1. Natural History of Sharks, Skates, and Rays
Early Chondrichthyes
MARE 394
Dr. Turner
Summer 2008

2. Defining Elasmobranchs
What kind of Thyes?...Chondrichthyes
What kind of Fish?...Cartilagenous Fish
What kind of Brates?...Vertebrates…
What kind of Dates?...Chordates…

3. Phylum Chordata 3 subphylum:
Subphylum Urochordata – tunicates, sea squirts
Subphylum Cephalochordata – lancelets
Subphylum Vertebrata – fishes, amphibians, reptiles, birds, mammals

4. Subphylum Urochordata
– Sessile, feeding
– Mobile, non-feeding
4 chordate traits:
Dorsal hollow nerve chord, notochord,
pharyngeal gill slits, post anal tail

5. Subphylum Cephalochordata
4 chordate traits:
Dorsal hollow nerve chord, notochord,
pharyngeal gill slits, post anal tail

6. Who did the what now?
“Well, whenever I'm confused, I just check my underwear. It holds the answer to all the important questions.” – Grandpa Simpson
Neoteny is the retention of juvenile traits in an adult
Specifically, paedomorphosis is the developmental process by which these changes take place 6

7. Subphylum Vertebrata Characteristics that distinguish vertebrates:
Extensive skull
Backbone - a dorsal row of hollow skeletal elements (vertebrae) which enclose and protect nerve (spinal) cord

9. Fishes Simplest & oldest of all living vertebrates
“I wish, I wish I did not kill that fish” – Homer Simpson
Simplest & oldest of all living vertebrates
Sharks ( MYBP)
Most abundant vertebrates (by # & species)
~29,500 living spp of fishes (>482 families)
58% Marine; 1% diadromous fish travel
between salt & fresh water 9

10. Fishes Superclass Agnatha (jawless fish) Class Myxini (hagfish)
Class Cephalospidomorpha (lamprey)
Superclass Gnathostoma (jawed fish-cart)
Class Chondrichthyes (cartilaginous fish)
Superclass Osteichthyes (jawed fish-bony)
Class Actinopterygii (ray-finned fish)
Class Sarcopterygii (lobe-finned fish)

11. Super

12. Class Myxini (hagfish) - Cartilage skull - Lack jaws & vertebrae
Superclass Agnatha
Class Myxini
(hagfish)
- Cartilage skull
- Lack jaws & vertebrae
- All Marine (30 spp.)
- Scavengers
- Produce slime 12

13. Class Cephalospidomorpha
Superclass Agnatha
Class Cephalospidomorpha
(lamprey)
- Cartilage skull
- Lack jaws & vertebrae
- Freshwater & Marine (35 spp.)
- Parasites – attach to host
- Rasping tongue
- Some diadromous 13

14. (sharks, skates, rays, ratfish)
Superclass Gnathostoma
Class Chondrichthyes
(sharks, skates, rays, ratfish)
- Cartilage skeleton (jaws & vertebrae)
- Marine & few FW (750 sp.)
- Traces of bone in scales & teeth
- Buoyancy via liver – squaline oil
- Spiral valve – corkscrew intestine SA:V ratio; compact
- Internal fertilization of eggs
- Claspers – modified pelvic fins ♂
Placoid scales 14

15. (sharks, skates, rays, ratfish)
Superclass Gnathostoma
Class Chondrichthyes
(sharks, skates, rays, ratfish)
Skates differ from Rays:
Skates have a more muscular tail, two dorsal fins & often a caudal fin, lay eggs
Skates & Rays differ from Sharks:
Enlarged pectoral fins that attach to side of head, no anal fin, ventral gill openings, dorsal eyes & spiracles
Ratfishes (Chimaeras):
Possess an operculum or gill cover over gill slits, adults have no scales, ♂ clasper on head 15

16. Shark
Ray
Skate
Ratfish 16

17. Class Cephalospidomorpha
- Freshwater & Marine (35 sp.)
- Parasites – attach to host
- Rasping tongue
- Some diadromous

