Vertebrates

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Hagfishes have a cartilaginous skull and axial rod of cartilage derived from the notochord, but lack jaws and vertebrae

Slime glands

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Derived Characters of Vertebrates

During the Cambrian period, a lineage of craniates evolved into vertebrates. Vertebrates became more efficient at capturing food and avoiding being eaten.

Vertebrates have the following derived characters:

Vertebrae enclosing a spinal cord

An elaborate skull

Fin rays, in the aquatic forms.

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Lampreys represent the oldest living lineage of vertebrates. They are jawless vertebrates inhabiting various marine and freshwater habitats.

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Origins of Bone and Teeth

Mineralization appears to have originated with vertebrate mouthparts.

The vertebrate endoskeleton became fully mineralized much later.

Today, jawed vertebrates, or gnathostomes, outnumber jawless vertebrates.

Gnathostomes jaws might have evolved from skeletal supports of the pharyngeal slits.

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Hypothesis for the evolution of vertebrate jaws

Skeletal rods

Cranium

Gill slits

Mouth

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Other characters common to gnathostomes:

An additional duplication of Hox genes

An enlarged forebrain associated with enhanced smell and vision

In aquatic gnathostomes, the lateral line system, which is sensitive to vibrations.

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Chondrichthyans (Sharks, Rays, and Their Relatives)

Chondrichthyans (Chondrichthyes) have a skeleton composed primarily of cartilage.

The cartilaginous skeleton evolved secondarily from an ancestral mineralized skeleton.

The largest and most diverse group of chondrichthyans includes the sharks, rays, and skates.

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Chondrichthyans

Pelvic fins

Pectoral fins

(c) Spotted ratfish (Hydrolagus colliei)

(a) Blacktip reef shark (Carcharhinus

melanopterus)

(b) Southern stingray (Dasyatis americana)

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Most sharks

Have a streamlined body and are swift swimmers

Are carnivores

Have a short digestive tract; a ridge called the spiral valve increases the digestive surface area

Have acute senses.

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