Natural History of Sharks,
Skates, and Rays
Early Chondrichthyes
MARE 394
Dr. Turner
Summer 2008
Defining Elasmobranchs
What kind of Thyes?...Chondrichthyes
What kind of Fish?...Cartilagenous Fish
What kind of Brates?...Vertebrates…
What kind of Dates?...Chordates…
Phylum Chordata
3 subphylum:
Subphylum Urochordata – tunicates, sea
squirts
Subphylum Cephalochordata – lancelets
Subphylum Vertebrata – fishes,
amphibians, reptiles, birds, mammals
Subphylum Urochordata
– Sessile, feeding
4 chordate traits:
Dorsal hollow nerve chord, notochord,
pharyngeal gill slits, post anal tail
– Mobile, non-feeding
Subphylum Cephalochordata
4 chordate traits:
Dorsal hollow nerve
chord, notochord,
pharyngeal gill slits,
post anal tail
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
Subphylum Vertebrata
Characteristics that distinguish vertebrates:
Extensive skull
Backbone - a dorsal row of hollow skeletal
elements (vertebrae) which enclose and protect
nerve (spinal) cord
Fishes
“I wish, I wish I did not kill that fish” – Homer Simpson
Simplest & oldest of all living vertebrates
•Sharks (400-500 MYBP)
Most abundant vertebrates (by # & species)
~29,500 living spp of fishes (>482 families)
58% Marine; 1% diadromous fish travel
between salt & fresh water
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)
Super
Super
Superclass Agnatha
Class Myxini
(hagfish)
- Cartilage skull
- Lack jaws & vertebrae
- All Marine (30 spp.)
- Scavengers
- Produce slime
Superclass Agnatha
Class Cephalospidomorpha
(lamprey)
- Cartilage skull
- Lack jaws & vertebrae
- Freshwater & Marine (35 spp.)
- Parasites – attach to host
- Rasping tongue
- Some diadromous
Superclass Gnathostoma
Class Chondrichthyes
(sharks, skates, rays, ratfish)
Placoid scales
- 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 ♂
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
Shark
Ray
Skate
Ratfish
Class Cephalospidomorpha
- Freshwater & Marine (35 sp.)
- Parasites – attach to host
- Rasping tongue
- Some diadromous
Early Vertebrates
Earliest vertebrates - early Cambrian 530
mybp (million years before present)..
Early relatives of agnathans (jawless
fishes) first 500+ mybp
Early Fishes
Early Fishes
1. Ordovician (505-438 mybp)
2. Silurian (438-408)
3. Devonian (408-360)
4. Carboniferous (360-290)
5. Permian (290-240)
6. Triassic (240-205)
7. Jurassic (205-138)
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
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
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
Early Fishes
Lampreys – (Class Myxini) – anadromous
or freshwater, lawless, eel-like fishes
Predatory & non-predatory forms
Keratinized teeth on buccal funnel &
tongue
Early Fishes
Ostracoderms – some of the earliest fishes
Lack jaws, have paired fins, bony armor,
cartilaginous skeleton, heterocercal tail
(460 mya)
Early Fishes
Placoderms – Earliest jawed fishes
Heavy bony skeletons
No special affinities with modern fishes
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
Early Fishes
Acanthodii – “spiny sharks” – oldest known
jawed vertebrates (440 mybp)
Small (<20cm), large eyes, streamlined
bodies, dentine-tipped scales
Early Fishes
Chondrichthyes –
What are they Doctor?
Sharks, rays, & skates…
But that’s not important right now…
Early Fishes
Osteichthyes – bony fishes – loosely
defined group
Defined by common structures and lack of
characters that define chondrichthyes
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 ♂
Early Cartilagenous Fishes
Two sister taxa: Elasmobranchii (sharks,
rays, skates) & Holocephali (chimeras)
Evidence of Early Chondrichthyans
Easier group to define than bony fishes:
1) only approximately 850 species
2) fossil groups are poorly known
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
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
Evidence of Early Elasmobranchii
Two early forms:
Cladoselachian
Xenacanth
Order Cladoselachiformes
Lacked: claspers, an elongate skull,
amphistylic jaw suspension, no anal fin,
Had: triangular, paired fins, multicusped
teeth
Predator in marine systems
Order Xenacanthiformes
Had: 2 anal fins, tail diphycercal (pointed)
Predator in freshwater systems
Order Hybodontiformes
Ancestral to modern sharks
Appeared during Permian (260 mybp)
Fed on large, active invertebrates – first
with large, sharp teeth
Order Chimaeriformes
Appeared during Devonian (350 mybp)
with Elasmobranchs
Modern forms during Jurassic (170 mybp)
Modern Cartilagenous Fishes
Monophyletic group
Common origin – distinct from bony fishes
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)
3. 4-7 internal & external gill openings
4. Vertebral column is notochord
supported by calcified vertebrae
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)
Modern Cartilagenous Fishes
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
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
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
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
External Covering
Slow-moving sharks more “armored” dorsal spines
Rays – barb/sting
Skates – denticles
Teeth are modified placoid scales
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 – hardshelled inverts
Pointed in front/flattened in back – small
inverts (grasping and crushing)
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
Movement
Large heterocercal tail
Counter-current heat exchangers in
pelagic sharks (Lamnidae)
Pectoral fins (Mylobatidae) – fly through
the water
Movement
Homocercal
Heterocercal
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
Ampullae of Lorenzini
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
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
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