Animal Diversity II: Vertebrates – Study Notes

Chapter 25: Animal Diversity II – Vertebrates

Case Study: Fish Story

This case study introduces the discovery of a living coelacanth, a fish previously thought to be extinct for 80 million years, highlighting the importance of chordate classification and evolutionary relationships.

• Coelacanth Discovery: In 1938, a strange fish was caught in South Africa and identified as a coelacanth, a member of the chordate phylum.

• Chordate Phylum: Includes frogs, dogs, snakes, humans, and coelacanths.

• Significance: Demonstrates evolutionary continuity and the persistence of ancient lineages.

25.1 What Are the Key Features of Chordates?

Chordates (Chordata) are a diverse taxonomic group that includes humans, birds, apes, tunicates, and lancelets. All chordates share several distinctive anatomical and developmental features.

• Deuterostome Development: Chordates develop with the anus forming before the mouth, a trait shared with echinoderms.

• Four Key Features (present at some life stage):

  • Dorsal nerve cord: A fluid-filled, hollow cord running along the back, which thickens at the anterior end to form the brain.

  • Notochord: A stiff rod providing body support and muscle attachment; present in embryos and replaced by the vertebral column in vertebrates.

  • Pharyngeal gill slits: Openings in the pharynx; function as gills in aquatic species or appear as grooves in embryos.

  • Post-anal tail: Extension of the body past the anus, containing muscle and nerve tissue.

• Human Chordate Features: Most features are visible only during embryonic development.

• Chordate Clades:

  • Tunicates (sea squirts)

  • Lancelets

  • Craniates (hagfishes and vertebrates)

25.2 Which Animals Are Chordates?

Chordates are classified into several groups based on their anatomical and reproductive traits.

Tunicates (Tunicata)

• Marine invertebrates, also called sea squirts.

• Immobile, filter-feeding, vase-shaped animals; body dominated by a pharynx with gill slits and mucus lining.

• Water enters via incurrent siphon, passes through gill slits, and exits via excurrent siphon.

• Adults are sessile; larvae swim and possess all four chordate features.

• Most are hermaphroditic and can reproduce both asexually and sexually.

Lancelets (Cephalochordata)

• Small, fishlike, invertebrate chordates that retain all four chordate features as adults.

• Live half-buried in sand; cilia in the pharynx draw water in for filter feeding.

• Separate sexes; reproduce sexually by releasing gametes into water.

Craniates

• Early craniates resembled lancelets but had brains, skulls, and eyes; lacked jaws.

• Modern craniates include hagfishes and vertebrates.

• Vertebrates replace the notochord with a vertebral column (backbone) made of bone or cartilage.

Table: Comparison of Craniate Groups

Group	Fertilization	Respiration	Heart Chambers	Body Temperature Regulation
Hagfishes (Myxini)	External	Gills	Two	Ectothermic
Lampreys (Petromyzontiformes)	External	Gills	Two	Ectothermic
Cartilaginous fishes (Chondrichthyes)	Internal	Gills	Two	Ectothermic
Ray-finned fishes (Actinopterygii)	External	Gills	Two	Ectothermic
Coelacanths (Actinistia)	Internal	Gills	Two	Ectothermic
Lungfishes (Dipnoi)	External	Gills and lungs	Two	Ectothermic
Amphibians (Amphibia)	External	Lungs	Three	Ectothermic
Reptiles (Reptilia)	Internal	Lungs	Three*	Ectothermic*
Mammals (Mammalia)	Internal	Lungs	Four	Endothermic

Additional info: Birds and crocodilians have four-chambered hearts and are endothermic (birds).

Hagfishes (Myxini)

• Lack jaws; use a tongue-like, tooth-bearing structure to grind food.

• Body stiffened by a notochord; skeleton limited to small cartilaginous elements.

• Exclusively marine; respire with gills; two-chambered heart; ectothermic.

• Secrete slime as a defense mechanism.

Vertebrates

• Have a backbone, paired appendages, and brains.

• Vertebral column supports the body, provides muscle attachment, and protects the nerve cord and brain.

• Jaws evolved ~425 million years ago, allowing for diverse feeding strategies.

• Paired appendages (fins, legs, wings) and increased brain complexity enabled adaptation to various environments.

25.3 What Are the Major Groups of Vertebrates?

Vertebrates are classified into several major groups, each with unique anatomical and physiological traits.

• Lampreys (Petromyzontiformes): Jawless, parasitic fish with a sucker mouth and cartilaginous segments protecting the nerve cord. Spawn in freshwater; adults die after spawning.

• Cartilaginous fishes (Chondrichthyes): Sharks, skates, and rays; possess jaws, cartilaginous skeleton, placoid scales, and internal fertilization. Ectothermic, two-chambered heart.

• Ray-finned fishes (Actinopterygii): Most diverse vertebrate group; bony skeleton, interlocking scales, swim bladder for buoyancy, two-chambered heart.

• Coelacanths and Lungfishes: Lobed fins with rod-shaped bones and thick muscle; lungfishes have both gills and lungs, can survive dry periods by burrowing in mud.

• Amphibians (Amphibia): First tetrapods on land; three-chambered heart; moist skin supplements inefficient lungs; external fertilization; aquatic larvae undergo metamorphosis.

• Reptiles (Reptilia): Adapted for terrestrial life; scales, amniotic eggs, internal fertilization, efficient lungs, three- or four-chambered hearts.

• Mammals (Mammalia): Endothermic, four-chambered heart, hair/fur, mammary glands.

Examples and Applications

• Frogs vs. Toads: Frogs have smooth, moist skin and long hind limbs for leaping; toads have bumpy, dry skin and shorter limbs for hopping.

• Reptile Adaptations: Amniotic egg allows for terrestrial reproduction; scales reduce water loss.

• Birds: Feathers, lightweight bones, and high metabolic rate enable flight and endothermy.

Key Terms

• Endothermic: Ability to maintain a constant internal body temperature independent of the environment.

• Ectothermic: Body temperature varies with the environment.

• Amniotic Egg: Egg with internal membranes that protect and nourish the embryo, a key adaptation for terrestrial life.

• Vestigial Structures: Remnants of ancestral features, such as limb bones in snakes.

Summary Table: Vertebrate Groups and Features

Group	Key Features	Heart Chambers	Temperature Regulation
Hagfishes	No jaws, slime defense	2	Ectothermic
Lampreys	Jawless, parasitic	2	Ectothermic
Cartilaginous fishes	Jaws, placoid scales	2	Ectothermic
Ray-finned fishes	Bony skeleton, swim bladder	2	Ectothermic
Coelacanths/Lungfishes	Lobed fins, lungs (lungfishes)	2	Ectothermic
Amphibians	Moist skin, metamorphosis	3	Ectothermic
Reptiles	Scales, amniotic egg	3/4	Ectothermic/Endothermic (birds)
Mammals	Hair, mammary glands	4	Endothermic

Additional info: Birds are classified as reptiles and are endothermic with four-chambered hearts.