Origin and Diversification of the Vertebrates

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Origin and Diversification of the Vertebrates
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Who are the living vertebrates? - Jawless fish
hagfish and lamprey - Fish with jaws cartilage
skeletons sharks and rays - Fish with jaws
bony skeletons all other fish (tuna, flounder,
bass, etc.) - Amphibians frogs and salamanders
cold-blooded, lay eggs in water - Reptiles
turtles, snakes and lizards cold-blooded, lay
eggs on land - Birds warm-blooded, feathers,
lay eggs - Mammals warm-blooded, hair, eggs
live birth, nurse young.
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Vertebrate Characteristics
- Backbone - Spinal cord - Heads - Tails -
Heart - "Gill" Slits - Segmented muscle on body
wall - Internal skeletons of hydroxyapatite -
Jaws - Two pairs of appendages
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Origin and Diversification of Vertebrates
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Echinoderms
Unique characters 5-fold symmetry in adults,
water-vascular system, a uniquely constructed
calcite skeleton. Shared novelties Embryonic
traits (Radial pattern of embryonic cleavage,
Deuterostome, Mesoderm formed by pouching)
Skin-based nerve network Bilateral,
cilia-covered larvae. Range Cambrian - Recent
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Hemichordata (Acorn Worms)
Unique characters Acorn worms are large(up to 2
m), burrowing worm- like filter-feeders with a
long muscular proboscis and a fleshy collar.
Shared novelties Adults are bilaterally
symmetric Closed circulatory system Paired
openings in the throat. Range Cambrian -
Recent
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Urochordata (Sea squirts)
Unique characters Tunicates are small, box-like
filter-feeding animals that live either alone or
in colonies cemented to the sea floor.
Shared novelties Notochord Hollow nerve cord
along back Tail Endostyle, an organ used for
filter-feeding. Range No fossil record
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Cephalochordata (Lancelates)
Unique characters Branchiostoma, also known as
the lancelet, is a small, free- living fish-like
animal that lives among sand grains and filter
feeds. Shared novelties Segmented muscle on
upper body wall. Range Cambrian - Recent
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Cephalochordates of the Burgess Shale - Pikaia
Unique characters Branchiostoma, also known as
the lancelet, is a small, free- living fish-like
animal that lives among sand grains and filter
feeds. Shared novelties Segmented muscles on
upper body wall. Range Cambrian (Pikaia from the
Burgess Shale) - Recent
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The Importance of Swimming The notochord, a
stiff rod of connective tissue, provides internal
support that permits efficient side-to-side
motion for swimming. When muscles contract, the
organism bends, rather than compressing like an
accordian. The tail and hollow nerve cord,
coupled with a closed circulatory system, are all
probably related to this more active swimming
life style.
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Craniates Class Myxini (Hagfish)The most
primitive known "vertebrate"
Unique characters Scavengers and carnivores that
actively feed by rasping at prey with a bony
tongue. They tie themselves into a knot to lever
a chunk out of prey. They can coat themselves
with mucous for defense. They contain no
vertebrae and no bone. Shared novelties A head
(cartilage brain case) Sense organs on the head
(weak eyes) A true heart True gills for
efficient oxygen retrieval from water Cartilage
gill supports to hold up these flimsy
sheets.Range Carboniferous-Recent
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Craniates Heterostracans The first truly
abundant fishes
Unique characters Jawless, armored body with
scales on the tail. The tail was the main source
of propulsion. Bottom feeding hunters and
detritus feeders. Still no vertebrae.
Shared novelties Improved sense organs (better
balance vision, lateral line system for motion
detection and probably electroreception (used to
hunt) Bone on the outer skull but not on the
braincase. Range Cambrian - Devonian
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• Why Bones of Calcium Phosphate?
• - Less soluble than calcite, not as subject to
  dissolution by metabolic acids.
• - Store of an important nutrient, phosphate.
• - May serve as insulation for electroreceptors.
• - Protection.

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The First True Vertebrates (no jaws) Lampreys
Unique characters The adult is a parasitic
bloodsucker. It is jawless, but its mouth has
many hooks for latching onto prey, then they use
the tongue to bore through the side of the prey.
No bone on the body - an evolutionary reversal.
