Superclass Gnathostomata

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WFB 232

Taxonomy, Week 2 31 Jan 2005

Superclass Gnathostomata

Jawed vertebrates, gill tissue, pectoral & pelvic fins, notochord replaced by vertebrae (or at least partially replaced)

Fossils date back to Silurian Period (~ 400 MYA)

4 classes (that we will cover this semester): Placodermi (Extinct), Chondrichthyes, Sarcopterygii, Actinopterygii

Class Placodermi (Extinct)
Similar to both Chondrichthyes and bony fishes (teleosts)

“Armored” head plates, hinged jaws, paired fins, scales/plates present in many species

Silurian to Carboniferous Periods (~400 – 300 MYA)

Class Chondrichthyes (Elasmobranchimorphii)

Chimaeras, ratfishes, sharks, skates, and rays

Cartilaginous skeleton, rigid fins, electrosensory receptors in “nose”

Been around a long time, and haven’t changed much (since Silurian Period, ~440 MYA)

Subclass Holocephali

Order Chimaeriformes

Chimaeras and ratfishes

Upper jaw/mouth fused to cranium

Mostly benthic feeders, variable shaped snouts and tails

2 dorsal fins, usually w/ spine in 1st dorsal

Teeth are grinding plates, males have claspers for mating

Jurassic Period (~200 MYA)

Subclass Elasmobranchii

sharks, skates, and rays

cartilaginous endoskeletons, but is calcified in many species

teeth: vary from grinding plates to ripping and shredding deciduous teeth

There are ~9 – 13 Orders, depending on which interpretation you prefer (see Bond P. 98): We will cover six in this course.
Order Heterodontiformes

Bullhead sharks

2 dorsal fins (spiny), anal fin, 5 gill slits

Order Orectolobiformes
Carpet sharks (incl. nurse, blind, whale, zebra sharks)

2 dorsal fins (not spiny), short mouth, well in front of eyes, 5th gill slit overlaps 4th, behind the origin of the pectoral fin

Feed on: other fish, benthos, zooplankton

Subclass Elasmobranchii (cont’d)

Order Carcharhiniformes

Ground sharks (incl. cat, hound, weasel, requiem, hammerhead sharks)

2 dorsal fins (not spiny), 5 gill slits (last 1-3 overlap pectoral fin),

Ovi-, ovo-, or viviparous development

Requiem: “chant for the dead” – black- & white-tip sharks

Includes some freshwater sharks (bull sharks in Lake Nicaragua, Mississippi R.)

Hammerheads: hearty appetites, feed on wide range of animals, incl. stingrays

Evolution of the head?

Largest order of sharks, >200 species

Order Lamniformes

Mackerel sharks (tiger, goblin, crocodile, megamouth, thresher, basking sharks)

Notorious sharks – incl. Great White Shark (Jaws)

2 dorsal fins (not spiny), 5 gill slits (last 2 overlap pectoral fin), mouth extends well beyond eyes

Order Hexanchiformes

Frill, cow sharks

1 dorsal fin (not spiny), 6-7 gill slits, mouth extends well beyond eyes
Order Squatiniformes

Angel sharks

Ray-like body shape, eyes dorsal, 2 dorsal fins (not spiny), 5 gill openings Terminal mouth, nostrils
Pectorals expanded, winglike, but not attached to head, move using tail

Order Pristiophoriformes

Saw sharks

Shark-shape body, snout flat blade w/teeth, eyes dorsal, 2 dorsal fins (not spiny)

5-6 gill openings, long barbels off “saw” (rostrum)

Order Pristiformes


Ray-shape body, rostrum flat blade w/teeth, eyes dorsal, 2 dorsal fins (not spiny)

Anterior vertebrae fused

Found in salt & fresh water (Lake Nicaragua)

Order Rajiformes

Rays (electric rays, guitarfishes, skates, stingrays, manta rays)

Anterior vertebrae fused, depressed body shape, gills ventral, enlarged pectoral fins, eyes dorsal, teeth “pavementlike”, live birth or encapsulated eggs

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