Ch.6

Question 1

Acanthodians

Answer 1

(acanthos = spine)
• Relationships to other Paleozoic fishes uncertain for decades
• Now known to be stem sharks

Characteristics:
• Dermal cranial bones
• Partially mineralized post-cranial skeleton
• Fin spines

Question 2

Chondrichthyes

Answer 2

chondros = cartilage, ichthyes = fish

Question 3

Acanthodii

Answer 3

“Spiny sharks”

Extinct class of gnathostomes (jawed fish)

Largely immovable spines supporting all fins except the tail, hence the name.

Dermal bones in the skull, a partially ossified internal skeleton and heterocercal caudal fin (vertebral column turns upward into the upper lobe of the fin)

Many had denticles covering the body like placoid scales of living Chondrichthyans.

Are stem Chondrichthyes, which means that acanthodians are not extinct and Acanthodii includes Chondrichthyes.

Question 4

Extant Chondrichthyes

Answer 4

• Chimaeriformes
• Chimaeras, ratfish
• 56 extant species
• One opercular opening per side (opercular is a bone that protects gills & aids in respiration- opens & closes to allow water to pass over gills through water current pressure)

• Neoselachii
• Selachii (sharks) 550 species
• Batomorphi (skates and rays) 700 species
• Each gill has its own opening, usually five, as many as 7

Question 5

Placoid scales

Answer 5

• Scales with same tissues as teeth
• Dentine core, enamel covering
• Other fish scales composed of other minerals and collagen

Different from scales on boney fish

Question 6

Cartilaginous skeleton of Chondricthyes

Answer 6

• Certain skeletal elements mineralized in different ways
• Vertebrae densely mineralized (dense internal calcification)
• Jaws get encrusted with small crystals (still calcification, the small prisms are called tesserae)

Skeletons are mineralized with the same mineral that osteichthyans use in bones. However, they deposit the minerals differently forming calcified cartilage.

The chondocranium (braincase) has a solid cartilaginous roof that protects the brain. This is in contrast to the dermal bones that cover the bring case of osteichtyans.

Question 7

Jaw suspension

Answer 7

• How the elements of the upper and lower jaw connect to the cranium
• Primitive condition for Chondrichthyes = autodiastylic (upper jaw attached to cranium & lower jaw attached to upper jaw)
• Upper jaw articulates to cranium (joints)
• Lower jaw suspended by multiple elements from hyoid arch

Question 8

Jaw suspension in extant Chondrichthyes

Chimaeriformes

Answer 8

• Holostylic = Upper jaw fused, lower jaw suspended by 2 hyoid elements

More rigid, but more support helps with the way that they feed, such as crushing prey between tooth plates. Pivots like a nutcracker.

Question 9

Jaw suspension in extant Chondrichthyes

Neoselachii

Answer 9

• Hyostilic = Upper jaw articulates with cranium via the mobile hyomandibula and flexible ligaments connect the anterior upper jaw to cranium, lower jaw suspended by 1-3 hyoid elements

More fluidity, allows for the mouth to be projected and retracted during feeding —> suction feeding

Question 10

Teeth

Answer 10

• Chondrichthyans have continuous replacement
• As teeth wear or break, they fall out
• Functional tooth – the exposed tooth actively being used
• Tooth file – single series of developing replacement teeth
• Tooth whorl – complete set of all functional and replacement teeth

Question 11

Tooth forms

Answer 11

• Chimaeras have textured tooth plates
• Plates are not replaced, rather refurbished
• Additional tissue is added to existing plates
• Some sharks are heterodont – have differently shaped teeth
• Sharks with broad teeth may have serrations along the margins
• Some sharks have cusplets
• Huge variety of shapes

Question 12

Claspers

Answer 12

• Reproductive structure
• Extend posteriorly from males’ pelvic fins
• A groove transports sperm to female during copulation

Question 13

Chimaeriformes

Answer 13

• Originated during Late Jurassic
• 56 extant species
• Pectoral fins used in propulsion (and lateral undulations of body)
• Most species live in deep seas (>500 meters)
• Eat invertebrates and small fish (but tooth plates can crush some hard-bodied prey)
• Oviparous
• heterocercal Caudal fin
• Many have venom gland associated with a mobile dorsal spine.

Question 14

Neoselachii

Answer 14

Extant sharks, skates, and rays
(neo = new, selachos = cartilaginous fish)

Question 15

Batomorphi

Answer 15

• Originated during Permian
• 703 species of skates, rays, sawfishes, electric rays
• Propulsion from enlarged pectoral fins
• Most live on floor of marine environments
• Many are durophagous – eat prey with hard shells
What unties is that they are depressed —> dorsally flattened

Question 16

Skates vs Rays

Answer 16

Skates
• Tail thick, short, with two dorsal fins, no fin spine
• Oviparous

Rays
• Tail narrow, no dorsal fins, with one or two barbed spines, can be very long
• Viviparous

Question 17

Selachii

Answer 17

• Originated during Triassic
• 574 species
• Propulsion comes from tail (different compared to chimaeriformes and batomorphis that use pectoral fins) their pectorals provide stability
• Range in size from 22 centimeters to 19 meters
• Most live in marine environments
• Not all are predators, some are filter feeders

Question 18

Neoselachii sensory systems

Answer 18

• Excellent senses of smell
• Most have electroreception
• Torpediniformes rays electrogenic: (batoformi)
• Produce electricity from modified branchial muscles (fresh produce more voltage than marine)
• Sensitive lateral line system (different water, pressure from pray, chroma really well b/c in deep water)
• Vision also strong in low light
• Abundant rod cells in the retina
• Well developed tapetum lucidum (layer of organic crystals behind retina reflect light, which reflects light back into the retina, allowing light to pass to the retina twice)

Question 19

Glow in the dark sharks

Answer 19

• Some squaliform sharks are bioluminescent – they actively produce light via photophores in their skin

• Three different groups of sharks are biofluorescent – structures in their skin reflect light

Counter shading

Question 20

Shark swimming

Answer 20

• Propulsion generated from undulations of spinal column and tail
• Large, fast-swimming sharks have endothermal heterothermy
• Most of the body is not kept warm
• The swimming muscles generate heat
• Blood flow patterns keep the heat centred around those muscles
• Keeps muscles warm, more efficient
• Caudal fins are adapted to mode of life
• Larger caudal fins displace more water = more efficient for long- distance swimming
• Heterocercal tails generate thrust back and down, helps keep the shark even in water

Question 21

Reproduction

Answer 21

• Oviparity and viviparity distributed across group
• Most are lecithotrophic – embryos nourished by yolk in egg or in uterus
• Some viviparous species are matrotrophic with placenta-like nourishment for the embryos

Question 22

Conservation

Answer 22

• 36% of elasmobranch species are threatened by extinction
• 70% of the large predatory species
• Main threat comes from fishing
• Intentional fishing
• Accidental bycatch
• Protections are expanding
• Recovery complicated by slow maturation and reproductive rates

Question 23

Key Concepts

Answer 23

• Acanthodians are now known to be stem sharks, and all sharks belong to Acanthodii

• Chondrichthyes have placoid scales, cartilaginous skeletons, constantly replaced teeth, and males have claspers for reproduction

• Chimaeriformes is a lineage of deep sea species that have tooth plates and use pectoral fins for propulsion

• Batomrophi is a lineage of dorso-ventrally flattened species that eat hard prey and use enlarged pectoral fins for propulsion

• Selachii is a lineage showing ranges of size and feeding styles that use tails for propulsion