Download Lecture 2: Vertebrate Origins

Lecture 2: Vertebrate Origins
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Vertebrata are a subphylum of the Chordata
a. Chordate characteristics
i. Notochord at some stage of development.
ii. Dorsal hollow nerve cord.
iii. Pharyngeal gill slits present at some stage of development
iv. 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.
v. Muscular postanal tail
vi. Ventral heart with a closed circulatory system.
vii. Living bony or cartilaginous endoskeleton.
Chordata include (note: hemichordates are in a separate phylum – these are the
acorn worms)
a. Urochordata – Tunicates
b. Cephalochordata
c. Vertebrata
What defines a vertebrate?
a. Presence of vertebrae
i. They are cartilaginous in some fishes
ii. They are absent in hagfish
iii. Lampreys possess only rudimentary cartilaginous elements around the
nerve cord. Note: at one time these organisms were considered to be
degenerate.
b. Presence of a cranium (hence the original name for the group: Craniata)
c. Duplicated Hox gene (homeobox gene)
d. Presence of embryonic tissue called the neural crest, which give rise to the
epidermal placodes. These are the origin of the complicated sensory tissue
characteristic of vertebrates.
e. An interesting observation about vertebrates:
i. Most animals (invertebrates) are small. Vertebrates however tend to
be large. This means that diffusion is no longer sufficient for many
bodily functions. This necessitates specialized structures and systems
in vertebrates.
ii. Basal metabolic rates of vertebrates are much higher than those of
invertebrates. What does this mean?
iii. Vertebrates are capable of anaerobic metabolism.
What is the evolutionary history of the vertebrates?
a. There are 3 general hypotheses
i. Arthropod hypothesis
1. Arthropods are a major animal group – common and therefore
likely to have daughter groups.
2. They share some characteristics with the vertebrates.
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3. If you turn an arthropod upside down, you have the basic
vertebrate body plan.
4. The body is segmented.
5. There is a ventral nerve cord and a dorsal heart.
6. Problem – the exoskeleton.
7. This idea dates to 1818 by St. Hilaire.
ii. Annelid hypothesis
1. 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.
2. 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.
iii. Echinoderm – Hemichordate – Chordate Hypothesis.
1. Both of the above hypotheses suffer from the fact that annelids
and arthropods have spiral determinate cleavage while
chordates have radial indeterminate cleavage.
2. Both annelids and arthropods are protostomes while chordates
are deuterostomes.
3. Arthropods and annelids have shizocoelous coelom formation
while chordates have enterocoelous coelom formation.
4. Echinoderms have precisely the same characters as the
chordates: radial indeterminate cleavage, deuterostomes, and
enterocoelous coelom formation.
5. 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.
6. 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.
7. Not all basal deuterostomes were asymmetrical or pentaradial.
The calcichordata were bilaterally symmetrical, and may in
fact be specialized echinoderms.
Non-vertebrate Chordates
a. Urochordates
i. Tunicates (sea squirts)
1. 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.
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2. Adults however, look very different. How could this lead to
vertebrates?
a. Paedomorphosis – retention of juvenile morphology in
the reproductive adult. This is an example of
heterochrony.
b. Alternatively, we may be derived from the sessile adult
stage.
i. Chordates are unique in having innervation of 2
types: segmented innervation and nonsegmented innervation. It may be that we were
originally nonsegmented (like the sessile adults)
and later our morphology was over-run by the
newly derived segmented components.
ii. 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.
b. Cephalochordates
i. Fish-like in appearance and totally marine.
ii. Best know example is ampioxus (lancelet).
iii. Has segmented myomeres, and many homologies with vertebrates.
Conodonts
a. Perhaps the earliest ‘vertebrates’, although this issue has not been resolved.
The importance of bone
a. There is some question about when bone evolves as a vertebrate character.
i. Hagfish and lampreys have no bone (they do have inner ear ossicle)
ii. Nature of early bone has some implications for physiology – ion &
fluid regulation.
b. What is the function of early bone?
i. May serve a protective function. There were large invetebrate aquatic
predators, and the armor of ostracoderms and placoderms may have
prevented predation.
1. Unfortunately, the bony armor is below the skin, and thus
susceptible to injury.
2. Perhaps it was used as a mineral sink? This is related to an
early hypothesis about where vertebrates evolved.
c. Did vertebrates have a freshwater or saltwater origin.
i. Romer and Smith argued for a freshwater origin.
1. This is based on the idea that bone is a mineral sink.
2. Phosphates and calcium were probably a hot commodity in the
Silurian.
3. Bone armor may have prevented osmosis.
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4. Although all fossils were found in marine sediments, they
argued that they had washed down into the sea.
ii. All fossils are marine.
iii. All old vertebrate groups are marine.
iv. Kidney function was probably co-opted from other mineral regulation.
1. note: do fish drink?
v. Prevailing view today is that vertebrates have a marine origin.
Vertebrate ancestry
a. Ostracoderms
i. Oldest fossil vertebrates (except for conodonts)
ii. First discovered in Ordovician rock in Russia and the U.S.
iii. Belong to agnathan / cyclostomate group of vertebrates.
iv. Major radiation was in the Silutian and Devonian, but were extinct by
the end of the Devonian. Why?
b. Ostracoderm morphology
i. No jaws.
ii. No paired fins.
iii. Heavy bony armor.
c. Placoderms
i. Less developed bony armor
ii. Paired fins and thus probably more active swimmers.
iii. Had jaws and were thus capable of predaceous lifestyle
iv. First appear in Silurian, major radiation in the Devonian, and extinct
by the end of the Permian.
v. One Placoderm group, the acanthodians, had bony scales like modern
fishes.
vi. Placoderms may have given rise to, or had a common ancestor with 2
major groups: the Chondrichthyes and the Osteichthyes.
d. Chondrichthyes
i. No bone, probably underwent reduction from the Placoderm condition,
or may represent true underived vertebrate condition. Could this be an
example of neoteny or paedomorphosis? They have a living
endoskeleton, but it is made of cartilage.
ii. Completely predaceous life style.
iii. Well formed independent jaws – consider the teeth.
iv. They have a spiracle.
v. They have internal fertilization – eggs with cases, and yolks. Some are
ovoviviparous.
vi. The holocephalans (chimaeras) have an upper jaw that is fused to the
brain case, and a flap of skin that covers the gill region.
e. Osteichthyes
i. They have a bony skeleton, probably a retention of the ostracoderm or
placoderm condition.
ii. They have bony scales and opercula
iii. Origin was in the Devonian, they split almost immediately into 2
groups: the Actinopterygians and the Sarcopterygians.
1. Actinopterygians: Chondrosteans (sturgeons), Holosteans
(bowfins and Garpikes) and Teleosts (modern bony fishes)
2. Sarcopteryginas: Dipneusti (lungfishes), crossopterygians and
ceolocanths.
iv. Sarcopteryginas have intenal nostrils like all land vertebrates, they
have fleshy lobed paired fins, and were probably ancestral to the
labyrinthodontia.