five major extant classes of fish
myxiniformes, petromyzontiformes, chondricthyes, sarcopterygii, actinopterygii
What are the diagnostic features of the phylum Chordata?
1. embryos all have a notochord
2. Dorsal, hollow nerve cord.
3. Pharyngeal slits
4. Postanal tail
a flexible rod located between the digestive tube and nerve chord.
a. Provides skeletal support.
b. In most vertebrates, it's replaced by a jointed skeleton.
c. Remains of the notochord exist as disks between the vertebrae.
Water enters through the mouth and passes out through the slits in the pharynx, without going through the digestive system.
i. Slits function as suspension-feeding devices in many invertebrate chordates
ii. Slits have been modified in more evolved vertebrates for: Gas exchange, Hearing, Jaw support
What are the major taxonomic groups within the phylum Chordata?
subphyla: urochordata, cephalochordata, vertebrata
the tunicates, underwater saclike filter feeders with incurrent and excurrent siphons
from what did fish evolve?
fish arose from invertebrate chordate ancestors - possibly tunicates & lancelets
when did fish evolve?
chordates date from early to mid Cambrian (>540 million years ago)
first good fish fossils date from Ordovician (~460 million years ago)
these fossils are of external armour - characteristic of early jawless fishes
how did fish evolve?
1. ancestors (tunicates?) had:
sessile adult stage
free swimming larval stage for dispersal
2. larval stage became more active & more vertebrate-like
3. larvae became capable of reproduction and hence became adults (i.e. neoteny)
where did fish evolve?
generally believed to be in ocean because:
habitat of other chordates
marine deposits contain most early vertebrate fossils
first fish may have been anadromous (i.e. early life stages adapted for living in freshwater
What are vertebrates
most have a bony skeleton
distinct cranium - skull with brain (i.e., craniates)
Crainates are Chordates with a hard bone or cartilage skull
Head consist of a brain and sense organs, incl. eyes and a skull
Include hagfish (Myxini), lampreys (Petromyzontida) and jawed vertebrates (Gnathostomata
the jawless fish
These are the most primitive vertebrates.
Extant groups include hagfish (no skeleton, no notochord in adult); lamprey (early version of a vertebral column).
the jawed fish
Members of group have two pairs of fins.
Jaws and fins allowed fish to become active in pursuit of food and in biting off chunks of flesh.
Jaws evolved from modifications of skeletal elements of anterior pharyngeal gill slits.
What are the jawless fish?
superclas agnatha, extinct members include the ostracoderms (a general term for several groups of 'armoured' fish)
extant (living) members include the hagfish (class Myxini) and lampreys (class Cephalaspidomorphi)
fossils date from Ordovician period (~460 million years ago)
about 600 species have been described
abundant for ~100 million years (gone by 380 million years ago)
earliest fossils found in marine deposits, later ones also in freshwater
no jaw - instead had a muscular feeding pump
body armour (name means shell-skinned)- first evidence of true bone
some had paired fins, but not true fins with bony support
What are the general characteristics of the hagfish and lampreys?
cartilaginous or fibrous skeleton
no paired fins
no body armour (the ancestral condition?)
single median nostril
series of round gill openings - no true gill arches
Hagfish - some other interesting features
live in temperate seas, generally from 25-1500 m
scavengers - enter dead fish via gills, mouth or anus and eat from inside out
slime production to aid burrowing - a 50 cm long hagfish can fill an 8 litre bucket in minutes!
4 'hearts' and a simple kidney
often not included in the vertebrates: instead placed in phylum Chordata, subphylum Myxini
lampreys- some other interesting features
~40 species known
oral disc in adults - horny 'teeth' on disk and tongue.
some species are anadromous:
migrate between freshwater and ocean
larval stage ("ammocoete") filter-feeds on algae and stays in streams for 4+ yrs
adult migrates to ocean, where it feeds & matures for 2 years
adult latches onto sides of other fish and sucks out blood
other species are non-migratory and live entirely in freshwater:
larval stage ("ammocoete") filter-feeds on algae and lasts for ~6 yrs
adult stage only lasts for ~6 months, has poorly developed teeth and does not feed
How did jaws evolve?
