Crash Course Biology: Chordates and Vertebrates

Introduction

• Understanding the complex group of animals known as chordates.
• Phylum Chordata includes all vertebrates and several invertebrates.

Animal Classification

• Classification based on shared ancestry and evolutionary milestones.
• Traits track evolution, marking progress to complex animals like mammals.

Synapomorphic Traits of Chordates

1. Notochord
   • Cartilaginous structure between digestive tube and nerve cord.
   • Develops into the backbone in vertebrates.
2. Dorsal Hollow Nerve Cord
   • Central nervous system, unique to chordates.
3. Pharyngeal Slits
   • Function varies: filter feeding, gill slits, developmental structures in land vertebrates.
4. Post-anal Tail
   • Propulsion in aquatic animals; becomes coccyx in humans.

Evolutionary Timeline

• Traits appeared over 500 million years ago during the Cambrian explosion.
• Found in all three chordate subphyla, despite their differences.

Chordate Subphyla

1. Cephalochordata (Lancelets)
   • Retain all four characteristics throughout life.
2. Urochordata (Tail-cords)
   • Tadpole-like larvae with chordate traits.
   • Adult forms lose some characteristics.
3. Vertebrata
   • Includes a hard backbone, diverse classification.

Vertebrate Classes

• Myxini (Hagfish): Vertebrate with a skull but no vertebrae.
• Petromyzontida (Lampreys): Have cartilage backbone and complex nervous system.

Gnathostomes: Evolution of Jaws

• Jaws likely evolved from pharyngeal support structures.
• Chondrichthyes (Cartilage Fish): Sharks, rays, possess early calcified skeleton.

Osteichthyes: Bony Fish

• Mineralized endoskeleton.
• Divided into:
  • Actinopterygii (Ray-finned fishes)
  • Sarcopterygii (Lobe-finned fishes): Include lungfish and tetrapods.

Tetrapods

• Transition from water to land with limbs.
• Class Amphibia: Frogs, salamanders; life cycle involves water.

Amniotes: Adaptations for Land

• Eggs adapted for terrestrial life.
• Classes include reptiles, birds, mammals.

Reptilia and Aves

• Reptilia: Terrestrial, ectothermic, 3-chambered heart.
• Aves (Birds): Endothermic, 4-chambered heart, evolved from archosaurs.

Class Mammalia

• Amniotes with hair, three ear bones, and mammary glands.
• Embryo develops inside the mother, dispensing with the hard egg shell.
• Includes diverse species, demonstrating evolutionary success.

Conclusion

• The journey from simple chordate ancestors to complex mammals.
• Encouragement to review and reinforce the material covered.