Natural History of Sharks,
Skates, and Rays
Early Chondrichthyes
MARE 380
Dr. Turner
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 Tunicata – tunicates, sea
squirts
Subphylum Cephalochordata – lancelets
Subphylum Vertebrata (Crainiata) –
fishes, amphibians, reptiles, birds,
mammals
Subphylum Tunicata
– 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
“Crainiata”
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
Class Agnatha (jawless fish)
Subclass Myxinodea (hagfish)
Subclass Petromyzontida (lamprey)
Infraphylum Gnathostomata (jawed fish-cart)
Class Chondrichthyes (cartilaginous fish)
Class Osteichthyes (jawed fish-bony)
Subclass Actinopterygii (ray-finned fish)
Subclass Sarcopterygii (lobe-finned fish)
Class Agnatha
Subclass Myxinodea
(hagfish)
- Cartilage skull
- Lack jaws & vertebrae
- All Marine (30 spp.)
- Scavengers
- Produce slime
Class Agnatha
Subclass Petromyzontida
(lamprey)
- Cartilage skull
- Lack jaws & vertebrae
- Freshwater & Marine (35 spp.)
- Parasites – attach to host
- Rasping tongue
- Some diadromous
Infraphylum Gnathostomata
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 ♂
Infraphylum Gnathostomata
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
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 - (Subclass Myxinodea)– (550
mybp?) – marine, jawless, eel-like fishes;
scavengers
Single nostril, rudimentary eyes, ventral
mouth, tongue with rows of keratinized
teeth
Early Fishes
Lampreys – (Subclass Petromyzontida) –
anadromous or freshwater, lawless, eellike fishes
Predatory & non-predatory forms
Keratinized teeth on buccal funnel &
tongue
360 mybp
Early Fishes
Ostracoderms – some of the earliest fishes
Lack jaws, have paired fins, bony armor,
cartilaginous skeleton, heterocercal tail
(460 mya)
Early Fishes
Acanthodii – “spiny sharks” – some of the
oldest known jawed fishes (440 mybp)
Small (<20cm), large eyes, streamlined
bodies, dentine-tipped scales
Early Fishes
Placoderms – also some of the earliest
jawed fishes
Heavy bony skeletons
No special affinities with modern fishes
380 mybp?
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
Osteichthyes – bony fishes – loosely
defined group
Defined by common structures and lack of
characters that define chondrichthyes
420 mybp
Early Fishes
Chondrichthyes –
What are they Doctor?
Sharks, rays, & skates…
But that’s not important right now…
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
morphology among scales & spines of
thelodonts & acanthodians (spiny sharks)
at this time
Evidence of Early Elasmobranchii
True sharks - 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
(350 mybp)
Order Cladoselachiformes
Lacked: claspers, an elongate skull,
amphistylic jaw suspension, no anal fin,
Had: triangular, paired fins, multicusped
teeth
Predator in marine systems
Jaws…Then & Now
Devonian shark: Snout typically short and rounded;
jaws longish and located at the front of the head
Modern sharks: Snout typically longish and
pointed; jaws shorter and located underneath the
head
Jaws…Then & Now
Long jaws are structurally weaker than short ones
and less able to produce a powerful bite
Early sharks may have plucked prey from the
bottom or with forceps-like delicacy
Jaws…Then & Now
Early sharks' upper jaws were fixed to the braincase at both
the front and the back (amphistylic) form of jaw suspension
Most modern sharks the upper jaw is fixed to the braincase at
the back only (hyostylic) jaw suspension.
Ancient sharks may have been less able to protrude their jaws
than modern sharks, reducing their ability to suck prey into
their mouths and restricting the size of their food
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;
becomes 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
Spiracle
small round opening precede gills on
lateral sides of head
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
Feeding
Flattened, pavement-like teeth –
hard-shelled 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 in-between