Subphylum Vertebrata

The vertebrates are a large and diverse group
including the fishes and tetrapods.

Vertebrates possess the basic chordate
characteristics, but also a number of novel
homologous structures.

An alternative name for the group Craniata is
actually a better descriptor for the entire group
because all members possess a cranium, but
some jawless fishes lack vertebrae.
Important developments of the
Vertebrates

Musculoskeletal system. Vertebrates possess
an endoskeleton, which is much more
economical in materials than the exoskeleton of
invertebrates.

It forms a jointed scaffolding for the attachment
of muscles. Initially the endoskeleton probably
was cartilaginous (it still is in jawless fishes and
sharks) and later became bony in many groups.
Important developments of the
Vertebrates
 Bone
is stronger than cartilage, which
makes it a better material to use for
muscle attachment in places where
mechanical stress may be high.
 Bone
may have evolved initially as a
means of storing minerals and was later
adapted for use in the skeleton.
Important developments of the
Vertebrates

Various aspects of vertebrate physiology have
been upgraded also to meet increased
metabolic demands.

For example the pharynx, which was used for
filter feeding in primitive chordates has had
muscles added that make it a powerful water
pumping organ.

With the conversion of the pharyngeal slits to
highly vascularized gills the pharynx has
become specialized for gas exchange.
Important developments of the
Vertebrates

The ancestors of vertebrates switched from filter
feeding to more active feeding, which required
movement and the ability to sense the
environment in detail.

With these changes came the need for a control
center to process information. The anterior end
of the nerve cord consequently became
enlarged into a brain.
Important developments of the
Vertebrates
 The
vertebrate brain in fact developed into
a tripartite brain (with a forebrain,
midbrain, and hindbrain) that was
enclosed within a protective cranium of
bone or cartilage.
Important developments of the
Vertebrates
 Sense
organs have also become highly
developed among the vertebrates.
 These
include complex eyes, pressure
receptors, taste and smell receptors,
lateral line receptors for detecting water
vibrations, and electroreceptors that detect
electrical currents.
Important developments of the
Vertebrates
 The
development of the head in
vertebrates with its array of sense organs
appears to have been driven by the
evolution of new embryonic tissues that
give rise to cells that play an important role
in the formation of sensory structures.
Important developments of the
Vertebrates
A
factor that may have played a major role
in the evolution of the vertebrates is the
duplication of Hox genes.
 Hox
genes play a major role in embryonic
development and vertebrates have four
sets, jawless vertebrates have two sets
whereas invertebrates and amphioxus
have only one.
Hox genes
 Because
a single hox gene influences the
expression of many other structural genes
a change in when and where a hox gene
is turned on may lead to major
morphological changes in the phenotype
such as the addition or loss of legs, arms,
antennae and other structures.
http://evolution.berkeley.edu/evolibrary/images/mutantfly.jpg
Induced ectopic eyes
In Drosopila (arrowed)
From Induction of Ectopic Eyes by
Targeted Expression of
the eyeless Gene in Drosophila
Georg Halder,* Patrick Callaerts,*
Walter J. Gehring.
Science. Vol. 267 24 March 1995
Hox genes
 The
duplication of the Hox genes appears
to have occurred around the time
vertebrates originated and it may be that
this gene duplication freed up copies of
these genes, which control development,
to generate more complex animals.
Hox genes
 One
group of animals in whose evolution
hox genes are hypothesized to have
played a major role is snakes.
 It’s
suggested that the hox genes
controlling the expression of the chest
region in lizard ancestors of snakes
expanded their zone of control in the
developing embryo.
Hox genes
 As
the hox genes for thoracic development
increased their influence, limb
development was suppressed at the same
time giving the limbless condition we see
in snakes today.
Early vertebrate ancestors
 Fossils
of early chordates are scarce, but
a few are known including Pikaia from the
Burgess Shale (approx 580 mya) that
appears to be an early cephalochordate
and has a notochord and segmented
muscles.
Figure 23.10
15.8
Pikaia
Early vertebrate ancestors
 Another
fossil from China Haikouella
lanceolata about 525mya.
 This
fossil has a notochord, pharynx, and
a dorsal nerve cord which are chordate
characters, but also pharyngeal muscles,
eyes, a head, gills and a brain which are
vertebrate traits.
Haikouella lanceolata
Haikouella lanceolata
Jawless early vertebrates
A
wide variety of armored jawless fishes
called ostracoderms are known from the
Ordovician (approximately 490-440 mya)
up to near the end of the Devonian period
(about 360 mya).
 These
fish in many cases lack paired fins
and so probably were not precision
swimmers.
Figure 23.14
15.10
Ostracoderms
Jawless early vertebrates
 The
ostracoderms were heavily armored
and jawless with narrow, fixed mouths.
They appear to have been mainly filter
feeders that used their pharyngeal
muscles to pump water.
 Ultimately,
the ostracoderms were
outcompeted by fish that possessed the
next big evolutionary development: jaws.
Early jawed vertebrates