18. Early Vertebrates
Earliest vertebrates - early Cambrian 530 mybp (million years before present)..
Early relatives of agnathans (jawless fishes) first 500+ mybp

19. Early Fishes

20. Early Fishes 1. Ordovician (505-438 mybp) 2. Silurian (438-408)
3. Devonian ( )
4. Carboniferous ( )
5. Permian ( )
6. Triassic ( )
7. Jurassic ( )
8. Cretaceous (138-63)
9. Cenozoic (63-24)
10. Quaternary (24-0)
A. Hemicycapsis, B. Pterapsis, C. Cyathapsis,
D. Drepanapsis, E. Coccosteus, F. Helodus,
G. Cladoselache, H. Raja, I. Chimaera

21. Early Fishes
Conodonts – (550 mybp) – known from small (<2mm) teeth found in fossil deposits
Large eyes and eel-like bodies,
notochord
Closer to jawed fishes than
lamprey & hagfishes

22. Early Fishes
Hagfishes - (Class Cephalaspidomorpha)– (550 mybp?) – marine, jawless, eel-like fishes; scavengers
Single nostril, rudimentary eyes, ventral mouth, tongue with rows of keratinized teeth

23. Early Fishes
Lampreys – (Class Myxini) – anadromous or freshwater, lawless, eel-like fishes
Predatory & non-predatory forms
Keratinized teeth on buccal funnel & tongue

24. Early Fishes Ostracoderms – some of the earliest fishes
Lack jaws, have paired fins, bony armor, cartilaginous skeleton, heterocercal tail (460 mya)

25. Early Fishes Placoderms – Earliest jawed fishes Heavy bony skeletons
No special affinities with modern fishes

26. Placoderms
Most likely sister group to the combined lineages of Acanthodii, Chondrichthyes, and Osteichthyes; share:
1) jaws with common structure
2) Two pairs of paired fins w/ bony girdles
3) three semicircular canals in inner ear

27. Early Fishes
Acanthodii – “spiny sharks” – oldest known jawed vertebrates (440 mybp)
Small (<20cm), large eyes, streamlined bodies, dentine-tipped scales

28. Early Fishes Chondrichthyes – What are they Doctor?
Sharks, rays, & skates…
But that’s not important right now…

29. Early Fishes Osteichthyes – bony fishes – loosely defined group
Defined by common structures and lack of characters that define chondrichthyes

30. Early Cartilagenous Fishes
Chondrichthyan fishes most successful measured by historical endurance; ability to survive extinctions
Defined by cartilagenous skeleton mineralized by calcifications (tesserae) and modification of mixopterygia (claspers) in ♂

31. Early Cartilagenous Fishes
Two sister taxa: Elasmobranchii (sharks, rays, skates) & Holocephali (chimeras)

32. Evidence of Early Chondrichthyans
Easier group to define than bony fishes:
1) only approximately 850 species
2) fossil groups are poorly known

33. Evidence of Early Chondrichthyans
Scales & spines from early chond. Identified in Lower Silurian (430 mybp) to Devonian (350 mybp)
Difficult to nail down due to similar mophology among scales & spines of thelodonts & acanthodians (spiny sharks) at this time

34. Evidence of Early Elasmobranchii
Appeared in middle Devonian (350 mybp); rays appeared during Jurassic (200 mybp)
Few well preserved specimens; difficult to piece together evolution
Two early forms: cladoselachian & xenacanth sharks

35. Evidence of Early Elasmobranchii
Two early forms:
Cladoselachian
Xenacanth

36. Order Cladoselachiformes
Lacked: claspers, an elongate skull, amphistylic jaw suspension, no anal fin, Had: triangular, paired fins, multicusped teeth
Predator in marine systems

37. Order Xenacanthiformes
Had: 2 anal fins, tail diphycercal (pointed)
Predator in freshwater systems

38. Order Hybodontiformes
Ancestral to modern sharks
Appeared during Permian (260 mybp)
Fed on large, active invertebrates – first with large, sharp teeth

