Vertebrate Origins
Vertebrates are but a single
subphylum within the chordates.

What defines a chordate?




Notochord at some stage of development.
Dorsal hollow nerve cord.
Pharyngeal gill slits present at some stage
of development
Endostyle (becomes thyroid gland in
vertebrates). It is a ciliated glandular
groove on the floor of the pharynx, that
aids in filter feeding by secreting mucus,
and just as in the thyroid, it is able to
concentrate iodine.
Vertebrates are but a single
subphylum within the chordates.



Muscular postanal tail
Ventral heart with a closed circulatory
system.
Living bony or cartilaginous endoskeleton.
Chordata Include:

a. Urochordata – Tunicates
As
envisioned
by Pough et
al.
As envisioned originally
by Romer.
Chordates include:Cephalochordates
Cross section
through
Amphioxus, a
cephalochordate.
Cephalochordates include:
Vertebrates

What defines a vertebrate?

Presence of vertebrae!




They are cartilaginous in some fishes.
They are absent in hagfishes.
Lampreys possess only rudimentary
cartilaginous elements around the nerve cord.
Note, at one time these organisms were
considered to be degenerate.
Presence of a Cranium (hence the original
name of the group: Craniata.
Vertebrae and
cranium for the
group
Vertebrata.
Note: these
mammalian
structures are
highly derived.
What defines a vertebrate?


Presence of duplicated Hox gene
(homeobox gene)
Presence of embryonic tissue called the
neural crest, which give rise to
epidermal placodes. These are the
origin of the complicated sensory tissue
characteristic of vertebrates.
An interesting observtion
about vertebrates:




While most animals are small, vertebrates
are relatively large. Thus diffusion is no
longer sufficient for most bodily functions.
This necessitates specialized structures and
systems in vertebrates.
Basal metabolic rates in vertebrates are
higher than other animals.
Vertebrates are easily capable of anaerobic
metabolism.
What is the evolutionary history of the
vertebrates?

3 hypotheses

Arthropod hypothesis



Arthropods are a major animal group –
common and therefore likely to have daughter
groups.
They share some characteristics with the
vertebrates.
If you turn an arthropod upside down, you
have the basic vertebrate body plan.
What is the evolutionary history of the
vertebrates?

3 hypotheses

Arthropod hypothesis




The body is segmented.
There is a ventral nerve cord and a dorsal
heart.
Problem – the exoskeleton.
This idea dates to 1818 by St. Hilaire.
What is the evolutionary history of the
vertebrates?

3 hypotheses

Annelid hypothesis


Semper and Dohrn noted in 1875 that annelidshave the
same basic body plan as vertebrates, only upside down,
and they have an excretory system that is remarkable
similar to that of some chordates.
Problem – the nerve cord is ventral and bifurcates to go
around the pharyngeal tube to a dorsal brain. If you
turn the organism upside down, the brain is ventral and
the mouth dorsal … a situation which does not show up
in any vertebrate.
What is the evolutionary history of the
vertebrates?

3 hypotheses

Echinoderm - Hemichordate – Chordate
Hypothesis hypothesis

Both of the above hypotheses suffer from the
fact that annelids and arthropods have spiral
determinate cleavage while chordates have
radial indeterminate cleavage.
What is the evolutionary history of the
vertebrates?

3 hypotheses


Both annelids and arthropods are
protostomes while chordates are
deuterostomes.
Arthropods and annelids have shizocoelous
coelom formation while chordates have
enterocoelous coelom formation.
What is the evolutionary history of the
vertebrates?

3 hypotheses


Echinoderms have precisely the same
characters as the chordates: radial
indeterminate cleavage, deuterostomes, and
enterocoelous coelom formation.
Also, some echinoderm bipinnaria larvae
resemble closely the tornaria-like larvae of
some chordates in that both have sensory cilia
at the anterior end, both have a complete
digestive system with ventral mouth and
posterior anus, and both have ciliated bands in
loops.
Diagramatic side views of larvae of A: acorn worm, B:
starfish, and C: sea cucumber. Black lines represent
ciliated bands. The digestive tracts are stipled. All are
bilaterally symmetric.
What is the evolutionary history of the
vertebrates?

3 hypotheses


It is important to remember that the
echinoderms we see today are probably very
dissimilar from the echinoderms that were the
actual ancestors to the chordates. Early
echinoderms for example were not pentaradial.
The diversity of echinoderms today is but a
fraction of what was once there.
Not all basal deuterostomes were asymmetrical
or pentaradial. The calcichordata were
bilaterally symmetrical, and may in fact be
specialized echinoderms.
Calichordate – fossil
deuterostome
Possible early chordate from MidCambrian Burgess Shale Pikaia
Non-vertebrate Chordates
Urochordates
Tunicates (sea squirts)

Sea squirts have sessile filter feeding adults
and free swimming planktonic larvae. Larvae
look similar to amphioxus – basic vertebrate
body plan. Have pharyngeal gill slits,
notochord, dorsal hollow nerve cord, muscular
post anal tail
Urochordates

Adults however, look very different.
How could this lead to vertebrates?


