Vertebrate Characteristics

Vertebrate Synapomorphies

• Vertebrates have a series of separate bones or cartilage joined together firmly as a backbone, known as vertebrae.
• Anterior division of neural tube is comprised of forebrain, midbrain, and hindbrain, which forms the brain.
• Cephalization refers to the anterior cluster of specialized sensory organs, including paired eyes, ears, and nose.
• Cranium is a composite structure of bone or cartilage that supports sensory organs and encloses the brain.

Vertabral Column

• A series of separate bones or cartilage joined together firmly as a backbone, comprising centrum, intervertebral disks, and neural arch.
• Centrum is the cylindrical body of the vertebrae, enclosing the notochord.
• Intervertebral disks are thin compression pads, usually made of cartilage.
• Neural arch is the dorsal structure that encloses the spinal cord.
• Neural spine extends from the neural arch.
• Hemal arch is the ventral structure that encloses blood vessels.
• Hemal spines extend from the hemal arch.

Early Vertebrates

• Early vertebrates, such as Haikouella and Haikouicthys, had strengthened notochord and rudimentary vertebrae.
• In more derived fish and terrestrial vertebrates, the vertebral column enlarges and replaces the function of the notochord.
• Notochord disappears in most adult vertebrates, but mammals maintain a component of embryonic notochord in the gel-like core of intervertebral disks called nucleus pulposus.

Gills

• Gills are complex, folded respiratory organs with lamellae folds on the pharyngeal pouches, accompanied by blood capillaries.

Hypothetical Vertebrate Origins

• Agnathans, including cyclostomes, are a group of vertebrates that lack jaws.
• Myxinoidea, or hagfish, are deep-sea, burrowing scavengers that eat dead or dying invertebrates and other fish.
• Hagfish have teeth-like processes on their tongue, can knot their bodies to escape predators, and release a mucus defensively.
• Petromyzontiformes, or lampreys, are half parasitic and have a suctorial oval mouth, rough tongue, and medial fins.

Protovertabrates

• Haikouella and Yunnanozoon are protovertebrates that have protovertebrae around the notochord, pharyngeal bars with gill filaments, and a dorsal nerve chord.
• Haikouichthys has a simple skull, ear, nose, and eye capsules, gill bars, and v-shaped myomeres.

Conodonts

• Conodonts have enigmatic teeth-like mineralized mouth elements, large eyes in optic capsules, and v-shaped myomeres.

Ostracoderms

• Ostracoderms have a lateral line system, dermal exoskeleton, bony plates fused into a head shield, and cartilage endoskeleton.
• They are typically minnow-sized, have dentin-like structures, and are likely bottom feeders.

Pteraspidomorphi

• Pteraspidomorphi have paired nasal openings, head shields, two semicircular canals, and small bony plates.