The origin of jaws was a hugely significant event
in the evolution of the vertebrates and the
success of the Gnathostomes [the jawed
vertebrates, “jaw mouth”] is obvious.

The first jawed vertebrates were the placoderms
heavily armored fish which arose in the late
Silurian (about 410mya) and possessed not only
jaws, but paired pelvic and pectoral fins that
gave them much better control while swimming.
Dunkleosteus Skull http://en.wikipedia.org/wiki/Placodermi
An 8-11 meter long super predator of the Devonian period
Coccosteus
http://en.wikipedia.org/wiki/File:Coccosteus_BW.jpg
Figure 23.17
15.13
Early jawed fishes of the Devonian (400 mya).
Evolution of Jaws
 Vertebrate
jaws are made of cartilage
derived from the neural crest, the same
material as the gill arches (which support
the gills).
 Jaws
appear to have arisen by
modification of the first cartilaginous gill
arches, which aid in gill support and
ventilation.
Evolution of Jaws
 The
advantages of possessing jaws are
obvious.
 However,
structures must benefit the
organism at all times or they will not be
selected for.
 What
use would a proto-jaw have been
before being fully transformed?
Evolution of Jaws

Mallatt (1996,1998) has suggested that jaws
were originally important for gill ventilation, not
grasping prey.

Gnathostomes have much higher energy
demands than agnathans. They also possess a
series of powerful muscles in the pharynx.
These muscles allow them to both pump water
across the gills and suck water into the pharynx.
Evolution of Jaws
 It
is likely that selection initially favored
enlargement of the gill arches and the
development of new muscles that enabled
them to be moved and so pump water
more efficiently.
 Once
enlarged and equipped with muscles
it would have been relatively easy for the
arches to have been modified into jaws.
Evolution of Jaws

Being able to close the mouth would have enabled the
muscles of the pharynx to squeeze water forcefully
across the gills.

Selection would have favored any change in gill arches
and musculature that enhanced water movement over
the gills.

Thus, Mallatt suggested that the mandibular branchial
arch enlarged into protojaws because it allowed the
entrance to the pharynx to be rapidly opened and closed.
Evolution of Jaws
 Selection
would have favored enlargement
and strengthening of the mandibular arch
to tolerate the forces exerted on it by the
strong pharyngeal muscles.
 Once
the proto-jaws can be rapidly closed
they can also take on a grasping function
and new selective forces would quickly
have driven jaw elaboration.
Figure 23.16
15.12
Note resemblance between upper jaw (palatoquadrate cartilage) and lower jaw
(Meckel’s cartilage) and gill supports immediately behind in this Carboniferous shark
Evolution of Jaws
 Equipped
with jaws for grabbing and
holding prey and powerful pharyngeal
muscles that could suck in prey
gnathostomes could attack moving prey.
 An
enormous diversification of
gnathostomes followed.
Living fishes
 The
living fishes (not a monophyletic
group) include:




