BIOL 112 Lecture 33

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No class Wednesday
Course Evaluations open Nov. 19^th – Dec. 4^th

Chordata: Craniates

Lampreys

• cartilaginous vertebral column
• retained notochord
• Earliest vertebrates from Cambrian explosion
• Small (~2 cm long)
• No eyes, appendages, jaws, or armor

From an evolutionary point, the first mineralized (calcified) structures on these organisms were teeth and other dental elements.

Gnathostomes: Jawed vertebrates

1. Jaw, thought to have evolved from pharyngeal slit supports, gives huge advantages
   • teeth embedded in solid jaw (not soft tissue)
2. Large brains
3. More developed sensory systems

Hox genes played large role in jaw development

Chondrichthyes

χονδριχθυσ

sharks, rays, chimeras

Skeleton composed predominately of cartilage

Bone evolution hypothesized to be an exaptation:

• calcium from blood migrates into cartilage (ossification)
• originally could have been a storage mechanism
• added advantage of hardening cartilage
• started with feeding structures (keratin teeth, skull cap, jaw) and then advanced down vertebrae

Other characteristics:

• well-developed sensory system for detection of prey
• (in sharks) an acute sense of smell (olfaction) and detection of electrical activity in prey (electroreception via Ampullae of Lorenzini)
• Lateral line; sensitive to pressure waves in water (similar to hearing receptor cells in ear)