Important developments of the Vertebrates: brain and sense organs

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Important developments of the Vertebrates brain
and sense organs

• The ancestors of vertebrates switched from filter
  feeding to more active feeding, which required
  movement and the ability to sense the environment
  in detail.

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Important developments of the Vertebrates brain
and sense organs

• The need to gather and analyze information led to
  the development of multiple sense organs 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.

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Important developments of the Vertebrates brain
and sense organs

• The development of sensory structures and
  increased mobility generated the need for a
  control center to process information.
• The anterior end of the nerve cord consequently
  became enlarged into a brain.

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Important developments of the Vertebrates brain
and sense organs

• 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.

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Human Brain
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Brain structure

• In the most primitive forms of brains the
  forebrain is associated with the sense of smell,
  the midbrain with vision and the hindbrain with
  balance and hearing.
• From this primitive condition the size and
  complexity of the brain has greatly increased.

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Hindbrain

• The Hindbrain has two portions
• Most posterior portion is the Medulla oblongata.
  It operates primarily at the reflex level.
  Reflex centers for respiration, heartbeat, and
  intestinal movement are found in the medulla
  oblongata.
• The medulla oblongata also relays signals from
  the inner ear and is a major pathway through
  which signals pass to and from higher areas of
  the brain.
• Damage to the medulla, not surprisingly, is
  life-threatening.

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Hindbrain cerebellum

• The anterior portion of the hindbrain includes
  the cerebellum (present only in jawed
  vertebrates), which is highly folded and
  convoluted.
• The cerebellum integrates sensory information
  (touch, vision, positional, hearing) with motor
  input to maintain the organisms equilibrium (its
  position and equilibrium in relation to gravity).
• The cerebellum also coordinates motor movements,
  both reflex movements and directed movements.

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Hindbrain cerebellum

• If the cerebellum is removed an organisms
  movements become uncoordinated and uneven.
• In humans damage to the cerebellum causes a
  condition called dysmetria in which someone
  reaching for a target with their hands (or feet)
  overshoots or undershoots it.

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Cerebellum

• The size of the cerebellum is proportional to its
  role.
• In fish, the cerebellum is proportionally
  enlarged in part because it must process lots of
  input from the lateral line system, but also
  because fish must orient themselves in three
  dimensions and equilibrium and balance are thus
  very important.
• In bottom-dwelling fish and those that are not
  active swimmers the cerebellum is relatively
  small.

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Midbrain

• The midbrain develops in step with the eyes and
  is the part of the brain that receives visual
  information.
• The roof of the midbrain (the tectum) is the part
  that receives visual information (and also
  lateral line and auditory input). The floor of
  the midbrain (the tegmentum) initiates motor
  output based on the input received.
• The midbrain is often the most prominent portion
  of the brain in fish and amphibians

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academic.emporia.edu/sievertl/verstruc/fbrain.htm
Frog brain model
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Forebrain diencephalon and telencephalon

• The forebrain has two major parts the posterior
  diencephalon and the anterior telencephalon.
• The diencephalon includes the pineal gland,
  pituitary gland, thalamus and the hypothalamus.

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Diencephalon Hypothalamus

• The hypothalamus plays a major role in
  homeostasis, the regulation of the bodys
  internal physiological balance including such
  aspects as temperature, water balance, appetite,
  blood pressure and sexual behavior.
• It achieves this by the release of hormones
  either produced in the hypothalamus itself or
  stimulating the release of hormones from the
  pituitary gland.

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Diencephalon Pituitary Gland

• The posterior pituitary gland stores and releases
  two hormones produced by the hypothalamus
  oxytocin and antidiuretic hormone (ADH).
• In humans oxytocin stimulates uterine
  contractions in childbirth and also milk
  production.
• ADH acts on the kidneys to increase water
  retention and reduce urine volume.

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Diencephalon Pituitary Gland

• The anterior pituitary (AP) produces many
  hormones that in turn control the activity and
  hormone release from other endocrine glands
  including the gonads, adrenal glands, and
  thyroid.
• Growth hormone is produced in the AP and has a
  direct effect on growth. Excess production can
  lead to gigantism and deficient production to
  dwarfism.

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Diencephalon pineal gland and thalamus

• The pineal gland affects skin pigmentation by
  affecting melanocytes. It also plays a role in
  regulating biological rhythms.
• The thalamus is the major coordinating center for
  incoming sensory impulses from all over the body
  and it relays the information to the cerebral
  cortex.

