Deuterostomes Part 2: Tetrapods – Structure, Evolution, and Diversity

The Deuterostomes: Overview

Introduction to Deuterostomes

Deuterostomes are a major lineage of animals distinguished by their embryonic development, in which the anus forms before the mouth. This group includes both invertebrate and vertebrate phyla, with significant evolutionary innovations.

• Key Deuterostome Groups: Echinodermata, Hemichordata, Chordata

• Chordata Subgroups: Cephalochordata, Urochordata, Vertebrata

Non-Vertebrate Deuterostomes

Echinodermata

• Examples: Sea stars, sea urchins, sea cucumbers, brittle stars, feather stars

• Key Features: Radial symmetry (as adults), water vascular system, calcareous endoskeleton

Cephalochordata

• Common Name: Lancelets

• Characteristics: Small, fish-like marine animals; retain notochord throughout life; filter feeders

Urochordata

• Common Name: Tunicates

• Characteristics: Larvae resemble chordates; adults are sessile filter feeders; notochord present only in larval stage

Agnathans: Jawless Vertebrates

Myxini (Hagfishes)

• Features: Cartilaginous skull, lack jaws and vertebrae, produce slime as defense, marine bottom-dwellers

Petromyzontiformes (Lampreys)

• Features: Cartilaginous segments around notochord, some parasitic, found in marine and freshwater habitats

Gnathostomes: Jawed Vertebrates

Major Clades

• Chondrichthyes: Cartilaginous fishes (sharks, rays)

• Osteichthyes: Bony fishes, including Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes)

Key Innovations

• Jaws and mineralized skeleton

• Lungs or lung derivatives

• Lobed fins (precursors to limbs)

Tetrapoda: The Origin of Limbs

Key Tetrapod Adaptations

• Four limbs and feet with digits

• A neck for independent head movement

• Fusion of pelvic girdle to backbone

• Loss of gills in non-aquatic species

• Ears for detecting airborne sounds

Evolutionary Transition

• Tetrapods evolved from lobe-finned fishes (Sarcopterygii)

• Early tetrapods were aquatic and 'fish-like'

• Key fossil: Tiktaalik roseae shows both fish and tetrapod traits (e.g., fins with wrist bones, neck, ribs)

Cladograms and Nested Subsets

Cladograms illustrate evolutionary relationships, showing tetrapods as a subset of bony fishes (Osteichthyes).

Lissamphibia: Modern Amphibians

Major Clades

• Caudata (Urodela): Salamanders (~750 species)

• Salientia (Anura): Frogs (>7,000 species)

• Gymnophiona (Apoda): Caecilians (~200 species)

Salamanders

• Some lack lungs (respire through skin)

• Paedomorphosis: retention of larval traits in adults (e.g., axolotl)

Frogs

• Powerful hind legs, lack tails as adults

• Some species survive freezing temperatures (e.g., Rana sylvatica)

Caecilians

• Legless, burrowing, nearly blind

• Chemo-sensory tentacles for environmental detection

Life History and Ecology

• Amphibians often have aquatic larvae and terrestrial adults ('both ways of life')

• Metamorphosis: transition from larva to adult

• Some frogs show direct development (no larval stage)

Amphibian Decline

• Chytrid fungi (Batrachochytrium dendrobatidis and B. salamandrivorans) cause chytridiomycosis, a major factor in global amphibian declines

• Fungi attack keratinized skin, leading to death in adults

Amniota: The Amniotic Egg and Terrestrial Adaptation

Definition and Diversity

• Amniotes are tetrapods with an amniotic egg, including reptiles (birds are nested within reptiles) and mammals

• Over 25,000 living species; fossil record extends over 300 million years

Amniotic Egg Structure

• Contains extraembryonic membranes: amnion, chorion, allantois, yolk sac

• Protects embryo and allows development on land

Membrane	Function
Amnion	Encloses embryo in fluid
Chorion	Gas exchange
Allantois	Stores waste, gas exchange
Yolk sac	Provides nutrients

Key Innovations

• Amniotic egg (sometimes shelled, sometimes retained internally)

• Less permeable skin

• Rib cage ventilation of lungs

Summary Table: Major Tetrapod Groups

Group	Key Features	Examples
Lissamphibia	Moist skin, aquatic larvae, metamorphosis	Frogs, salamanders, caecilians
Reptilia	Scaly skin, amniotic egg, ectothermy (except birds)	Lizards, snakes, turtles, crocodilians, birds
Mammalia	Hair, mammary glands, endothermy	Humans, whales, bats, monotremes

Key Terms and Concepts

• Deuterostome: Animal whose anus forms before the mouth during development

• Notochord: Flexible rod supporting the body in all embryonic and some adult chordates

• Paedomorphosis: Retention of juvenile features in the adult form

• Amniotic Egg: Egg with specialized membranes for terrestrial development

• Cladogram: Diagram showing evolutionary relationships

Additional info: The notes above integrate and expand upon the provided slides, adding definitions, examples, and summary tables for clarity and completeness.