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Chordates: Origin & Characteristics
[Academic Script]
Subject:
Zoology
Course:
B.Sc. 2nd Year
Paper No. & Title:
Z-201B
Animal Diversity-II
Topic No. & Title:
Topic – 1
Chordates:
Origin & Characteristic
Lecture Title:
Chordates :
Origin & Characteristic
Academic Script
1.Introduction:
Chordates occupy a greater variety of habitats and show
more complicated mechanisms of self-maintenance than any
other group in the whole animal kingdom. Chordates derive
their name from one of their synapomorphies, or derived
features indicating their common ancestry.
The chordate is the animal phylum, with which we all are
familiar, as it includes us – the human beings, along with
other vertebrates. It is also true that all chordates are not
vertebrates.
Origin:
Several
hypotheses
are
produced
by
zoologists
while
attempting to find out the evolutionary relationships of the
chordates. We will try to understand different theories of origin
of chordates and their characteristics in this programme.
It is said that the chordates are evolved from animals like
Amphioxus. We may look farther backwards to understand the
origin of the whole Chordate phylum from still earlier
ancestors. They may have existed in the earliest Paleozoic
times. The great difficulty of such an inquiry in itself is a
stimulus and a challenge.
The Fish-like forms developed in early Ordovician strata, as
Scales and bones have been found from the fossil collection of
this period. So the origin of the Chordates must be sought far
back in the Cambrian period, perhaps nearly 600 million years
ago. It is believed that Chordates evolved during the time of
Cambrian Explosion, when Invertebrates were also evolving.
Now we must think about which of the main lines of
Invertebrates shows the closest affinity with the Chordates.
Almost every phylum in the Animal kingdom is suggested,
including the Arachnids and Nemertines to have affinity with
Chordates.
Because of their body segmentation, it has been suggested
that
Annelids
Chordates.
and
However
Arthropods
the
have
detailed
closer
study
affinity
suggests
with
many
differences between Chordates and these groups.
Invertebrates like Arthropods possess segmentation affecting
most organ systems, while in Chordates, only dorsal myotomal
region is segmented. Even the mesoderm is not divided in its
ventral region in most Chordates.
In Annulates, the organisms marked with rings, the nerve cord
is present on ventral side and the ‘brain’ on the dorsal side.
The Vertebrate nerve cord is dorsal to the gut & the anterior
part is modified in the comparative ‘brain’. The circulatory
system in Annelids is closed and open in Arthropods, while all
Chordates possess closed circulatory system.
Patten and Gaskell carried out such theories to extreme and
tried to show relationship of Chordates with the Eurypterids as
well as Arachnids of the Cambrian and Silurian periods. These
animals show a certain resemblance to some early fossil
fishes, for example, the Cephalaspids of the Devonian period.
The modern Annulates differ from that of Chordates according
to the embryonic development point of view of evolution, like.
1) In Annulates, the arrangement of blastomeres during
cleavage is spiral hence known as Spiralia, while in Chordates,
it is irregular, hence called Irregularia.
2) Gastrulation occurs by immigration in Annulates, while it
occurs by invagination in Chordates
3) Regeneration of ATP is accomplished from phosphoarginine
by its kinase in all Arthropods and Molluscs. This method is
primitive
as
it
occurs
in
many
Bacteria,
Protozoa,
Coelenterates and Platyhelminths. Both the systems are found
in Annelids, Echinoderm and Tunicates, whereas in Amphioxus
and Chordates, phosphocreatine and its kinase provide the
sole phosphagen system.
2.Evolution:
In
Chordates,
Echinoderms,
Tunicates
and
Hemichordates, the opening of gastrula, the blastopore,
becomes the anus and the mouth is a new opening. So these
animals are grouped together as Deuterostomia, distinct from
the other Non-Vertebrates, where the blastopore forms the
mouth, hence they are called Protostomia. The mechanism for
providing energy liberation also shows systematic difference.
According to Chamberlin, all modern Chordates possess the
glomerular kidneys to remove excess water from the body.
They might have evolved from some fresh water forms. But
the early fossils of Chordates were recovered from marine
sediments and even modern Protochordates are all marine
forms. Myxinoids and Sharks possess the glomerular kidneys.
It supports the theory of marine origin of Chordates.
As they have similarities in embryonic development, type of
coelom and larval stages, Chordates might have evolved from
the Deuterostome ancestors like Echinoderm, Hemichordates
and Pogonophorans.
