Becoming Vertebrate Chordates The Missing Link A Timeline

Becoming Vertebrate Chordates: The Missing Link

A. Timeline Perspective 450 mya: paired appendages and jaws 420 mya: lungs 400 mya: four limbs 375 mya: amniotic egg 300 mya: endothermy 200 mya: dinosaurs 100 mya: modern species

B. General Trends in Vertebrate Evolution 1. Divergent Evolution a. Closely related evolutionary lines that are subjected to different forces or natural selection tend to become more different. b. Ex) Clam and Octopus

2. Convergent Evolution a. Different evolutionary lines that encounter extremely similar forces of natural selection tend to become more similar as they evolve. b. Ex)

C. Options for Support 1. Coelum a. limits size b. limbs can’t be attached 2. Exoskeleton a. size limited by weight b. must molt to grow

3. Cartilage a. unlimited growth potential b. requires additional support outside of an aquatic environment. 4. Bone a. unlimited growth potential b. allows for complex feeding and movement.

c. Bone Lets Vertebrate Be Complex (1) Paired appendages (a) Allow for better balance and controlled movement. (b) Connected to the bones of the spinal column on limb girdles. - pectoral = arm, clavicle, scapula and spine - pelvic = legs, hips and spine

(2) Jaws (a) Develop from pharyngeal slits used for feeding. (b) jaws with plate-like teeth jaws with teeth beaks platypus bill baleen

D. Invertebrate Chordates 1. Animals that have a notochord and pharyngeal slits. 2. Notochord a. A flexible rod-like structure that forms the main support of the body in the lowest chordates. b. Replaced by the spine in higher vertebrates.

3. Pharyngeal Slits a. Openings in the “neck” that can develop into more complex structures. b. Become gills, vocal cords, etc. In higher vertebrates.

3. Lancets a. aka: amphioxus b. cephalized c. very “fish-like”

4. Tunicates and Lancets a. Marine chordates, filter feeders with no jaws or appendages. b. Tunicates (1) aka: sea squirts (2) Not cephalized (3) Often sessile as adults