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Telencephalon

• The telencephalon (or cerebrum) includes two
  expanded lobes, the cerebral hemispheres (which
  in many mammals are greatly folded) and the
  olfactory bulbs.
• The receipt of olfactory information is a major
  role of the telencephalon and in species in which
  olfactory information is important the olfactory
  bulbs are greatly enlarged.

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Cerebral hemispheres

• The cerebral hemispheres in reptiles and
  especially in birds and mammals are enlarged 5 to
  20-fold over those of non-amniotes of comparable
  size.
• The enlarged size of the cerebral hemispheres
  allows more and faster processing of sensory
  information and thus greater intelligence.

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Cranial nerves

• Not all nervous inputs and outputs to and from
  the brain travel via the spinal cord.
• A series of cranial nerves connect directly into
  the brain.
• Most have Roman numerals for names and many of
  them directly connect to sensory structures and
  other structures in the head.

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Cranial nerves

• The cranial nerves are
• Cranial nerve 0 Nervus terminalis runs to blood
  vessels of the olfactory epithelium.
• Cranial nerve I Olfactory nerve connects with
  olfactory cells in the mucous membranes of the
  olfactory sac.
• Cranial nerve II Optic nerve connects to the
  eyes.
• Cranial nerves III and IV and VI connect to
  extrinsic eye muscles.
• Cranial nerve V Trigeminal nerve branches into
  three nerves that connect to the eye, jaws and
  the skin of the head. Cranial nerve VII also
  innervates the face as well as the taste buds.

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Cranial nerves

• Cranial nerve VIII Auditory nerve connects to the
  inner ear.
• Cranial nerve IX Glossopharyngeal nerve connects
  to taste buds and parts of the throat.
• Cranial nerve X Vagus nerve serves areas of the
  mouth, pharynx and most of the viscera.
• Cranial nerve XI supplies some jaw muscles and
  the trapezius.
• Cranial nerve XII Hypoglossal nerve innervates
  tongue muscles
• Cranial nerves arise from both neural crest cells
  and from ectodermal placodes in the embryo

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Neural crest cells

• Neural crest cells are groups of special cells
  derived from the embryonic dorsal tubular nerve
  cord.
• Early in development these cells separate from
  the neural tube before it closes.
• They assemble into cords above the neural tube
  and migrate along distinct pathways to various
  permanent locations where they differentiate into
  a variety of structures.

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Image Source http//www.niaaa.nih.gov/publication
s/arh25-3/175-184.htm
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Neural crest cells

• Neural crest cells give rise to among other
  structures
• Schwann cells
• Some components of the peripheral nervous system
• Odontoblasts (give rise to dentin)
• Dermis of facial region (from which many skull
  bones are produced)
• Beak of birds
• Some chromatophore cells
• Connective tissue of the heart
• Parts of the meninges

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Ectodermal Placodes

• Ectodermal placodes (with some exceptions in
  fish) are thickenings of the surface ectoderm
  that sink inwards and develop into various
  sensory structures.
• Paired olfactory placodes that form at the tip of
  the head develop into odor receptors that connect
  to the brain.
• Paired optic placodes produce the lens of the eye.

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Ectodermal Placodes Vestibular apparatus

• Some dorsolateral placodes (in fish) give rise to
  the lateral line system.
• The otic placode (one of the dorsolateral
  placodes) forms the vestibular apparatus in the
  inner ear.
• The vestibular apparatus plays a major role in
  both balance and hearing.

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Ectodermal Placodes Vestibular apparatus

• There are three semicircular canals (arranged at
  roughly 90 degree angles to each other) and two
  connecting structures (the sacculus and
  utriculus) in the vestibular apparatus
• The canals are fluid filled and respond to
  rotation when the head is tilted. The
  information about orientation and motion is then
  delivered to the brain for interpretation.

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http//goodrich.med.harvard.edu/pictures/BRODEL34s
maller.bmp
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Ectodermal Placodes Vestibular apparatus

• In some fishes and in reptiles, birds and mammals
  a section of the vestibular apparatus (the
  lagena) is specialized for sound reception.
• In terrestrial vertebrates the lagena is usually
  elongated and in most mammals it becomes coiled
  forming the cochlea.

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Ectodermal Placodes Vestibular apparatus

• In mammals sound vibrations are transferred from
  the eardrum via the inner ear bones (malleus,
  incus and stapes) to the cochlea.
• The vibrations cause hair cells in the fluid
  filled cochlea to move and this movement is
  converted into nerve signals that are then
  transmitted to the brain where they are
  interpreted as sounds.

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http//www.tchain.com/otoneurology/images/master-e
ar.jpg
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Significance of neural crest cells and ectodermal
placodes

• The vertebrate head is mostly a collection of
  parts that are derived from neural crest or
  ectodermal placode tissue.
• These unique tissues and their mode of embryonic
  production distinguish vertebrates from all other
  chordates.

