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Cards In This Set
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Which agnathan is thought to be the ancestor of gnathostomes?
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It is unknown.
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In what period (and approximately how many years ago) did the gnathostomes first appear?
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The Silurian Period, approximately 450 m.y.a.
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What are jaws?
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Bony elements fused or articulating with the cranium and possessing teeth
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Define: Key Innovation
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Key Innovation: a novel trait that permits exploitation of novel resources and tends to be associated with bursts of adaptive and taxonomic diversity. Example of a key innovation is the development of jaws.
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What were the two earliest groups of jawed vertebrates? Are they currently around?
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The two earliest groups of jawed vertebrates were the Placoderms and the Acantodians. Both groups are extinct.
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What are jaws derived from?
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Gill arches
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What is Meckel's cartilage?
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Meckel's cartilage, or the mandibular cartilage, formed the lower jaw.
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What formed the upper jaw?
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The palatoquadrate cartilage/bone
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What did jaws permit?
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1) larger food items to be eaten2) allowed for the development of teeth and then of different types of teeth3) Allowed for active defence against predators4) allowed armour to be reduced since there was active defence against predators (3)) there no longer needed as much passive defence against predators
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2 major groups of fossil gnathostomes and features of each.
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Placoderms- tons during the Silurian-Carboniferous periods; dorso-ventrally flattened bodies, bony head shields covering the anterior most 1/3 to 1/2 of the body; over 50 genera recognized from the fossil record. Best known group = Arthrodires "jointed-neck"- had a unique hinge between the end of the braincase and the first cervical vertebrae (craniovertebral joint), also the only placoderms with true teethAcanthodi- "spiny sharks"- had spiny finlets along the dorsal and ventral surgaces of the body, extinct by the mid-Permian; body form suggests they were found more in the water column- pelagic; at least 20 genera from fossil record
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Define "Teleostomi"
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A group composed of the Osteichthyes and the Acanthodi. The acanthodi share several features with the osteichthyes including otoliths, lateral line canal, branchiostegal rays, interhyal bone
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What chracteristic unifies the Chondrichthyes?
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A cartilagenous skeleton
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What two groups of fishes are found within the Chondrichthyes?
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1) The elasmobranchs- sharks, skates and rays2) the Holocephali- the chimaeras (rat and rabbit fishes)
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When did sharks first appear in the fossil record?
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During the Ordovician Period, approximately more than 450 m.y.a.
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What shared, derived traits are found in the Chondrichthyans that distinguishes them from other jawed fishes?
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1) cartilagenous internal skeleton2) 5-7 gill slits (Holocephali have 4 gill openings but a single gill cover)3) "placoid scales" (Holocephali have no scales)4)presence of a spiracle5) enlarged olfactory capsules6) a pronounced and overhanging snout7) serial tooth replacement8) presence of claspers in mature males9) internal fertilization
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