Vertebrate Origins

Question 1

vertebrate evolution

Answer 1

Descended from marine protochordate ancestor

Question 2

Suspension feeders with ciliary pumps

Answer 2

Pre-verts (ancestral chordates)

But ciliary pump ineffective for larger animals

Question 3

First vertebrates

Answer 3

Larger than protochordate ancestor, more Energetic demand with larger size

Question 4

Proposed major events of vertebrate evolution

Answer 4

1. Pharynx evolution
2. Appearance of gills
3. Appearance of mineralized tissue
4. Evolution of jaws

Question 5

Pharynx evolution

Answer 5

Shift from ciliary pump (protochordate) to muscular pharyngeal pump (craniates/verts)

Changes:

• pharynx of vert ancestor developed encircling muscle –>muscle contracts, decrease lumen size
• change in pharyngeal arch support: collagen to cartilage

Result:
Water squeezed out of pharynx; elastic recoil
Increased efficiency, more efficient suspension and sediment (deposit) feeding than ciliary pump
Would enable larger size

Question 6

Appearance of gills

Answer 6

Absent in protochordates, better oxygen delivery

Ex. Lancelets (protochordates) diffuse oxygen across skin which works well for their very small size

Question 7

Appearance of mineralized tissue

Answer 7

In general: deposition of Ca salts

Cell deposition of Calcium and Phosphorus in matrix of connective tissue

First appeared in pharynx (conodonts) and skin (ostracoderms)

Question 8

Evolution of jaws

Answer 8

Jaws from anterior pharyngeal arches

Shift from suspension feeder/sediment feeder to “grasping feeder”

Predator and scavenger

Wary prey and difficult to obtain prey now accessible

Question 9

Conodonts

Answer 9

Fish-like in appearance

Conodont elements: tooth-like; found in muscular pharynx

S &M elements: anterior, prey acquisition, attached to tongue or moving cartilage plates

P elements: posterior, grinding, crushing

Possessed notochord and myotomes

NO BONE – just mineralized tissue

Question 10

Myotomes

Answer 10

Somites forming lateral wall skeletal muscle groups, mesoderm origin

Question 11

Evidence that conodonts must be vertebrates

Answer 11

Conodont elements made of CaPO4, calcified cartilage, enamel, dentin

Dentin from odontoblasts (neural crest origin)

Neural crest cells = apomorphic trait of craniates (extant taxa)

Dentin only in vertebrate clades

Question 12

Ostracoderm fishes

Answer 12

First undisputed vertebrates: bone

Dermal bone

Armored jawless fishes: all are extinct taxa

Taxon Ostracodermi now considered paraphyletic – phylogeny of ostracoderms not resolved

Had mineralized dermal exoskeleton

Bony head shield (dermal bone beneath epidermis), smaller bony plates on trunk

Plates composed of bone, dentin, enamel

Question 13

Origin of bone

Answer 13

Apomorphic; no homologue in other taxa but…

Composed of Ca salts
Produced by specialized cells
Secreted into matrix formed by specialized cells

ex: have skeletal support system that is characterized by these features: mollusks, echinoderms, arthropods

Some protochordate ancestor developed ability to form bone

Question 14

Proposed selective advantages of bone:

Answer 14

1. Protective armor
2. Insulation/protection of sense organs in skin (electroreceptors/mechanoreceptors)
3. Mineral storage

Question 15

Insulation/protection of sense organs in skin (electroreceptors/mechanoreceptors)

Answer 15

Insulator = non-conductor, would enhance transduction

Transduction: cells convert one kind of signal/stimulus to another

Context: transport signals to CNS

ostracoderms had bony tubercles with distinct openings; sensory function?

Electrical field channeled to hair cells in tubercles

Question 16

Mineral storage

Answer 16

Deposition of Ca salts; mobilized when needed

Question 17

Hyperotreti (hagfish)

Answer 17

Ancestry largely unknown

Little or no cartilage; no bone or mineralized tissue, no vertebral elements –> do not fossilize well

Eel-like scavengers

Slime= anti-predator, gill-clogging

Question 18

Evolution of Bone essay

Answer 18

the hard fraction of vertebrate bone is calcium phosphate (hydroxyapatite) rather than calcium carbonate (calcite or aragonite) of invertebrate skeletons

possibly more stable under conditions of physiological stress associated with active lifestyles

Bursts of activity lead to lactic acid production and fluctuations of blood pH and prolonged acidosis. Calcium carbonate of invert bone tends to dissolve under these conditions making it unsuitable for an active lifestyle. Calcium phosphate makes bone matrix more stable.