Jaws evolved from modifications of skeletal elements of anterior pharyngeal gill slits
What are the evolutionary advantages of jaws?
allow manipulation of food items
enable a more diverse diet
provide some defense
extinct classes jawless fish
class Placodermi - plate-skinned fish
class Acanthodii - spiny sharks
extant (living) classes jawless fish
class Sarcopterygii - lobed fin fish
class Chondrichthyes - sharks, skates, rays
class Actinopterygii - ray finned fish
Placoderms (class Placodermi)
first fossils from late Silurian period (~420 million years ago) and dominated waters in late Devonian period (~400 million years ago)
first appeared in marine habitats and then in freshwater
died out completely in lower Carboniferous period (~350 million years ago)
What were the diagnostic characteristics of the placoderms?
jaws but no ability to extrude
no tooth replacement, and tooth structure different from derived fishes
bony armour (plate-like, with large plate around head)
paired true fins, but no anal fin
dorsoventrally compressed body
Spiny sharks (class Acanthodii)
lived from ~450 million years ago (Ordovician period) until ~280 million years ago (Permian period)
stout median and paired spines
a cartilaginous skeleton
a large head and large eyes
Evolution of subclass Elasmobranchii
- the elasmobranchs
arose in late Silurian period and common by Devonian (~400 million years ago)
modern forms in place by Jurassic period (~144 million years ago)
three major radiations of elsmobranchs
cladoselachian sharks (extinct)
hybodont sharks (extinct)
modern elasmobranchs (extant)
First elasmobranch radiation: Cladoselachian sharks (extinct)
three cusped teeth (multicusped)
fins broad based and stiff, with long radial elements
no anal fin
homocercal caudal fin
body supported by notochord only
often had spines in front of fins
Second elasmobranch radiation: hybodont sharks (extinct)
replaced cladoselachian sharks in Permian period (~250 million years ago)
humped tooth (hybodont)
fins both flexible and mobile
caudal fin heterocercal
Third elasmobranch radiation:
Coming of the modern sharks
first appeared in the Jurassic period (~200 million years ago)
a ventral mouth with a protrusible upper jaw
a rapid tooth replacement system
ceratotrichia supporting the fin
calcified vertebrae instead of a notochord
changes enabled improved feeding and locomotion
What are the diagnostic characteristics of the modern elasmomobranchs?
16 orders and ~850 species of modern elasmobranchs with:
a cartilaginous skeleton
usually 5-7 gill slits that are not covered by an operculum
spiracles to draw in water
paired ventral nostrils
a sub-terminal mouth
hyostylic jaw suspension enabled by:
an upper jaw formed by palatoquadrate cartilage
a lower jaw formed by Meckel's cartilage
the jaws being loosely attached to brain case
consequently, modern elasmobranchs have a protrusible upper jaw
What are the diagnostic characteristics of the modern elasmomobranchs? (continued)
fin rays are unsegmented, soft and epidermal
placoid scales - tiny tooth-like structures
enlarged liver for buoyancy
spiral valve intestine
electrical field receptors - the Ampullae of Lorenzini
single cloaca (urogenital and anal opening)
internal fertilization - males have claspers for holding and inseminating female
What adaptive features are shown by the modern elasmobranchs?
adapted for energy-efficient movement:
mostly predatory, carnivorous and large, with well-developed non-visual senses
adapted for energy-efficient movement:
static buoyancy mechanism e.g. enlarged liver containing the buoyant oil squalene
What adaptive features are shown by the modern elasmobranchs? (continued)
reproduction may be:
oviparous - lay eggs externally
viviparous - live-bearing
mature late (generally 6-18 years)
produce few, relatively large young
long-lived, slow growing
What are the major extant orders of the class Chondrichthyes, subclass Elasmobranchii?
16 extant orders
Including: Heterodontiformes (bullhead sharks), Orectolobiformes (carpet sharks), Lamniformes (mackerel sharks), Carcharhiniformes (ground sharks), Hexanchiformes (frilled and cow sharks), Squaliformes (dogfish sharks), Pristiophoriformes (sawsharks), Torpediniformes (electric rays), Rajiformes (skates and relatives), Myliobatiformes (stingrays and relatives)