clades of eugnathostomata

Question 1

describe placoderms. derived traits?

Answer 1

• Earliest gnathostomes 
	• Mid-silurian fossils (430 mya), but probably an early ordovician (465 mya) origin 
	• Very successful in devonian (419-359 mya) - both marine and fresh water 
	• Diverse 
	• Body heavily armoured 
	• Derived traits include: 
		○ Jaws (homoplastic) 
		○ Lateral appendages 
		○ Heterocercal tail

Question 2

four placoderm features (detail and describe)

Answer 2

1. Autostylic jaw suspension (homoplasy)
   • p.q anchored firmly to neurocranium
   • No hyoid arch (autapomorphic)
   • Jaws had little later motion, no suction
   
   2. Medial jaw musculature
      • Jaw musculature medial to pq
      • Suggests independent evolution of jaws
   3. Placoderm “teeth”
      • Modified dermal bone, not real teeth
      • Could slice and pierce
   4. Ossified braincase and vertebral column
      • “top” devonian marine predators
      • Dunkleosteus -“Top” Devonian marine predator in
      Alberta

Question 3

placoderm example?

Answer 3

bothriolepis - bottom dwelling devonian antiarch (basal placoderms)

Question 4

describe Acanthodians

Answer 4

• Also known as spiny sharks
• Not a true shark
• Greek: acantha = spine
• Spines in front of numerous fins (as many as 6 pairs in addition to pectoral and pelvic pairs)
• Late Ordovician to Early Permian
• Most diverse in Early Devonian
•  One of the earliest eugnathostomes & fin-bearers
•  Considered a basal (stem) Chondrichthyan group
•

Question 5

general features of acanthodians? body shape?

Answer 5

fusiform body with heterocerecal tails, spiny fins are not homologous