BSC 2011: Biological Diversity - Vertebrates 1

Phylum Chordata and Vertebrate Diversity

General Characteristics of Chordates

• All chordates share four key characteristics at some point in their lives:
  • Notochord: A single flexible rod that provides support.
  • Dorsal hollow nerve cord: Forms the central nervous system.
  • Pharyngeal slits: Gaps that connect the inside of the throat to the outside.
  • Postanal tail: An extension of the body past the anus.
  • Endostyle: A ciliated groove in the pharynx that aids in feeding.

Subphylum Overview

Cephalochordata: Lancelets

• Composed of 26 marine species.
• Filter feeders, performing gas exchange across their surface.
• Usually sessile but can swim out of their burrow.

Urochordata: Tunicates

• Contains ~3,000 marine species.
• Larvae are tadpole-like, showing all four chordate traits.
• Adults are typically sessile, retaining only pharyngeal slits and are filter feeders through siphons.
• Tunicates are the closest living relatives of vertebrates.

Vertebrate Distinctions

• Vertebrates possess all chordate characteristics, in addition to:
  1. Vertebral column: The notochord is replaced by a bony or cartilaginous column of interlocking vertebrae.
  2. Endoskeleton: Composed of either cartilage or bone that grows with the animal.
  3. Cranium: A protective structure for the brain made of bone or cartilage.
  4. Complex internal organs: Including a liver, kidneys, endocrine glands, and a heart with at least two chambers.

Classification of Vertebrates

Jawless Fishes (Agnatha)

• Includes hagfishes and lampreys.
• Characteristics:
  • Hagfishes: Marine, finless, jawless; known for reduced vertebrae and cartilaginous skull; act as deep-sea scavengers.
  • Lampreys: Lack hinged jaws but possess a notochord and a rudimentary vertebral column; can be parasitic as adults, using an oral disk to attach and feed.

Gnathostomes: Jawed Vertebrates

• Developed jaws from pharyngeal arches.
• Featured groups include:
  • Chondrichthyes: Cartilaginous fishes (sharks, skates, rays).
  • Osteichthyes: Bony fishes (includes ray-finned and lobe-finned fishes).

Class Chondrichthyes: Cartilaginous Fishes

• Skeleton made of cartilage.
• Unique features include dynamic lift from fins and a two-chambered heart with single circuit circulation.
• Various reproduction strategies:
  • Oviparous: Lay eggs.
  • Ovoviviparous: Eggs retained inside female, no placenta.
  • Viviparous: Development within the uterus with placental nourishment.

Class Actinopterygii: Ray-finned Fishes

• Features include a bony skeleton, operculum that covers gills, and a swim bladder for buoyancy.
• Characteristics are essential for distinguishing from cartilaginous fishes.

Class Sarcopterygii: Lobe-finned Fishes

• Contains coelacanths and lungfishes; possess fins supported by skeletal extensions leading to limbs.
• Important in the evolutionary transition to tetrapods (four-limbed animals).

Tetrapods

• Tetrapods share a common ancestor with lobe-finned fishes, adapting limbs for life on land.
• Development of lungs or lung derivatives, allowing respiration outside water.
• Key adaptations include structures to prevent desiccation, support, gas exchange, and locomotion on land.

Class Amphibia

• Comprising frogs, salamanders, caecilians; they rely on water for reproduction.
• Fertilization is generally external, with aquatic larvae stages.
• Breathing adaptations: buccal pumping, skin absorption of oxygen, and gills.
• Amphibians possess a three-chambered heart, aiding in circulation.

Summary of Adaptations

• Land Adaptations: Adaptations for desiccation prevention, support, gas exchange, and reproduction on land developed around 380 million years ago, exemplified by fossil Tiktaalik, known as a “fishapod.”
• Evolutionary Timeline: Major evolutionary advances observed throughout the Paleozoic era transition, including significant fossil records from Silurian to Carboniferous periods.

Conclusion

• Vertebrates display a vast range of adaptations that allow them to thrive in diverse environments, underscoring the evolutionary significance of this phylum in biological diversity.