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The role of hox genes in the evolution of the
Vertebrates

• A factor that may have played a role in the
  evolution of the vertebrates is the duplication
  of the Hox gene complex.
• Hox (short for hemeobox) genes are master control
  genes that regulate the expression of a hierarchy
  of other genes during development.

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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.

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http//evolution.berkeley.edu/evolibrary/images/mu
tantfly.jpg
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Induced ectopic eyes In Drosopila
(arrowed) From Induction of Ectopic Eyes
byTargeted Expression ofthe eyeless Gene in
Drosophila Georg Halder, Patrick Callaerts,
Walter J. Gehring. Science. Vol. 267 24 March
1995
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Hox genes

• Invertebrates and amphioxus have only one set of
  hox genes, the living jawless vertebrates have
  two sets, but all jawed vertebrates have four
  sets.

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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.

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Hox genes

• One group of animals in whose evolution hox genes
  are hypothesized to have played a major role is
  snakes.
• Its suggested that the how genes controlling the
  expression of the chest region in lizard
  ancestors of snakes expanded their zone of
  control in the developing embryo.

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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 wee in snakes today.

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Geological Time Scale

• Precambrian 4,500-542mya
• Paleozoic 542-200 mya
• Mesozoic 200-65 mya Age of Dinosaurs
• Cenozoic 65mya to present Age of Mammals

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Paleozoic

• Cambrian 542-488 mya. first appearance of
  chordates
• Ordovician 488-444 mya
• Silurian 444-416 mya
• Devonian 416-359 mya
• Carboniferous 359-299 mya
• Permian 299-251 mya
• Triassic 251-200 mya

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Mesozoic

• Jurassic 200-146 mya
• Cretaceous 146-65 mya
• Camels Often Sit Down Carefully, Perhaps Their
  Joints Creak

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Early vertebrate ancestors

• Fossils of early chordates are scarce, but a few
  are known including Pikaia from the Burgess Shale
  (approx 505 mya) that appears to be an early
  cephalochordate and has a notochord and segmented
  muscles.
• Unlike living cephalochordates it has a pair of
  sensory tentacles. It was small, about 5cm long.

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15.8
Pikaia
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Pikaia
http//proto5.thinkquest.nl/jre0294/pikaia20plaa
tje.jpg
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Early vertebrate ancestors

• Another fossil from China is Haikouella
  lanceolata about 525mya, which places it in time
  at the base of the vertebrate radiation and a
  likely vertebrate ancestor. It was about an inch
  long (lt3cm).
• Haikouella possesses all the chordate characters
  and also a suite of vertebrate characters
• Dorsal nerve cord with a relatively large brain
• Gills
• Head with possible eyes
• Pharyngeal muscles and gills
• Myomeres

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Haikouella lanceolata
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Haikouella
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Haikouichthys and Myllokunmingia

• Two other Chinese fossils from the early Cambrian
  are clearly early vertebrates. These are
  Haikouichthys and the very similar (perhaps
  identical) Myllokunmingia.
• As in the case of Haikouella, both of these
  animals were also small (lt3cm).

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Haikouichthys and Myllokunmingia

• Both Haikouichthys and Myllokunmingia lacked bone
  and cranial elements, but both possessed
• Gill bars and gills,
• V-shaped myomeres,
• A head
• A heart
• Large eyes
• An ear
• Possible vertebrae

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Myellokunmingia
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Conodonts

• For almost 150 years tiny, tooth-like
  microfossils have been important index fossils in
  geological studies.
• These conodont elements are extremely common in
  rocks from the late Cambrian through the end of
  the Triassic. It was unclear what organism they
  belonged to until the early 1980s.

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Conodont elements
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Hill/Encyclopedia /images/CE157400FG0010.gif
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http//www.toyen.uio.no/palmus/galleri/montre/mic0
1.jpg
Conodont (Manticolepis subrecta) elements
composed of calcium-phosphate, and are tiny
(0.1-0.2mm), toothlike structures from the
Devonian
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Conodonts

• In the 1980s the discovery of Carboniferous era
  conodont fossils in Scotland and later in South
  Africa solved the mystery.
• These fossils were of a soft-bodied, slender,
  laterally compressed animal with a complete set
  of conodont elements in its pharynx.

