Elmer lecture 7 amniotes amphibians upload

Major Transitions: Amphibians and the Colonisation of Land

• Presented by: Dr. Kathryn Elmer

• Course: L2A 'Radiations and Extinctions'

Class Overview

• Deep Phylogeny of Early Vertebrates

  • What is an amniote?

  • Key innovations in amniotes

• Focus on Amphibians

  • Early amphibians

  • Modern amphibians (Lissamphibia: caecilians, caudates, anura)

  • Key innovations in modern amphibians

Evolution of Amphibians

• Historical Context

  • During the Devonian and Carboniferous periods, tetrapod lineages diversified, leading to early amphibians.

  • Most early lineages eventually became extinct.

  • Amphibians and amniotes diverged approximately 370 million years ago.

Defining Amniotes

• Anamniotes: Includes fishes and amphibians; typically lay eggs in water.

• Amniotes: Derived from the Greek term for the "membrane surrounding the fetus."

  • Comprises reptiles, birds, and mammals.

  • Key Innovation: Ability to lay eggs on land rather than in water, which represents a significant evolutionary adaptation.

Geologic Time Scale of Amphibians

• Overview of the significant periods leading up to amphibian diversification and extinction events.

  • Paleozoic to Mesozoic eras saw major developments in amphibian evolution.

Phylogenetic Relationships

• Lissamphibia (modern amphibians) encompasses:

  • Caecilians (Gymnophiona): Legless, fossorial amphibians.

  • Salamanders (Caudata): Characterized by reduced skulls and developed tails, primarily found in the Northern Hemisphere.

  • Frogs and Toads (Anura): Notable for their diverse adaptations and reproductive strategies.

Evolutionary Relationships

• Relationships with Other Groups

  • Importance of ecological niche openings post-extinction events.

• Extinct Amphibian Groups

  • Example: Gerobatrachus, a transitional fossil linked to modern amphibians.

Key Innovations in Amphibians

• Innovations that have contributed to amphibian diversity and success:

  • Reproductive Strategies: Includes various modes of fertilization and development.

  • Defensive Toxins: Anti-predator adaptation prevalent in many salamanders, newts, and toads.

  • Adaptations to Habitat: Such as direct development in some species, allowing for survival in terrestrial environments.

Fossil Record and Modern Diversity

• Fossil Evidence: Poorly represented in certain groups (e.g., caecilians) but essential for understanding the evolutionary history of amphibians.

• Ecological Diversification Post-K-Pg Extinction: The Cretaceous-Paleogene extinction event opened new ecological niches that facilitated diversification, particularly among frogs.

Reproductive Strategies in Frogs and Toads

• Diverse Modes of Reproduction: Adaptations such as the gastric brooding of certain species, which allows for unique life cycles.

• Notably, the Darwin's frog incubates young in male vocal sacs.

Summary of Key Points

• Overall Themes:

  • The complex phylogenetic history of amphibians from early tetrapods to modern lissamphibians.

  • Innovations that allowed amphibians to thrive in new environments and their ongoing evolution.

  • The importance of fossil records in understanding amphibian diversification and extinction events.