Chordates Overview

Introduction to Chordates

  • Chordates include all animals post-Enchinodermata
  • Previous groups: Invertebrates (Porifera, Cnidaria, Mollusca, Platyhelminthes, Annelida, Nematoda, Arthropoda, Echinodermata)
  • Chordates:
    • True tissues
    • Bilateral symmetry
    • Deuterostomes (anus develops first)

Phylum Chordata Overview

  • Divided into two groups of invertebrates (cephalochordata, urochordata)
  • Majority are vertebrates (with a backbone)
  • Shared key characteristics of chordates include:
    • Notochord
    • Dorsal hollow nerve cord
    • Pharyngeal gill slits
    • Muscular post-anal tail

Key Characteristics of Chordates

  • Notochord:

    • Flexible rod for skeletal support
    • Becomes spine in vertebrates

  • Dorsal Hollow Nerve Cord:

    • Develops into the brain and spinal cord
    • Develops from rolling ectoderm

  • Pharyngeal Gill Slits:

    • For respiration or feeding
    • Develop into gills in non-tetrapods or become parts of head/neck in tetrapods

  • Muscular Post-Anal Tail:

    • Tail moves behind the anus
    • Used for swimming in aquatic species

First Two Groups within Chordates

Cephalochordata:
  • Example: Lancelets
  • Retain chordate characteristics throughout life
  • Small suspension feeders (1-2 inches)
Urochordata:
  • Example: Tunicates (Sea squirts)
  • Display chordate characteristics only in larval stage
  • Adult tunicates have an incurrent and excurrent siphon for feeding

Transition to Vertebrates

  • All remaining chordates (after cephalochordata and urochordata) are vertebrates
  • Vertebrates defined by having a backbone, complex nervous system, and a skull

Early Vertebrate Groups

Myxini (Hagfish):
  • Jawless vertebrates, scavengers
  • Have a rudimentary vertebrae and cartilaginous skull
  • Produce a defensive slime
Petromyzontida (Lampreys):
  • Another group of jawless vertebrates, parasites
  • Buccal funnel and teeth-like structures for attachment

Gnathostomes (Jawed Vertebrates)

  • Nophistomes group includes organisms with hinged jaws
  • Evolution of jaws theorized to originate from modified skeletal rods
  • Includes chondrichthyes (sharks, rays) which possess:
    • Cartilaginous skeleton
    • Bony teeth

Characteristics of Gnathostomes

  • Enlarged forebrain enhancing smell and vision
  • Lateral line system for sensing vibrations

Chondrichthyes (Sharks and Rays)

  • Streamlined body for swift swimming
  • Diverse reproductive strategies (oviparous, ovoviviparous, viviparous)
  • Possess a cloaca for multiple systems' openings

Characteristics of Fishes

  • Fish breathe through gills protected by operculum
  • Swim bladder for buoyancy control
  • Lateral line system for balance and vibration detection
  • Most are oviparous

Osteichthyes (Bony Fishes)

Actinopterygii (Ray-finned fishes):
  • Bony skeleton with rays supporting fins
  • Common examples: salmon, trout, tuna
Actinistia (Lobe-finned fishes):
  • Have muscular fins allowing locomotion both in water and on land
  • Ancestors of amphibians
Dipnoi (Lungfishes):
  • Adapted to breathe air through lungs
  • Capable of surviving dry periods by burrowing into mud

Conclusion:

  • The diversity of chordates shows the adaptability with key characteristics that facilitate life in varied environments, bridging aquatic and terrestrial ecosystems across evolutionary history.