bio20 Chapter 32

Chapter 32: An Overview of Animal Diversity

Introduction

• Focuses on animal diversity and characteristics defining animals

• Presented by Nicole Tunbridge and Kathleen Fitzpatrick

Key Concepts of Animal Characteristics

General Characteristics

• Animals are multicellular, heterotrophic eukaryotes.

• Develop from embryonic layers.

• Tissues are formed from layers of embryonic cells.

• Besides defining features, exceptions exist for most criteria defining animals.

Nutritional Mode

• Animals are heterotrophs (unlike plants that are autotrophs) and obtain energy by consuming other organisms.

• Distinction from fungi: animals digest food internally, while fungi digest externally.

Animal Cell Structure

Cell Characteristics

• Animals consist of eukaryotic cells without cell walls; instead, they are supported by structural proteins like collagen.

• Unique animal tissues include nervous and muscle tissues.

Tissues and Functions

• Tissues: groups of similar cells acting as functional units.

Reproduction and Development

Reproductive Traits

• Primarily sexual reproduction; diploid stage dominates the life cycle.

• Diploid zygotes undergo cleavage without growth.

• Formation of blastula followed by gastrulation.

Development Stages

• Most animals exhibit at least one larval stage that is morphologically and behaviorally distinct from adults.

• After metamorphosis, larvae resemble adults but are sexually immature.

Genetic Control

• All animals possess developmental genes, notably Hox genes, regulating morphology and gene expression.

Evolutionary History of Animals

Timeline and Evidence

• History spans over 770 million years; 1.3 million living species identified.

• Chemical evidence of early sponges found in 710 million year old sediments.

Origin of Multicellular Animals

• Closest living relatives to animals are choanoflagellates.

• Morphological evidence indicates the common ancestor of animals resembled choanoflagellates.

Ediacaran Period

Fossil Records

• First accepted macroscopic animal fossils date from about 560 million years ago (Ediacaran biota).

• Some fossils classified as molluscs, sponges, or cnidarians; early evidence of predation observed.

Ediacaran Animals

• Not all fossils resemble modern organisms, yet some display predation adaptations.

Cambrian Explosion

Rapid Diversification

• Occurred between 535-525 million years ago; marks significant diversification of animal forms.

• Introduction of large fossils with hard skeletons.

• Most fossils were bilaterians, distinct with bilateral symmetry and complete digestive tracts.

Factors Influencing Cambrian Diversity

• New predator-prey relationships.

• Increased atmospheric oxygen.

• Evolution of regulatory gene complexes.

Body Plans of Animals

Symmetry

• Animals can be compared based on symmetry: radial or bilateral. Radial symmetry is often linked to a sessile lifestyle, while bilateral symmetry correlates with active movement.

Tissue Organization

• Animals are grouped into diploblastic (two germ layers: ectoderm, endoderm) and triploblastic (with a mesoderm layer) based on tissue organization.

Developmental Modes

Protostome vs. Deuterostome

• Two categories of developmental modes exist:

  • Protostome Development: Spiral and determinate cleavage; mouth forms from the blastopore.

  • Deuterostome Development: Radial and indeterminate cleavage; anus develops from the blastopore.

Coelom Formation

• Coelom forms in protostomes by solid mass splitting; in deuterostomes, it forms from archenteron wall.

Phylogenetic Patterns

Animal Relationships

• All animals share a common ancestor, with sponges as the basal group.

• Eumetazoa includes all animals with tissues.

• Most phyla belong to Bilateria, with the chordates being the sole vertebrate group.

Cladal Divisions

• Bilaterians distributed into three clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa.

• Ecdysozoans secrete exoskeletons and shed during growth (ecdysis).

• Lophotrochozoans characterized by lophophore feeding structures and trochophore larval stages.

Future Directions in Animal Systematics

• Current research questions whether ctenophores are basal metazoans and whether acoelomate flatworms constitute basal bilaterians.

• Systematics is an evolving discipline influenced by continuous research and discoveries.