Shared novelties Vertebrae surrounding
notochord (made of cartilage) Dorsal and anal
fins Endostyle turns into thyroid. Range
Carboniferous - Recent
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Osteostracans (extinct armored jawless fishes
Unique characters Similar to heterostracans,
with a bony head shield, scales on the tail,
propulsion from the tail, well developed sense
organs, and elaborate plumbing for gill system.
They were active swimmers. Many were bottom
feeders. Shared novelties Paired pectoral
fins (source of forelimbs) Braincase covered in
bone. Range Silurian - Devonian
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Invertebrate-Vertebrate Evolutionary Transition
Overview - Recent studies in vertebrate
evolution (e.g., based on studies in
developmental genetics and paleontology) suggest
that evolution of the vertebrate brain may have
had a surprisingly early start in invertebrate
ancestors, long before the mineralized skeleton
that makes vertebrates so distinctive - The true
innovation that launched the lineage of fish and
other vertebrates seems to have been new kinds of
embryonic tissue, which could form new sensory
organs - This allowed protovertebrates, such as
Haikouella and Myllokunmingia, to embark on a new
way to make a living- as predators
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Craniates that may be important transitional
fossils Haikouella and Myllokunmingia
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Amphioxus - One way to track vertebrates
evolutionary history is to analyze their closest
living relative Molecular and anatomical research
both indicate that this is Amphioxus
- Paleontologists have long suspected that
vertebrate diverged from a lancelet-like relative
sometime in the Cambrian period - Meanwhile,
molecular studies of gene similarities between
lancelets and today's vertebrates suggest that
the vertebrate lineage goes all the way back to
750 million years ago!
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Brain and Bone - Until very recently, the
earliest undisputed vertebrates were a mere 475
million years old. - These small, jawless fish
(heterostracans?) with bodies completely covered
in bony plates of armor are thought to have fed
on sea-floor invertebrates and to have used their
armor to defend against predators. - Fossils
retaining the imprint of the brain reveal that
these fish had already evolved many of the major
features of modern vertebrate brains, such as
divisions into forebrain, midbrain, and
hindbrain. - If these armored fishes represent
the earliest vertebrates, they suggest that
brains and bone evolved together. - But, with no
obvious intermediates among either ancient or
living creatures, biologists were hard put to
explain the origins of the vertebrate skeleton
and nervous system. - In 1983, Northcutt and
Gans argued that the key to vertebrate evolution
was the invention of a head, which in turn was
made possible by the evolution of a new kind of
embryonic cell.
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An Overview of Early Development of the
Vertebrate Embryo
- Various regions of the three germ layers
develop into the rudiments of organs during the
process of organogenesis - A number of kinds of
morphogenetic changes, including folding,
splitting, condensation (clustering) of cells
etc. occur within the layered embryonic tissues
and represent the first evidence of organ building
Gastrulation in a Vertebrate Embryo
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Neural Tube Formation in a Vertebrate Embryo
- The organs that first begin to take shape in
the embryos of vertebrates are the neural tube
and notochord - The notochord is formed from
condensation of the dorsal mesoderm just above
the archenteron, and the neural tube originates
as a plate of dorsal ectoderm just above the
developing notochord - The neural plate soon
undergoes folding, actually rolling itself into
the neural tube, which will eventually become the
CNS
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Early Embyogenesis cont.
- Unique to vertebrate embryos, a band of cells
called the neural crest cells developsalong the
border where the neural tube pinches off from the
ectoderm - Cells of the neural crest later
migrate to various parts of the embryo, forming
pigment cells of the skin, some of the bones and
muscles of the skull, etc.
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- Neural crest supposedly gave vertebrates the
flexibility to build a new kind of body, one that
included the complex sense organs, big brains,
and powerful pumping throats seen for the first
time in lampreys and fossil jawless fish. -
Along with the new body plan came an ecological
shift, as vertebrates evolved from small, passive
filter feeders to large, active predators that
darted about hunting their prey. - In short,
developmental changes produced new structures and
presented the opportunity to the animal to start
doing something else - This developmental
revolution may have also sparked the origin of
bone. - Neural crest cells build the
electroreceptors that line the bodies of fish
once these receptors evolved, the researchers
theorized, neural crest started building
mineralized bone around them to insulate them
from the rest of the body. - Later, the bone
spread out to form a protective coat of armor, as
seen in the early bony fish.