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http//www.le.ac.uk/gl/map2/abstractsetc/conanimal
s.jpg
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Conodonts

• The fossils showed clear evidence that conodonts
  were vertebrates. There were V-shaped myomeres,
  a notochord, caudal fin rays, and what appeared
  to be a postanal tail and a dorsal nerve cord.
• In addition, histological examination of conodont
  elements showed they contained a variety of
  mineralized vertebrate dental tissues cellular
  bone, calcium phosphate crystals, calcified
  cartilage, enamel and dentin.

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Conodonts

• Because dentin is laid down by odontoblasts, the
  presence of dentin in conodont elements is
  indirect evidence of neural crest tissue, which
  is a uniquely vertebrate characteristic.
• Condonts animals were mostly 3-10cm long although
  some may have been as big as 30cm.

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Conodont
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Conodonts

• There is evidence of wear on conodont elements
  which suggests they were used to crush and slice
  food.
• Recent fossil evidence also shows the conodont
  elements were attached to tongue-like or
  cartilaginous plates that could be moved in and
  out of the mouth presumably to impale and catch
  food items.
• This and the animals large eyes suggests that
  conodonts actively selected larger food items and
  likely were predators.

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Ostracoderms Jawless early vertebrates

• A wide variety of armored jawless fishes
  collectively referred to as ostracoderms (from
  the Greek ostrac a shell and derm skin) are known
  from the very late Cambrian and early Ordovician
  (488-444 mya) up to near the end of the Devonian
  period (359 mya).

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Ostracoderms

• First vertebrates to possess bone and also the
  first to possess an intricate lateral line system
  and an inner ear with two semicircular canals.
• Ostracoderms were encased in bony plates (with
  skin in between the plates so they could flex).
  The bony plates of the head in many cases were
  large and often fused into a head shield
• They did not have a well developed endoskeleton
  and it was usually of cartilage. Given the lack
  of bony vertebrae in fossils, presumably the body
  was stiffened by a notochord.

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Silurian marine fish fauna. Mostly agnathans,
but also (10) a gnathostome an acanthodian
called Nostolepis . www.palaeos.com .
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Ostracoderms

• Most ostracoderms were small (10-35 cm in length)
  and most lacked paired fins so they probably were
  not precision swimmers.
• The ostracoderms were jawless with narrow, fixed
  mouths. They appear to have been mainly filter
  feeders that used their pharyngeal muscles to
  pump water.

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Ostracoderms

• Because most ostracoderms were small, filter
  feeders, many were dorsoventrally flattened and
  most lacked fins it is likely that they were poor
  swimmers and almost certainly were bottom
  dwellers that extracted food from sediments.

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Ostracoderms

• The phylogenetic relationships of the various
  ostracoderm groups are still being figured out.
• Major groups include the Pteraspidomorphs,
  Osteostracans and Anaspids.

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Pteraspidomorphs

• Most Pteraspidomorphs had head shields formed by
  the fusion of large bony plates. The rest of the
  body behind the head is covered with small plates
  and scales.
• None possessed paired fins, but some had spines
  that projected from the head shield.
• Pteraspidomorphs occur from the Ordovician to the
  late Devonian.
• They possessed paired nasal openings and a
  vestibular apparatus with two semicircular canals.

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(A Pteraspidomorph)
(An Anaspid)
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Anaspids

• Appear late in the Silurian and possess much more
  flexible body armour made up of small plates and
  a hypocercal tail (with an extended ventral lobe)
  which suggest a trend towards more open-water
  swimming.

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A Pteraspidomorph
15.10
Ostracoderms (it should read anaspid not anapsid
in the caption on the right).
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Osteostracans

• Osteostracans were also heavily armoured and
  possessed a large head shield.
• Unlike pteraspidomorphs, there were in some
  species anterior lobes that projected from the
  head shield (and are now believed to be
  homologous to the pectoral fins of gnathostomes).
  These would have enhanced stability in swimming.

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Osteostracans

• The Osteostracans are considered to be the
  closest known relatives of the gnathostomes.
• Shared derived characters linking them to the
  gnathostomes include cellular dermal bone,
  pectoral fins with a narrow base, large orbits
  and calcified cartilage.

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A Pteraspidomorph
15.10
Ostracoderms (it should read anaspid not anapsid
in the caption on the right).
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http//www.nature.com/nature/journal/v443/n7114/im
ages/443921a-f1.0.jpg
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Silurian marine fish fauna. Mostly agnathans,
but also (10) a gnathostome an acanthodian
called Nostolepis . www.palaeos.com .
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Ostracoderms

• Ultimately, the ostracoderms were outcompeted by
  fish that possessed the next big evolutionary
  development jaws.
• By the end of the Devonian the ostracoderms had
  become extinct. The conodonts survived into the
  Jurassic and a few other agnathans have survived
  to today.