Simply comparison of two groups is not enough to understand
the evolution. Extending the same method, we may divide the
whole world of Metazoa into two groups: 1) The Spiralia
including Arthropods, Annelids, Molluscs, and Platyhelminthes
and 2) The Irregularia including Chordates, Echinoderms,
Brachiopods, Polyzoans, Graptolites and Phoronis.
At first sight we cannot believe that there may be any
similarities in the animals belonging to these groups, but when
we find similarities in the fundamental organization, we
become more convinced that the ancestors of Fish-like animals
are to be found here.
By study of living relics of early Chordates, it is possible to
trace the history of this strange change with some plausibility,
although some zoologists like Brien still find that ‘The idea of
bringing such unlike animals together is amazing’. Fossils of
the earliest Vertebrates are known from the Silurian-Devonian
period i.e. about 400 million years ago.
3.Theories of Origin:
Over the years, different theories have evolved to explain
the Origin of Chordates. We will understand them in details.
1. Echinoderm Origin
by Johannes Muller (1860)
The theory is based on the comparative studies of larval
stages of Echinoderms and Hemichordates. Tornaria larva of
Hemichordates
resembles
Bipinnaria, Auricularia,
Echinoderm
larvae
such
as
Dipleurula and Doliolaria, which all
possess ciliary bands and apical tuft of cilia. Johannes Muller,
W. Garstang and DeBeers proposed that Echinoderm larvae
gave rise to Chordates by neoteny.
Further, like Chordates, Echinoderms are also Deuterostomia
and possess mesodermal skeletal elements. The discovery of
fossil
period,
Echinoderms
confirms
called Calcichordata from
Echinoderm
ancestry
of
Ordovician
Chordates.
Calcichordates were asymmetrical animals which demonstrate
affinities with both Echinoderms and Chordates but their
skeleton is made of CaCO3, where as in Vertebrates the bones
are made of hydrated Calcium and Phosphate.
The Calcichordates had large pharynx with a series of gill slits,
each covered with flaps for filter feeding, a small segmented
body and a post anal tail. A perforated pharynx for filter
feeding appears to have evolved in diverse groups of animals
during Cambrian-Ordovician periods when Planktons were
abundant in water.
2. Hemichordate Origin ( Stomochordata )
by Romer (1959)
This theory suggests that ancestral Deuterostomes were
sedentary
tentacle
feeders
whose
mucous-laden
ciliated
tentacles served to trap Plankton as they were waved in water
as
do
the
modern
Lophophorates
Hemichordates, Cephalodiscus
&
Pterobranch
& Rhabdopleura.
Due
to
mutation pharyngeal gill slits evolved in these ancestors, the
pharynx became sieve-like to trap Plankton as the water
current passed through it. Extant Pterobranch possess both
ciliated arms and pharyngeal gill slits.
Tornaria
larva
of
Hemichordates
show
phylogenetic
relationship with Echinoderm larvae and it also shows affinities
with Chordates.
3. Urochordate Origin
by W. Garstang (1928) and N.J. Berrill (1955)
They
gave
Urochordates
importance
which
to
carries
the
tadpole-like
typical
Chordate
larva
of
characters,
namely, a notochord in tail along with segmented myotomes,
Dorsal hollow nerve cord, Sense organs and Pharyngeal gill
slits.
Garstang suggested that Chordates evolved from some sessile
filter feeding Urochordate. The larval stage evolved into adult
by neoteny and lost the sedentary adult stage.
4. Cephalochordate Origin
by Chamberlain (1900)
He
studied
the
Cephalochordates
primitive
and
and
advanced
proposed
that,
characters
while
of
extant
Cephalochordates possess all the Chordate characters in
typical state, they also show some primitive features of nonChordates, such as, absence of heart, head, sense organs,
respiratory pigment, filter-feeding mode of food capture and
excretion by solenocytes.
The current consensus is that Chordates are monophyletic,
meaning that the chordata contains the only descendants of a
single common ancestor which is itself a Chordate, and the
nearest relatives of Craniates are Cephalochordates.
Fossils of 60 specimens from mid-Cambrian of the earliest
Chordate, Pikaia gracilens have been discovered from Burgess
Shale in British Columbia, Canada. The Amphioxus-like fossils
show streamlined, ribbon-shaped, 5 cm long body having
notochord in the posterior two-third of body and myomeres. It
has a small head with two tentacles and gill slits in the neck
region.
