3 - Earth History and Implications for Vertebrate Evolution

Zoology 224 Course Notes: Earth History and Implications for Vertebrate Evolution

Learning Objectives

The primary objectives of this course are to understand past continental configurations and explain how these configurations have affected climate, environment, and the evolution of vertebrates. A strong grasp of earth history is crucial for comprehending vertebrate evolution, as it provides a historical and geological context to examine development over time.

Geological Time Scale Overview

In ZOOL 224, it is essential to know the major eras of the geological time scale, which include:

  • Precambrian

  • Palaeozoic

  • Mesozoic

  • Cenozoic

While having a detailed knowledge of all epochs and ages is not necessary for this course, important epochs will be emphasized markedly in bold font in this section. It is advisable to download a geological time scale for reference, such as the one available at the Geological Society of America's website (https://www.geosociety.org/documents/gsa/timescale/timescl.pdf).

Terminology Note

  • Upper/Lower: Refers to sedimentary strata.

  • Early/Late: Used when describing the time of deposition.

  • Capitalization of these terms follows the Geological Society of America designations for clarity.

Earth History and Vertebrate Evolution

Origins of Vertebrates

  • Vertebrates originated during the Palaeozoic Era, about 540 million years ago (mya), and continue to evolve today. Throughout this era, continental arrangements significantly affected climate and localized habitats.

  • The shifting positions of continents influenced vertebrate evolution through factors such as:

    • Isolation of lineages that allowed for independent evolution

    • Loss of isolation, allowing competitors to invade previously isolated areas

  • Approximately 300 mya, all continents coalesced into a single landmass known as Pangaea, where terrestrial vertebrates first evolved. Pangaea began to split approximately 150 mya into northern (Laurasia) and southern (Gondwana) regions

Key Historical Earth Changes

  • Approximately 100 mya, South America and Africa became isolated until 18 mya, when Africa collided with Eurasia, facilitated by the Arabian landmass.

  • About 3 mya, the emergence of Central America formed a land bridge between North and South America.

  • The Great American Biotic Interchange occurred about 3 mya linked to the connection between North and South America. Marine vertebrates faced restriction due to the Great American Schism, as the Atlantic and Pacific Ocean connection was severed.

Climate Influences on Vertebrate Evolution

  • Changing oceanic circulation significantly impacts climate; for instance, the current restriction of the Arctic Ocean blocks warmer waters from reaching the Arctic, creating cooler climates characterized by extensive ice coverage.

  • The Atlantic Meridional Overturning Circulation (AMOC) drives the Gulf Stream, transporting warm marine waters northward to northern Europe while pushing cold waters south.

  • Melting glaciers from polar regions release freshwater, lowering the density of seawater and hindering the AMOC, with downstream effects on northern climates.

Changes in Sea Levels

  • Eustatic sea level changes directly influence land availability for organisms. Rising sea levels can flood continental areas, forming epicontinental and epeiric seas. This limits land for terrestrial life and can separate connected areas. For example, during the Cretaceous, the Western Interior Seaway (WIS) separated North America into eastern and western portions.

  • Conversely, the expansion of epeiric seas provides increased habitat for marine life. Epeiric rising sea levels also create climate changes, transitioning continental climates (characterized by dry and varied temperatures) to wetter, more maritime climates due to proximity to oceans.

Summary of the Palaeozoic Era

Time Frame: 541 – 252 million years ago

  • The Cambrian Period begins the Palaeozoic Era, with the ‘Cambrian Explosion’ marking the appearance of most major animal phyla in the fossil record, including Phylum Chordata.

  • The first vertebrates, jawless fishes (agnathans), emerged during this time, and the evolution of jaws followed, marking a significant advancement in vertebrate evolution.

  • The Devonian Period is recognized as the ‘Age of Fishes’, characterized by the abundance and diversity of early jawed fishes, including placoderms and acanthodians, as well as the origins of terrestrial vertebrates (early tetrapods).

  • The position of landmasses shifted significantly throughout the Palaeozoic, with continental placements influencing climate and habitat interaction.

  • Major climatic events include a transition from a hot dry climate to cooler, moister conditions marked by glaciation events about 450 mya, stimulating the rise of terrestrial plants, which contributed to further atmospheric changes by sequestering carbon dioxide.

Fossil Records and Evolutionary Developments in the Palaeozoic

  • Evidence for early vertebrate life, such as jawless fishes, is mainly derived from bone fragments. However, the Devonian yielded better fossil records, with significant findings of diverse jawed fishes and placoderm fossils from locations including North America and Australia.

  • Although placoderms thrived during the Devonian, they faced extinction by the end of the period, and two notable jawed vertebrate lineages, the acanthodians and Chondrichthyes (cartilaginous fishes), also flourished and underwent evolutionary changes.

  • In the terrestrial sphere, amphibians appeared and diversified, along with early representatives of amniotes (Predecessors to mammals and reptiles), marking the end of the Palaeozoic with the most catastrophic extinction recorded, where approximately 95% of marine organisms perished and a significant proportion of tetrapod families became extinct due to rapid warming and ocean acidification.

Mesozoic Era Overview

Time Frame: 252 – 66 million years ago

  • The Mesozoic is often synonymous with dinosaurs, but it also witnessed the evolution of various vertebrate lineages. This era marked the dominance of Sauropsida (dinosaurs, birds, and reptiles) while the Synapsida (ancestors of mammals) remained with diminished diversity compared to their contemporaries.

  • At the start of the Mesozoic, the continents formed Pangaea, permitting unrestricted movement of vertebrates across terrestrial environments. By the Jurassic, Pangaea divided into Laurasia and Gondwana, creating new marine and terrestrial ecosystems.

  • The climate during this era was characterized by warmth and higher sea levels, leading to expansive epicontinental seaways, facilitating diverse habitats.

  • The end of the Cretaceous saw the widespread extinction, notably among dinosaurs. However, many mammals and aquatic fishes successfully navigated the Cretaceous–Palaeogene (K–Pg) boundary.

Cenozoic Era Overview

Time Frame: 66 million years ago to present

  • Following the K–Pg extinction event, small mammals diversified, leading to the Cenozoic being termed the “Age of Mammals”. Environmental fragmentation resulted in accelerated divergence among vertebrate lineages, giving rise to many modern species and families.

  • Changes in climate and habitat emergence following the Late Mesozoic fragmentation compelled vertebrates to adapt to new environments, with significant climatic variations across different latitudes and ecosystems.

  • Two significant geological events in the late Cenozoic included:

    • Collision of Africa and Eurasia 18 mya, permitting faunal interchange across previously isolated regions.

    • Formation of the land bridge linking North and South America 3.5 mya, fostering intercontinental migrations but ultimately isolating certain marine faunas due to the Great American Marine Schism.

    • The Cenozoic period witnessed significant changes in vertebrate faunas, with many modern families establishing themselves by the Eocene period. Teleostei dominated the waters, while notable mammalian forms and unique species experienced rapid evolution but faced extinction before the present era.