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Supportng Research Involving Lancelets
- Lancelets don't have a true neural crest, but
they do have cells in the same position as neural
crest cells, and they express some of the same
genes that neural crest cells express before they
begin to migrate. - These cells also migrate,
but only as a sheet moving on the surface of an
embryo, not as small clusters traveling inside
it they haven't managed to break loose and
wander - These observations suggest that one
innovation of vertebrates was the wandering
neural crest it opened up the potential to get a
potentially complex vertebrate head." -
Interestingly, the swollen bud on the front end
of the lancelet nerve cord bears a striking
similarity to the vertebrate brain. - Thus, the
same genes that organize major regions of the
forebrain, midbrain, and hindbrain of vertebrates
express themselves in a corresponding pattern in
this small cluster of cells in the lancelet's
nerve cord.
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Slicing the Lancelet Brain
- A detailed examination of the neuroanatomy of
lancelets by Lacalli indicates that the nervous
nerve cord is divided like a vertebrate brain. -
In the regions of the lancelet nerve cord where
forebrain and midbrain genes are being expressed,
the neuronal structure matches that of the
vertebrate forebrain and midbrain. -Lacalli
claims that clusters of neurons in the lancelet
brain seem to perform the same functions as their
vertebrate counterparts--even though in the
lancelet these clusters may be made up of only a
handful of neurons.
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Lancelet Brain cont.
- Lacalli alos claims that lancelets have a
rudimentary limbic system a cluster of nerve
cells in the lower part of the brain that
interact with the cerebral cortex - He has found
lancelet neurons whose structure and organization
resemble those of vertebrate limbic neurons and
that are located in the corresponding parts of
the midbrain and forebrain. - He suggests that
the common ancestor of vertebrates and lancelets
used its protolimbic system to switch between its
handful of behaviors, such as swimming and
feeding.
- The very beginning of the limbic system is to
be found in Lacalli's work he shows that there
is the precursor of the hypothalamus, a crucial
part of the limbic system.
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The Advent of Predators - Although there are
tremendous similarities between lancelets and
vertebrates, many anatomists believe that the
head of vertebrates represents a huge
evolutionary and developmental step. - And its
the differences in vertebrate and lancelet brains
that become critical for understanding vertebrate
evolution - For example, lancelets apparently
have no sense of smell - One of the parts of the
vertebrate brain that's missing from the lancelet
nerve cord is the telencephalon - Gans and
Northcutt's suggest that early vertebrates
shifted from filter-feeding to predation, and a
key innovation in this process might well have
been the beginning of a nose. - A lancelet
doesn't need to sniff out its prey, but as the
early vertebrates became predators, smell became
an asset - They would also benefit from eyes to
see prey and sophisticated control of their
bodies to chase prey down.
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Evidence of the Filter Feeding-Predatory Shift
- The discovery of Haikouella - In some ways
these fossils look like lancelets, but they also
have a few key vertebrate traits unnecessary for
filter feeders, such as eyes and muscle blocks.
- These clues suggest that Haikouella is poised
at the transition from invertebrate to
vertebrate, closer to vertebrates than even the
lancelet. - That makes another feature of their
anatomy significant The fossil nerve cord has an
even larger swelling than does that of the
lancelet it appears that they do have a brain.
- If so, this fossil discovery pushes the origin
of a vertebrate-like brain back to more than 530
million years ago.
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The Evolution of Bone - In a recent paper in
Biological Reviews of the Cambridge Philosophical
Society, Donoghue, Forey, and Aldridge create a
new evolutionary tree for vertebrates that for
the first time incorporates a mysterious group of
animals called conodonts. - These creatures left
behind vast numbers of enigmatic little fossils
in the shapes of cones and thorns, ranging in age
from 510 million to 220 million years old. -
Over the years "conodonts have been attributed to
almost every major phylum you can think
of Finally in the 1980s new fossils began to
emerge with the conodont elements lodged in soft
tissue. - Now researchers envision conodonts as
eel-shaped predators with a pair of giant eyes
and a gaping mouth filled with the tooth-like,
bony conodont elements, which are made of dentine
and other ingredients of the vertebrate skeleton.