Other
Chordate-like
fossils
are
Cathaymyrus from
early
Cambrian sediments in China and Palaeobranchiostomata from
early Permian from South Africa that appears to be more
similar to Amphioxus.
5. Combined theory
by E.J.W. Barrington (1965)
He combined all the theories and proposed that the common
ancestor of Echinoderms and Chordates was, a sessile ciliary
arm feeder that lived in the Plankton-rich environment of the
Cambrian period.
According to this theory, the Modern Crinoidea also called
Echinodermata, Pogonophora and Pterobranch Hemichordates
evolved from a similar ancestor by retaining the original mode
of feeding. Perhaps because they have continued to inhabit the
same environment as occurred in the ancestral days.
4.Further Transition:
However, Pharyngotremy must have evolved in a large
number of groups at the same time, which must have been
much more superior method of food gathering by filtering
water
through
pharynx,
as
compared
to
ciliatedarm
feeding. Hence, the sedentary Protoascidians of that time, lost
ciliated arm feeding and adopted pharyngeal filter feeding as
the only method of food gathering.
Sometime later, when the Plankton population in water
declined, free-swimming tailed larvae of Urochordates did not
metamorphose.
They
became
neotenic
adult,
as
free-
swimming mode was superior in food searching at the time of
food scarcity.
By perfection and expansion of Chordate characters, which
were
already
present
in
the
Ascidians
tadpole
larva,
Cephalochordate-like ancestors were evolved. We have fossils
of such primitive Chordates, for example, Pikaia gracilens from
mid-Cambrian.
All of the earliest Chordate fossils have been found in the
Early Cambrian Chengjiang fauna
and include two species
that are regarded as Fish, which implies that they are
Vertebrates.
Because
the
fossil
record
of
Chordates
is
poor,
only molecular phylogenetics offers a reasonable prospect of
dating their emergence. However, the use of molecular
phylogenetics
for
dating
evolutionary
transitions
is
controversial.
It
has
also
proved
difficult
to
produce
a
detailed
classification within the living Chordates. Attempts to produce
evolutionary "family trees" give results that differ from
traditional classes, because several of those classes are not
monophyletic. As a result vertebrate classification is in a state
of flux.
5.Characteristics:
Now let us understand the characteristics of chordates.
Characteristics of Chordates: Phylum Chordata is a group
of animals which are either Vertebrates or one of the several
closely related Invertebrates. They are united for at least
some period of their life cycle, by having a notochord, a
hollow dorsal nerve cord, gill slits and an endostyle and a
post-anal tail. We will understand them in detail.
A. Notochord:
The notochord is an elongated, rod-like structure, which is
developed from the mesoderm. It is made up of skeletal tissue
and located dorsal to the gut tube and ventral to the nerve
cord. It is a flexible structure that forms the main support of
the body in the lowest Chordates, such as the Lancelet, a
primitive backbone. From a similar structure in embryos of
higher Vertebrates, the vertebral column is developed.A semiflexible rod is running along the length of the animal.
Chordates, which lack bone, muscles work against the
notochord to move the animal. Mostly, all Chordates have a
notochord at some stage in their lives, but in some, such
as Tunicates the notochord is lost in the adult, whereas in
others, such as the Vertebrates the notochord is present in the
embryo, but in later stages is largely replaced and surrounded
by the vertebrae, or backbones. The notochord should not be
confused with the backbone or vertebral column of most adult
Vertebrates. The notochord appears early in embryogeny and
plays an important role in promoting or organizing the
embryonic development of nearby structures. In most adult
Chordates
the
notochord
disappears
or
becomes
highly
modified. In some Non-Vertebrate Chordates and Fishes the
notochord persists as a laterally flexible, but incompressible
skeletal rod, that prevents telescopic collapse of the body
during swimming.
B. The Nerve cord:
The nerve cord of chordates develops dorsally in the body as a
hollow tube above the notochord. This is in contrast to
organisms such as Annelids and Arthropods, in which the main
nerve cord is solid and located ventrally. In most species, it
differentiates during embryogeny anteriorly into the brain and
a spinal cord that runs through the trunk and tail. The
Chordate nerve cord is hollow, with pairs of nerves branching
from it at intervals and running to the muscles. Together, the
brain and the spinal cord, form the central nervous system, to
which peripheral sensory and motor nerves are connected.
C. The Visceral Clefts and Arches:
Gill slits are, at some stage of life, found in all chordates. They
have been modified extensively in the course of evolution.
