tonnes and preying primarily on other marine life. (Fig. 8)

  • Colonization of land featured prominently, including trees that could reach 8 m in height and development of the first forests.

  • Plants began forming seeds and further spread throughout land environments.

Mass Extinction Events

  • Series of extinctions began at the middle of the Late Devonian, leading to the loss of over 50% of marine genera (approximately 80-85% of marine species).

  • Extinctions transpired over 10 million years from 370 mya to 360 mya.
      - Causes:
         - Initial extinction associated with impact evidence suggested by deposits containing Iridium, tektites, and shocked mineral grains.
         - Second extinction believed a result of anoxic ocean waters caused by nutrient influx leading to algal blooms and subsequent oxygen depletion.

  • Notable gap in fossil records termed Romer's Gap indicated a long recovery period post-extinction.

  • Personal hypotheses: asteroid impacts for 370 mya and anoxic seas for 360 mya.

Carboniferous Period (359 – 299 mya)

Geological Context

  • Characterized by glaciation (Icehouse climate), low sea levels, and extensive mountain building.

  • Continental blocks approached final assembly into Pangaea during the late Carboniferous.

Climate and Life

  • The early Carboniferous climate remained warm; however, southern Gondwana began a cooling trend as it settled at the South Pole.

  • Conifer trees evolved during this period, thriving under dryer climates and contributing to significant coal deposits.

  • Evolution led to amphibians abundant both on land and in seas.

  • Minor extinction event in the middle Carboniferous linked to climatic and sea level changes without impacting overarching evolutionary directions.

Permian Period (299 – 251 mya)
Geological Overview

  • Completion of land assembly into Pangaea, resulting in variable dry climates.

  • Marine and terrestrial life thrived, fostering significant biodiversity post-Carboniferous.

Flora and Fauna

  • Evolution of conifer trees suited to dry climates became widespread.

  • Fauna included a wide diversity of aggressive species including ancestral reptiles and the first completely terrestrial organisms.

Great Permian Extinction

  • Marked by an unprecedented mass extinction event eliminating approximately 96% of marine species and 70% of terrestrial species.

  • Misleading statistics: 30% of terrestrial species remaining post-event still suggests 99.5% of individual members of surviving species were lost.

  • Chapter 18 will provide detailed insights into the causes of this catastrophe.

Triassic Period (251 – 201.6 mya)

Geological Changes

  • Separation of Pangaea commenced but at a slow pace, starting with North America drifting away (Fig. 14).

  • Creation of new shallow water environments through rifting activities conducive to biological innovations.

Climate Conditions

  • The prevalent climate during the Triassic remained hot and dry, with no glaciations.

Life and Extinctions

  • Mesozoic Era termed the age of reptiles; emergence of diverse reptilian species including early dinosaurs and pterosaurs.

  • Extinction event occurring at the end of Triassic involved substantial losses, particularly among marine organisms. Current hypotheses imply vast shifts in climatic conditions and volcanic activity as potential causes.

Jurassic Period (201.6 – 145.5 mya)

Geological Setting

  • Continued rifting of Pangaea; the new landmasses became identifiable as current continents (Fig. 16).

Life During Jurassic

  • Characterized as the Golden Age of Dinosaurs, with moderate background extinctions occurring.

  • Plentiful marine biodiversity and elevated temperatures due to high levels of greenhouse gases.

Cretaceous Period (145.5 – 65.5 mya)

Geological and Biological Context

  • The last and longest period of Mesozoic (80 my) marked by continuous high sea levels and rich biodiversity.

  • The term 'Cretaceous' refers to the era's chalky deposits derived from marine organisms (Fig. 19).

  • Pivotal for marine life and the establishment of contemporary geological deposits.

Extinction Event

  • Commencement of the extinction event marking the end of the Cretaceous approximately 65.5 mya; critical for the transition into the Cenozoic Era.

Cenozoic Era (65.5 mya – now)

Tertiary Period (65.5 – 2.6 mya)

  • Movement of continents to present locations; gradual cooling towards an Icehouse climate with continental ice sheet developments by around 35 mya.

  • Post-Cretaceous-Tertiary mass extinction facilitated the diversification and dominance of mammals.

  • Few instances of marine extinctions compared to high mammal diversification.

Quaternary Period (2.6 mya – now)

  • Continued Icehouse conditions marked the Quaternary with glacial/interglacial cycles.

  • The top subdivision, Holocene Epoch, identifies the period from roughly 12,000 years ago to today, correlating with human development and dominance.

Holocene Mass Extinction

  • Indicates a current ongoing extinction event associated with human impact; detailed in Chapter 20.

Summary of Important Facts:

  • Detailed guide for understanding significant mass extinction causes and environmental contexts during each period in the Paleozoic and Mesozoic eras.
      - Emphasis on major extinction hypothesized causes such as environmental cooling and anoxic waters, among others.
      - Comparative analysis of life adaptations and biodiversity impacts across geological timescales.