39. Order Chimaeriformes
Appeared during Devonian (350 mybp) with Elasmobranchs
Modern forms during Jurassic (170 mybp)

40. Modern Cartilagenous Fishes
Monophyletic group
Common origin – distinct from bony fishes

41. Modern Cartilagenous Fishes
Beyond cartilage have several traits in common….
1. Simple box-like cranium
2. Upper jaws (palatoquadrate cartilage) not fused to cranium; lower jaw is a single element (Meckel’s cartilage)
internal & external gill openings
4. Vertebral column is notochord supported by calcified vertebrae

42. Modern Cartilagenous Fishes
5. Pectoral & pelvic fins are supported internally by a girdle skeleton ; externally by rays (lepidotrichia) of flexible connective tissue
6. Basal skeleton on ♂ anal fins – claspers (paired copulatory organs)
7. Most have covering of small placoid scales (dermal denticles)

43. Modern Cartilagenous Fishes

44. S-U-C-C-E-S-S Success due to adaptive characteristics: 1) buoyancy
2) respiration
3) external covering
4) feeding
5) movement
6) sensory systems
7) osmoregulation
8) reproduction

45. Buoyancy -no swimbladder -combination of methods to reduce density
Cartilage less dense than bone (1.1 vs 2.0)
Large, oil-filled liver (0.8) (water 1.0)
Hydrodynamic lift from heterocercal tail & pectoral fins

46. Respiration 3 basic means of respiration
1) Two-pump system (like teleosts) – pump O2 water across gills in slow-moving, bottom oriented sharks
2) Ram ventilation – push water across gills during swimming; fast-moving sharks
3) Spiracles – used to bring water across gills; small round opening precede gills on lateral sides of head – on top of head in rays – almost absent in pelagic sharks

47. External Covering All have placoid scales
Rays – few rows on back; sometimes modified into spines
Sharks – skin overlapping into lightweight, protective coat
fast-swimming sharks have channels between scales to minimize turbulence

48. External Covering Slow-moving sharks more “armored” - dorsal spines
Rays – barb/sting
Skates – denticles
Teeth are modified placoid scales

49. Feeding Most are specialized predators – teeth dictate type of feeding
Triangular, blade-like teeth – large fish & marine mammals
Long, thin, pointed – whole fish
Rows of small, sharp teeth – small inverts
Flattened, pavement-like teeth – hard-shelled inverts
Pointed in front/flattened in back – small inverts (grasping and crushing)

50. Feeding
Teeth continually shed & replaced; may loose 30,000 in lifetime
Jaws loosely attached to cranium - can throw jaws or create suction
Large stomach & spiral valve intestine

51. Movement Large heterocercal tail
Counter-current heat exchangers in pelagic sharks (Lamnidae)
Pectoral fins (Mylobatidae) – fly through the water

52. Movement
Homocercal
Heterocercal

53. Sensory Systems
Odor – olfaction detects dissolved chemicals in the water
Low frequency sounds – inner ear and lateral line system; “hear” and “feel” sound waves respectively
Ampullae of Lorenzini – pit organs filled with an electrically conductive gel used for detecting weak electrical currents & magnetic fields

54. Ampullae of Lorenzini

55. Osmoregulation
Osmoregulators – regulate internal salt concentration approximately 1/3 seawater
Utilize large quantities or organic salts (urea & trimethylene oxide)
Invade marine, estuarine (brackish) and freshwater systems

56. Reproduction
Osmoregulatory and and reproductive systems likely evolved simultaneously;
Long gestation periods of embryos (in egg or ♀, would not be possible without ability to withstand high concentrations of waste

57. Reproduction
Unlike most bony fishes (teleosts) put most energy into relatively small number of large, active, young (ecological term?)
Wide variety of means to this end:
egg laying (oviparity) to live bearing (viviparity); all stages inbetween