Transcript Origin and Diversification of the Vertebrates

Origin and
Diversification of
the Vertebrates
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.
Vertebrate Characteristics
- Backbone
- Spinal cord
- Heads
- Tails
- Heart
- "Gill" Slits
- Segmented
muscle on body
wall
- Internal skeletons
of hydroxyapatite:
- Jaws
- Two pairs of
appendages
Origin and Diversification of
Vertebrates
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
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
Urochordata (Sea squirts)
Unique characters: Tunicates
are small, box-like filterfeeding 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
Cephalochordata (Lancelates)
Unique characters:
Branchiostoma, also known as
the lancelet, is a small, freeliving fish-like animal that lives
among sand grains and filter
feeds.
Shared novelties: Segmented
muscle on upper body wall.
Range: Cambrian - Recent
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
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.
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
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
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.
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
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
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
Craniates that may be important transitional
fossils: Haikouella and Myllokunmingia
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!
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.
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
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
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.
- 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.
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.
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.
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.
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 it’s 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.
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.
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.
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.
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.
Re-examining Vertebrate Phylogeny
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.
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
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
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
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)
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.
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.
Osteichthyes (Bony fish): Ray-finned fish
Shared Novelty
uniting bony fish:
Fins made of bony
spines connected by
poorly muscled
webs.
Range: Silurian Recent
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
Overview of Vertebrate Phylogeny