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Conodonts
Conodonts are the enigmatic, microscopic and
phosphatic remains of a group of primitive
chordates. They are mainly tooth-like in shape
and functioned as a food-gathering apparatus.
They are extinct, having ranged from the Cambrian
through the Triassic Periods of the Paleozoic
Era.
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Conodonts cont. - This new information seemed to
elevate conodonts to the status of chordate
predators, but paleontologists have fought over
exactly what sort of chordate they might be. -
Donoghue et al. tried to resolve the debate with
a massive study of both fossil and living
creatures, analyzing 103 different traits in 17
different groups of chordates, ranging from
lancelets to jawed vertebrates. - Their results
show that after the vertebrate lineage split from
lancelets, the first group to branch away were
the hagfish lampreys are only slightly less
primitive. - Conodonts, surprisingly, turn out
vertebrates, even closer to living jawed fish
than to lampreys or hagfish.
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Re-examining Vertebrate Phylogeny
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Conodonts cont. - Only after the rise of
conodonts did the armored jawless fish, the
ostracoderms, appear, and from one of their
ranks, the jawed fish eventually evolved. -
According to the new phylogeny, hagfish and
lampreys offer a good representation of what the
most ancient vertebrates were like unarmored and
without mineralized skeletons. - And conodonts
represent the first appearance of a mineralized
skeleton. - The conodont skeleton is believed to
be the primitive vertebrate skeleton -
Mineralization began not in the skin of fish but
in the mouths of conodonts, and it presumably
made them fiercer predators.
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Invertebrate-Vertebrate Transition - There are
now fossil fish known from the Cambrian that are
more advanced than hagfish, so it must have
happened during the Cambrian explosion. - The
transition appears to have happened in the ocean
- all invertebrate next of kin, all
non-vertebrate chordates, and the most primitive
living vertebrates all come from the ocean. -
Jawless, armored fish appeared in the Cambrian,
were present but not diverse in the Ordovician,
and flourished in the Silurian and Devonian. -
However, other fishes soon began to appear
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Placoderms the most primitive jawed fish
Unique characters Heavy armor on head and front
of the trunk, scales on the tail, no teeth, just
plates of bone for shearing food, heavy fish
(probably slow swimmers). Shared novelties
Jaws Paired pelvic fins (source of hindlimbs)
Paired nasal openings. Range Silurian -
Carboniferous
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Acanthodians earliest known jawed fishes
Unique characters Their fins were supported by
erectable spines. Some filter fed, others had
teeth. Highly manueverable swimmers propelled by
their tails. Shared novelties Teeth Advanced
jaw joint. Range Silurian - Permian
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Chondrichthyes Cartilaginous fish
Unique characters No bone except in their scales
(an evolutionary reversal). Fin and tail
structures suggest an active, highly efficient
swimming for a predatory life style. Sharks give
birth to live young.  This requires internal
fertilization of eggs.  They link up when
breeding by using claspers. Shared novelties
Regular pattern of tooth replacement Range
Silurian - Recent
Fossil rhinobatoid (guitarfish -- one of the
earliest rays)
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The Origin of Jaws and Teeth Jawless fishes have
gills and these flimsy sheets of tissue are
supported by gill arches. Many fish actively
pump water past the gills using muscles acting on
the gill arches. One hypothesis for the origin
of jaws is that the pair of gill arches furthest
in front were jointed and actively involved in
pumping water. These jointed arches developed a
pincher motion that was eventually used for
holding prey. Support the front gill arch in
living sharks supports the jaws and attaches them
to skull.
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Osteichthyes Vertebrates with skeletons made
entirely of bone Range Silurian - Recent
Shared novelties Skeleton completely composed
of bone, including skull, vertebral column, fins,
and ribs Swim bladder for buoyancy control.
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Osteichthyes (Bony fish) Ray-finned fish
Shared Novelty uniting bony fish Fins made of
bony spines connected by poorly muscled webs.
Range Silurian - Recent
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Osteichthyes (Bony fish) Lobe-finned fish
Unique Characters Torpeodo shaped body with
heavy scales, unusual bone with many pores,
perhaps for electroreceptive cells. Shared
novelties Paired pectoral and pelvic fins that
are fleshy and muscular Peculiar convoluted
dentin and enamel. Range Devonian - Recent
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Overview of Vertebrate Phylogeny