Vertebrate Zoology Chapter 6: Origin and Radiation of the Chondrichthyes

Vertebrate Zoology Chapter 6: Origin and Radiation of the Chondrichthyes

6.1 Acanthodii

• Etymology: Acantha = spine (Greek).
• Characterized by spines anterior to each fin (except the caudal fin).
• Commonly referred to as "spiny sharks".
• Temporal Range:
  • Appeared in the Silurian period and went extinct by the end of the Permian.
  • Initially marine; by the Devonian period, they were mainly freshwater species.
• Morphological Traits:
  • Lacked extensive bony armor seen in placoderms but retained dermal head bones.
  • Considered stem chondrichthyans.
• Noteworthy Fossil Example:
  • †Doliodus: A transitional form sometimes referred to as "sharkyopteryx" due to its mixture of traits illuminating chondrichthyan evolution.

Acanthodians

• Skeletal Structure:
  • Partially ossified internal skeletons.
  • Spines associated with fins.
  • Features a heterocercal tail fin.
  • Skin covered with dermal denticles.

Phylogeny and Classification

• New Organization:
  • All Chondrichthyans now classified under Acanthodii.
  • Acanthodians considered stem chondrichthyans.
  • Placoid scales are deemed a derived trait, along with pelvic claspers.
  • Classification structures are expected to evolve with the discovery of new fossils.

6.2 Class Chondrichthyes

• Overview:
  • Chondrichthyes are an ancient group of cartilaginous fishes that are not as diverse as bony fishes but have persisted relatively unchanged for over 350 million years.
  • Their key features include well-developed jaws, advanced sense organs, powerful swimming abilities, and streamlined body shapes.
• Origin Date:
  • Oldest unambiguous Chondrichthyans date to the Early Devonian period.
• Current Species Count:
  • Approximately 1,300 living species, all with cartilaginous skeletons.
• Major Taxonomic Groups:
  • Two major radiations: Euchondrocephali and Elasmobranchii (including both extinct and extant forms).
• Subclass Overview:
  1. Elasmobranchii (\sim 1,277 species):
  • Sharks and rays, characterized by multiple (usually 5-7) gill openings on the sides of the head.
  1. Holocephali (\sim 50 species):
  • Chimaeras: includes ratfish and ghostfish, noted for having one gill opening on the side of the head.

Key Features of Class Chondrichthyes

1. Cartilaginous Skeleton:
   • Some structures are partially calcified with calcium hydroxyapatite, resulting in a form of calcified cartilage (synapomorphy for class).
2. Heterocercal Tail:
   • Possesses an asymmetrical shape where the vertebral column extends into the larger upper lobe, providing thrust and lift during swimming.
3. Placoid Scales:
   • These dermal structures are derived from the body armor of placoderms.
   • Comprised of a tooth-like structure with an enamel exterior and a dentine interior.

Genetics and Cartilage Formation

• Bone Formation Genes:
  • Genes for bone formation are present except for those encoding secreted phosphoproteins, possibly explaining the prevalence of cartilage in Chondrichthyans.
• Significance of Elephant Shark Genome Analysis:
  • Identified a family of genes absent in the elephant shark but present in bony vertebrates, implying evolutionary importance in understanding bone-related diseases like osteoporosis.

Advantages of Cartilage

• Lightweight Structure:
  • Cartilage is about half the density of bone, aiding in buoyancy, allowing sharks to avoid sinking.
• Strength in Critical Areas:
  • Jaws and backbones are reinforced with calcium salts forming a calcified cartilage, giving them a strength comparable to bone while retaining the benefits of a lighter structure.
• Flexibility:
  • Facilitates maneuverability, allowing for sharper turns.
• Muscle Attachment:
  • Skeleton not serving as a primary muscle attachment; instead, muscles attach to the thick skin composed of a complex crisscrossed mesh of tough collagen fibers functioning as an external skeleton.

Reproductive Anatomy

• Claspers:
  • Present on pectoral fins of males, facilitating internal fertilization by conducting sperm into females through grooves on the claspers.
  • Believed to be a potential second synapomorphy of class Chondrichthyes.

Teeth Structure and Replacement

• Tooth Replacement Mechanism:
  • Each tooth is part of a whorl; developing teeth are organized for efficient replacement.
  • Young sharks replace teeth approximately every eight days.
  • Considered a "crown gnathostome feature" alongside placoderms.

Evolution of Jaw Suspension in Chondrichthyans

• Types of Jaw Suspension:
  • Transition from Autodiastylic (upper jaw attached to chondrocranium) to Amphistylic (jaw front articulates with chondrocranium; back connected via hyomandibula).
  • Three primary forms:
  1. Holostylic: Upper jaw fused to chondrocranium with two crushing plates (seen in Holocephali).
  2. Hyostylic (Shark): Upper jaw suspended by ligament; allows the jaw to protrude.
  3. Hyostylic (Ray): Upper jaw suspended by ligament; permits downward jaw protrusion.

6.3 Neoselachii Radiation

• Taxonomic Structure (Per Compagno's 2005 "Sharks of the World"):
  1. Subclass Elasmobranchii - includes various orders such as:
  • †Plesioselachus
  • †Order Phoebodontiformes
  • †Order Squatinactiformes
  • †Order Protacrodontiformes
  • Infraclass Cladoselachimorpha
  • Infraclass Xenacanthimorpha
  • Infraclass Euselachii (sharks and rays)
  1. Subdivision Batoidea (rays, skates, and sawfish).

6.4 Euchondrocephali and Chimaeriformes

Cladoselache

• Etymology: Greek for "branch-toothed shark".
• Morphological Features:
  • Marine predator from the Devonian (416-360 MYA).
  • Shark-like appearance, about 2 meters long, possessing a large terminal mouth and short snout.
  • Features three-pronged teeth arranged in a whorl, analogous to modern sharks.
  • Equipped with two dorsal fins, each preceded by a spine.
  • Possessed broad-based fins and a caudal fin with a strong upper lobe.
  • Lacked claspers, suggesting external fertilization.
  • Skin generally lacked placoid scales (presence of dermal denticles is debated or absent).