Chapter 34 Vertebrates
Chordates
• Chordates describes animals that have bilateral symmetry and belong to
clade known as deuterostomia
• Vertebrata is the subphylum of chordata
Shared characteristics of chordates
a. Notochord – flexible and fibrous tissue rod that is remnants of
vertebrae, becomes cushioned disks in vertebral column of humans
b. Dorsal, hollow nerve chord - primitive form of brain and spinal cord,
can be dorsal to notochord
c. Pharyngeal slits – water circulation; modified gas exchange
d. muscular tail
Tunicates
• Also know as Sea Squirts
• Have 2 life stages :
1. larval stage
-have bilateral symmetry
-free swimming
-notochord supports whole tail
*stage may only last hours or days
2. Adult life stage –
-lose tail, notochord, and most of nerve cord- which
becomes cluster of cells called ganglion
-water enters through oral siphon and exits atrial
siphon
-water will be ejected out of atrial siphon to clear
feces or digested material
-have circulation system with heart and blood
vessels
*Adults become sessile and can be solitary or colonial
Lancelets
• Adult and larvae stages, but no real change is
body shape or motility
• Notochord runs full length of body
• Muscles in V-shaped segments that pull side
to side for wiggly swimming motion
• Nervous system has hollow nerve cord that
divides into segmented nerves that branch
out and control muscle contraction
• Small sliver or arrow shaped organisms
• Inhabit sandy or muddy bottoms of warm
ocean waters
• Outer body surface is nearly transparent and
internal organs are visible
• Feed on diatoms or decaying food matter
• Characteristically hide in sand during day with
mouths sticking out to feed, swim about at
night
Craniates
• Next evolutionary checkpoint of
chordates was the development of a
head with eyes, sensory organs, and a
nerve cord
• Two sets of Hox genes compared to
one in tunicates and lancelets
• Neural crest – cells that appear near
dorsal margins of closing neural tube
- cells disperse and become parts
of teeth, bones or cartilage of the skull,
dermal layer of skin, and neurons or
sensory capsules in the eyes
• Example: Hagfish – notochord, eyes,
brain and ears, no vertebrae, no jaw
Vertebrates
• Branching from craniates,
development of transcription
factor genes ( Dlx) that formed the
elementary nervous system and
skeleton
• More extensive skull developed
and vertebrae formed over spinal
cord
• Lampreys are the earliest known
vertebrates
- cartilage skeletal network, jawless
• Conodonts – lamprey-like soft
body vertebrate, large eyes, and
early signs of mineralized dental
tissues
Gnathostomes
• Jawed vertebrates with teeth
- developed from modification of skeletal rods
between pharyngeal gill slits
• Other features of gnathostomes:
- additional duplication of Hox genes (4 cluster)
- enlarged forebrains – enhanced sense of
vision and smell
• Lateral line system – rows of microscopic organs
sensitive to vibrations in the water
Examples: sharks, rays, ray finned and lobe
finned fish
Tetrapods
• 365 million years ago fins of lobe fins developed into
limbs and feet
• Tetrapod – four feet
• Limbs supported body weight, feet had digits to
transmit muscle forces on ground
• Pelvic girdle attached to hind legs and fused with
backbone
• Appearance of one neck vertebra( up and down) then a
second vertebra (side to side)
• No gill slits, pharyngeal clefts became ears, glands, and
other structures
Example: amphibians
Amniotes
• Group of tetrapods named
for the amniotic eggs with 4
specialized membranes that
protect and nurture
developing embryo
• Other features include less
permeable skin and ability to
use rib cage to ventilate
lungs
Example: reptiles, birds,
and some mammals
Mammals
• One step further in evolution, mammals have
hair, fat layer to retain heat, and mammary
glands to produce milk to nurse young
• Larger more developed brains
• Differentiation of teeth
• Synapsids – early group of mammals lacking
hair, sprawling gate, and laid eggs
- temporal fenestra – hole behind eye
sockets
Mammals
• During Mesozoic era three lineages emerged:
1. Monotremes – egg laying, hair, produce
milk, no nipples (Ex: platypus, spiny
anteater)
2. Marsupials – mammals with pouch, live
young develop in pouch, hair, milk, nipples
(Ex: wombat, kangaroo)
3. Eutherians – placental mammals, hair,
milk, longer gestation period (Ex: primates,
lemurs, humans)
Primates
• Primates have hands and feet for grasping objects
and climbing
• Flat nails on digits instead of claws
• Large brain, flat face, short jaws
• Forward looking eyes, close together
• Big toe on each foot separate from other toes
• Opposable thumbs- mobile and separate from
other digits giving more precise manipulation and
“power grip”
Hominoids
• Features:
longer arms,
short legs, no
tail, larger
proportional
brain, and
social
organization
Hominids
• Homo sapiens – bipedal (upright on two legs)
larger distinct divisions of brain – complex
language, symbolic thought, and
manufacture and use of complex tools
reduced jaw bones and muscles
shorter digestive tract
Sahelanthropus tchadensis – oldest known
hominid that resembled primitive
characteristics of humans
Hominids
Two common misconceptions:
1. Hominids are not necessarily chimpanzees,
separate branch exists and each acquired own
characteristics after divergence from common
ancestor
2. Homonids are not an organized ladder
leading to modern day Homo sapiens
*several hominid species coexisted at
one time, disorganized alignment of
characteristics passed on and breaking
away from other groups- one gave rise to
homo sapiens others became extinct
Timeline of Hominids