Paedomorphosis – retention of juvenile
morphology in the reproductive adult. This
is an example of heterochrony.
Alternatively, we may be derived from the
sessile adult stage.
Tunicates and Hemichordates: A & B Pterobranchs, C: colonial
tunicate, D tunicate, E free swimming tunicate, and F acorn worm.
Longitudinal section through the head of an acorn worm
(Hemichordata)
Urochordates


Chordates are unique in having innervation of 2 types:
segmented innervation and non-segmented innervation.
It may be that we were originally nonsegmented (like
the sessile adults) and later our morphology was overrun by the newly derived segmented
components.
Also, chordates have allorecognition. Invertebrates do
not. However, echinoderms have allorecognition, as do
some colonial organisms. Perhaps it is a means of
preventing fusion of non-identical organisms. The
ancestors of echinoderms may have been colonial and
sedentary.
Contrast between visceral and somatic components.
Tunicate like larvae w/ somatic component retained in
adult, and true vertebrate w/ visceral in black.
Cephalochordates
Fish-like in appearance and totally marine.
Best know example is amphioxus (lancelet).
Has segmented myomeres, and many
homologies with vertebrates.
Generalized nonvertebrate
chordate design
compared with
hypothetical
primitive
vertebrate.
So, the
vertebrate family
tree looks
something like
this … in Romer’s
depiction.
Who are the earliest
Vertebrates?



Probably the conodonts, although the
issue has not been resolved.
Conodont fragments are abundant –
probably ‘teeth.’ These are referred to
as ‘conodont elements.’ Impressions of
complete conodont animals have been
found.
This raises the issue of what becomes a
fossil.
Fossilization?

Most fossils are mineralized tissue.
Bone is an excellent site for
mineralization, as are odontodes in the
skin. Teeth too are good.
There is some question about when bone
evolves as a vertebrate character.
Hagfish and lampreys have no bone (they
do have inner ear ossicle)

Nature of early bone has some implications
for physiology – ion & fluid regulation.
What is the function of early
bone?

May serve a protective function. There were
large aquatic invertebrate predators, and the
armor of ostracoderms and placoderms may
have prevented predation.


Unfortunately, the bony armor is below the skin
and thus susceptible to injury
Perhaps it was used as a mineral sink? This is
related to an early hypothesis about where
vertebrates evolved.
Did vertebrates have a freshwater or
marine origin?

Romer and Smith argued for a
freshwater origin.




Bone may represent a mineral sink.
Phosphates and calcium were probably a
‘hot’ commodity in the Silurian.
Bone armor may have prevented osmosis.
Although all fossils were found in marine
sediments, they argued the fossils washed
into the sea.
Did vertebrates have a freshwater or
marine origin?



All fossils are marine.
All old vertebrate groups are marine.
Kidney function was probably co-opted
from other mineral regulation functions.


(Do fish drink?)
Prevailing view today is that vertebrates
have a marine origin.
Vertebrate Ancestry

Ostracoderms




Oldest fossil vertebrates except conodonts.
First discovered in Ordovivian rock in
Russia and the U.S.
Belong to agnathan/cyclostome group.
Major radiation in the Silurian and
Devonian, but extinct by the end of the
Devonian.
Vertebrate Ancestry

Ostracoderm morphology



No jaws
No paired fins.
Heavy bone armor.
Vertebrate Ancestry

Placoderms




Less developed bony armor
Paired fins and thus probably more active
swimmers.
Had jaws and were capable of predaceous
life-style
First appeared in Silurian, major radiation
in Devonian, extinct by end of Permian.
Vertebrate Ancestry


One Placoderm group (acanthodians) had
bony scales like modern fishes.
Placoderms may have given rise to, or had
a common ancestor with 2 major groups:
the Chondrichthyes and the Osteichthyes.
Vertebrate Ancestry

Chondrichthyes


No bone, probably underwent reduction
from Placoderm condition, or may
represent true underived condition. Could
this be an example of neoteny or
paedomorphosis? They have a living
endoskeleton, but it is made of cartilage.
Completely predaceous life-style.
Vertebrate Ancestry



They have a spiracle.
They have internal fertilization.
The holocephalans (chimeras) have an
upper jaw that is fused to the brain
case, and a flap of skin that covers the
gill region.
Vertebrate ancestry

Osteichthyes



They have a bony endoskeleton, probably
a retention of the ostracoderm or
placoderm condition.
They have bony scales and opercula
Origin was the Devonian, they split almost
immediately into 2 groups: the
Actinopterygians and the Sarcopterygins
Vertebrate Ancestry

Actinopterygians.


Chondrosteans (sturgeons), Holosteans
(bowfins and garpikes) and Teleosts
(modern bony fishes).
Sarcopterygians.

Dipneusti (lungfish), crossopterygians and
ceolocanths.