the jawless fishes (e.g. lampeys),
cartilaginous fishes (e.g. sharks and rays),
bony, ray-finned fishes (most of the bony
fishes such as trout, perch, pike, carp, etc)
and
the bony, lobe-finned fishes (e.g. lungfishes,
coelacanth).
Figure 24.01
16.1
Figure 24.02
16.2
Living jawless fishes
 There
are a little more than 100 species of
living jawless fishes or Agnathans (the
term agnathan does not represent a
monophyletic group).
 These
belong to two classes the Myxini
(hagfishes) and the Cephalaspidomorphi
(lampreys).
Characteristics of agnathans
 Lack
jaws (duh!)
 Keratinized plates and teeth used for
rasping
 Vertebrae absent or reduced
 Notochord present
 Dorsal nerve cord and brain
 Sense organs include taste, smell,
hearing, vision.
Hagfishes: class Myxini

Hagfishes are a marine group of primarily
scavengers.

They use their keen sense of smell to find dead
or dying fish and invertebrates and rasp off flesh
using their toothed tongue.

As they lack jaws, they gain leverage by knotting
themselves and bracing themselves against
whatever they’re pulling.
Figure 24.03
16.3
Hagfishes

Hagfishes are unusual in that they have body
fluids, which are in osmotic equilibrium with the
surrounding sea. This is unknown in other
vertebrates, but common in invertebrates.

They are also unusual in having a low pressure
circulatory system that has three accessory
hearts in addition to a main heart.
Hagfishes
 Hagfishes
have a remarkable (and
revolting) ability to generate enormous
quantities of slime, which they do to
defend themselves from predators.
A
single individual can fill a bucket with
slime.
Hagfish clip
 Eddie
and the Hagfish
 http://www.youtube.com/watch?v=NYRr_
MrjebA
Lampreys: Class
Cephalaspidomorphi

Lampreys occur in both marine and fresh waters
and about half of all species are ectoparasites of
fish (the others are non-feeding as adults and
live only a few months).

Lampreys spawn in streams and the larvae
(ammocoetes) live and grow as filter feeders in
the stream for 3-7 years before maturing into an
adult. Feeding adults live a year or so before
spawning and dying.
Figure 24.05
16.5
Lampreys

Parasitic lampreys have a sucker-like mouth with
which they attach to fish and rasp away at them
with their keratinized teeth.

The lamprey produces an anticoagulant as it
feeds to maintain blood flow. When it is full the
lamprey detaches, but the open wound on the
fish may kill it. At best the wound is unsightly
and largely destroys the fish’s commercial value.
Sea lamprey close up of sucker and teeth
Figure 24.06
16.4
Figure 24.04
Introduced sea lampreys

Landlocked sea lampreys made their way into
the Great Lakes around 1918 and caused the
complete collapse of the lake trout fishery by the
1950’s.

Lamprey numbers fell as their prey base
collapsed and control efforts were introduced.
Trout numbers have since recovered somewhat,
but wounding rates are still high.
Sea lampreys in Lake Champlain

Lake Champlain also has large populations of
sea lampreys which spawn in the creeks that
empty into the lake.

Until recently, lampreys were believed to have
been introduced into Lake Champlain, but
genetic analyses indicate the population was
established perhaps as much as 11,500 years
ago by lampreys that migrated up the St.
Lawrence.
Sea lampreys in Lake Champlain
 As
is the case elsewhere there has been a
campaign to control lamprey numbers
primarily by using lampricides in steams.
 Controls
do reduce lamprey wounding
rates and after control rates have fallen
from 60-70 wounds per 100 fish examined
to as low as 30 wounds/fish.
Lamprey clip
 Invading
Species Awareness PSA - Sea
Lamprey
 http://www.youtube.com/watch?v=x-
KJZ22-wTQ