The visceral,
also
called
pharyngeal
or
gill,
clefts
and
arches are located in the pharyngeal part of the digestive tract
behind the oral cavity and anterior to the oesophagus,
meaning they are the openings between the pharynx and the
outside.
The visceral clefts:
They appear as several pairs of pouches that push outward
from the lateral walls of the pharynx eventually to reach the
surface to form the clefts. Thus the clefts are continuous, slitlike
passages
connecting
the
pharynx
to
the
exterior.
Pharyngeal slits are filter-feeding organs found in nonvertebrate
chordates
like
lancelets
and
tunicates,
and
hemichordates, living in aquatic environments. These repeated
segments
are
controlled
by
similar
developmental
mechanisms. Pharyngeal slits, resembling the gill slits, are
transiently present during the embryonic stages of Tetrapoda.
In Fishes and some Amphibians, the slits bear gills and are
used for gas exchange. In most land- living Chordates, the
"gill slits" are present only in embryonic stages.
The Visceral Arches:
The soft and skeletal tissues between adjacent clefts are the
visceral arches. The slits are supported by gill arches, which
have
also
been
highly
modified
in
various
groups
of Vertebrates. The embryonic fate of the clefts and slits varies
greatly depending on the taxonomic subgroup. In many NonVertebrate
Chordates,
such
as
Tunicates
and
Cephalochordates, the clefts and arches are elaborated as
straining devices concerned with capture of small
food
particles from the water. In typical Fish-like Vertebrates and
juvenile Amphibians, the walls of the pharyngeal clefts develop
into gills that are organs of gas exchange between the water
and blood. In adult Amphibians and the amniotic Tetrapods
like Reptiles, Birds and Mammals, the anterior most cleft
transforms into the auditory tube and middle ear chamber,
whereas
the
other
clefts
disappear
after
making
some
important contributions to glands and lymphatic tissues in the
throat region. The skeleton and muscles of the visceral arches
are the source of a great diversity of adult structures in the
Vertebrates. For example, in Humans and other Mammals,
visceral arch derivatives include, the jaw and facial muscles,
the embryonic cartilaginous skeleton of the lower jaw, the
alisphenoid bone in the side wall of the braincase, the three
middle ear ossicles, the skeleton and some musculature of the
tongue, the skeleton and muscles of the larynx and the
cartilaginous tracheal rings.
D. The Post-anal tail:
All Chordates have a post-anal tail, or extension of the
notochord and nerve cord past the anus. This feature is also
lost in the adult stages of many Chordates, such as Frogs and
Primates including Human beings.
6.Sub-phyla:
Now let us understand three Subphyla of Phylum Chordata
namely Tunicata, Cephalochordata and Craniata.
1. Tunicata is represented by Tunicates. Tunicate larvae have
both a notochord and a nerve cord which are lost in adulthood.
The strictly marine Urochordata or Tunicata are commonly
known Tunicates, Sea Squirts, and Salps. There are roughly
1,600 species of Urochordates. Most are small solitary animals
but some are colonial organisms. Nearly all are, sessile as
adults, but they have free-swimming active larval forms.
Urochordates are unknown as fossils.
2. Cephalochordata,
also
known
as
Amphioxus,
is
represented by Lancelets. They have a notochord and a nerve
cord, but no brain or specialized sensory organs, and a very
simple circulatory system. The group contains only about 20
species of sand-burrowing marine creatures.
3. Craniata.
Craniates
are
the
only
sub-phylum
whose
members have skulls. The Vertebrates and Hagfishes together
comprise
the
taxon
Craniata.
In
all
Craniates,
except
for Hagfish, the dorsal hollow nerve cord is surrounded
with cartilaginous or bony vertebrae and the notochord is
generally
reduced.
Hence,
Hagfish
is
not
regarded
as
Vertebrates.
The remaining Chordates are possibly, some odd extinct
groups.
The
Chordates
and
three
sister phyla,
the Hemichordata, the Echinodermata and the Xenoturbellida,
make up the Deuterostomes, one of the two superphyla that
encompass all fairly complex animals.
7.Summing Up:
The Phylum Chordata includes the well-known Vertebrates,
namely fishes, amphibians, reptiles, birds and mammals.
