Animals Part 2

ANIMALS PART II

TIMELINE / EVOLUTIONARY HISTORY OF ANIMALS

  • Cambrian Explosion: 535−525 million years ago (mya)

  • Ediacaran Animals: 560 mya

  • Early Land Animals: 365 mya
      - Origin and diversification of dinosaurs
      - Diversification of mammals

  • Geological Eras:
      - Neoproterozoic: 1000-542 mya
      - Paleozoic: 542-251 mya
      - Mesozoic: 251-65.5 mya
      - Cenozoic: 65.5 mya to Present

ANIMAL DIVERSITY

  • Over 1 million species named to date; many more exist.

  • Most named species are arthropods.

  • Common ancestor of all living animals probably existed between 700 and 770 mya.

NEOPROTEROZOIC ERA

  • Duration: 1 billion-542 mya

  • Early animal fossils found within this era include:
      - Ediacaran Biota: approximated to be around 560 mya.
      - Evidence of early animal embryos and predation have been discovered within Neoproterozoic rocks, showcasing biological interaction in early life.

PALEOZOIC ERA

  • Duration: 542-251 mya

  • Cambrian Explosion: Occurred from 535 to 525 mya; marked the earliest fossil appearance of many major groups of living animals (extant phyla).
      - Most fossils from the Cambrian explosion consist of bilaterians, characterized by:
        - Bilaterally symmetric form
        - Complete digestive tract
        - One-way digestive system

Cambrian Critters from the Burgess Shale

  • Choia: a type of sponge

  • Pikaia: an early chordate

  • Olenoides: a trilobite

  • Aysheaia: a velvet worm

EVOLUTIONARY HISTORY SUMMARY

  • Animal diversity increased throughout the Paleozoic, although it experienced interruptions due to mass extinctions.

  • By approximately 450 mya, animals began to affect terrestrial ecosystems.

  • Vertebrates transitioned to land around 365 mya.

CENOZOIC ERA (FROM 65.5 MYA TO PRESENT)

  • The Cenozoic era began after extensive mass extinctions affected both terrestrial and marine life, notably wiping out non-avian dinosaurs and marine reptiles.

  • Post-extinction, mammals thrived, increased in size, and occupied many available ecological niches.

ANIMAL CHARACTERIZATION

Body Plans

  • Defined as a set of morphological and developmental traits.

  • Symmetry types:
      - Radial Symmetry: A form of symmetry where body parts radiate from a central point; appears to have evolved first in the animal kingdom.
      - Bilateral Symmetry: Recognized by having:
        - Dorsal side (top)
        - Ventral side (bottom)
        - Left and right sides
        - Anterior end (head region)
        - Posterior end (tail region)
        - Often includes sensory structures, such as a brain, primarily concentrated in the anterior end.

Germ Layer Development

  • All triploblastic animals exhibit bilateral symmetry:
      - DIPLOBLASTIC:
        - Germ layers: ectoderm (outer) and endoderm (inner)
      - TRIPLOBLASTIC:
        - Germ layers: ectoderm, mesoderm (middle), and endoderm

Classification based on Tissue Types

  • Ectoderm: Germ layer that forms the outer covering and nervous system.

  • Endoderm: Innermost germ layer that lines the developing digestive tube (archenteron), gut, and other digestive organs.

  • Mesoderm: Intermediate layer contributing to the circulatory system, muscle, internal structures, bones, and most organs.

TISSUES

  • Definition: Tissues are collections of specialized cells, separated from other tissues by membranous layers.

  • Sponges: Lack true tissues.

  • Diploblastic Animala: E.g., Cnidaria, Ctenophora, having an ectoderm and endoderm; possess a gel-like middle layer termed mesoglea.

BODY CAVITIES

  • Most triploblastic animals have a body cavity:
      - True body cavity: Coelom, derived from mesoderm.
      - Coelomates: Animals with a true coelom.
      - Pseudocoelomates: Organisms with a similar body structure but not classified as coelomates—illustrating that a grade (similar body plans) is not necessarily a clade (includes all descendants of a common ancestor).

EMBRYONIC DEVELOPMENT

Classification of Development Types

  • Animals categorized as:
      - Protostome Development: Where the blastopore (first hole formed during early development) becomes the mouth.
      - Deuterostome Development: Where the blastopore converts into the anus, and a second opening becomes the mouth.

  • Etymology:
      - “Protostome” means “first mouth”.
      - “Deuterostome” means “second mouth”.

Cleavage Patterns

  • Cleavage: Refers to the pattern of early cell division:
      - Protostomes: Display spiral and determinate cleavage.
      - Deuterostomes: Characterized by radial and indeterminate cleavage, allowing for the possibility of identical twins via embryonic stem cells.

CLEAVAGE TYPES IN EMBRYONIC DEVELOPMENT

  • Spiral Cleavage: Occurs in protostomes.

  • Radial Cleavage: Found in deuterostomes.