General characteristics of Vertebrates
We need to be familiar with some of the traits to identify how
they are expressed in the Vertebrates, either in the laboratory
or in nature. Vertebrates may be characterized by 12 general
derived characteristics. They are,
1. Bilateral symmetry
2. Two pairs of jointed locomotor appendages
3. Skin - modified as scales, hair or feathers
4. Because of Metamerism, the ribs, vertebrae, muscles and
ganglia or peripheral nerves are formed
5. Coelom or body cavity is well-developed
6. Well-developed axial skeleton and appendicular skeleton
7. Highly developed brain enclosed by skull, and nerve cord
enclosed by vertebrae.
8. Well-developed sense organs i.e. eyes, ears, nostrils
9. Gills or lungs are located closely to the pharynx or throat.
10. Closed circulatory system with ventral heart and median
dorsal artery.
11. Genital and excretory systems closely associated to form
the urinogenital system
12. Digestive tract with two major digestive glands - liver and
pancreas
With few exceptions, Chordates possess Bilateral symmetry.
Body is divided into three parts namely Head, trunk and tail.
Chordates are well represented in marine, freshwater and
terrestrial habitats from the Equator to the high northern and
southern latitudes. Chordates other than Craniates, include
entirely aquatic forms.
The smallest Chordates, for example, are some of the
Tunicates and Gobioid fishes. They are mature at a length of
about 1 cm. The largest Chordates existed were Sauropod
Dinosaurs, which reached more than 20 meter height and
living blue whales, grow to about 30 meter length.
So in this programme, we learnt about the origin and general
characteristics of Chordates.
Downloads:
The Fossil record to Remember:
 The oldest fossil Chordates are of Cambrian age. The
earliest is Yunnanozoon lividum of China from the Early
Cambrian, 525 ma (= million years ago). This fossil was
recently described and placed with the Cephalochordates
(Chen et al., 1995).
 Another possible Cephalochordate is Pikaia (Nelson, 1994)
from the Middle Cambrian.
 These fossils are highly significant because they imply the
contemporary existence of the Tunicates and Craniates in
the Early Cambrian period during the so-called Cambrian
Explosion of animal life.
 Two other extinct Cambrian taxa, the Calcichordates and
Conodonts, are uncertainly related to other Chordata
(Nelson, 1994). In the Tree of Life project, Conodonts are
placed as a subgroup of Vertebrates.
 During the Ordovician Period (510 - 439 mya), jawless or
Agnathan fishes appeared and diversified. These are the
earliest known members of Vertebrata, the Chordate
subgroup that is most familiar to us.
 Fossils representing major lineages of Fish-like Vertebrates
and the earliest Tetrapods (Amphibian) were in existence
before the end of the Devonian Period (363 mya).
 Reptile-like Tetrapods originated during the Carboniferous
Period (363 - 290 mya).
 Mammals differentiated before the end of the Triassic
Period (208 mya) and Birds before the end of the Jurassic
Period (146 mya).
Remember General characteristics of Vertebrates with
clear understanding:
One should become very familiar with these traits, and identify
how they are expressed in the Vertebrates, in the laboratory or in
nature.
Vertebrates
may
characteristics.
be
characterized
by
12
general
derived
1. Bilateral symmetry
2. Two pairs of jointed locomotor appendages, which can
include fins (pectoral and anal/dorsal fins, as well as the
forelimbs and hind limbs).
3. Outer covering of protective cellular skin, which can be
modified into special structures such as scales, hair and
feathers
4. Metamerism
found
in
skeletal,
muscular
and
nervous
systems. The structures can include ribs, vertebrae, muscles
and ganglia / peripheral nerves.
5. Well-developed coelom, or body cavity completely lined with
epithelium (cellular tissue), that may be divided into 2 to 4
compartments.
6. Well-developed internal skeleton of cartilage and bone,
separated
into
sternum)
and
axial
skeleton
appendicular
(skull,
skeleton
vertebrae,
ribs,
(girdles
and
appendages).
7. Highly developed brain enclosed by skull, and nerve cord
enclosed by vertebrae. This provides advanced neural
structures that are highly protected from damage.
8. Well-developed sense organs (eyes, ears, nostrils) located
on the head (cephalisation).
9. Respiratory system, including either gills or lungs, and
located closely to the pharynx or throat.
10. Closed circulatory system with ventral heart and median
dorsal artery.
11. Genital and excretory systems closely associated, utilizing
common ducts and pathways. Hence also known as the
urinogenital system
12. Digestive tract with two major digestive glands (liver and
pancreas) that secrete the bile and pancreatic juice into it.