Earth Sciences 2266F/G Lecture Notes - Lecture 9: Branchial Arch, Fish Scale, Mangiafuoco

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Lec 9
Chondrichthyes means cartilaginous fish
Within this type of fish: sharks, rays, dogfish
No body fossils found; people only find scales or teeth in Ordovician-Silurian
Earliest body fossil was found in the early Devonian
We don’t know if true jaws first appeared in sharks or Placoderms, but these are the 2 groups that
first showed jaws; however cannot use jaws as a defining feature that they share b/c other
animals have jaws as well
Sharks are used to study the origin of jaws
Sharks are dedicated to evolution in the aquatic system; no swim bladders, no bony plates or
skeleton, so their body has a specific gravity in seawater to remain floating with the sea
movement thus they are good swimmers with their adapted streamlined body
Without swim bladders, it’s impossible for sharks to evolve lungs, thus sharks cannot have been
the ones to evolve into land animals
Also lacks strong bony limbs needed to get onto land
If shark’s origin in the Ordovician is true, then it’s hard to understand b/c they have good
swimming ability and have jaws so why weren’t they dominating from the start?
They were suppressed – hard to explain by Darwinian theory
Only after the F-F mass extinction, sharks start to radiate
Early fossils have multicusped teeth structures
In modern sharks, their teeth is single bladed
But in the beginning of sharks, they had multicusped teeth
Today, not all sharks have this kind of single bladed teeth
E.g. if you go deeper, some sharks still have the multi-cusped teeth
These teeth are not very good for slicing flesh or meat – simply good for grabbing since there are
so many cusps; they can hold their prey
Modern sharks are good at slicing flesh
Oldest known shark skeleton
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In evolution, the lower jaw of shark still has a big piece of cartilage – evolved from the lower
half of the front 3 gill arch pairs
In other fish, once the jaw started to evolve, some of it started to ossify; they even start to ossify
in Placoderm fish
Only in sharks do we see the cartilage structure from the original cartilaginous gill arch support
Autostylic: ancient sharks have this
Hyostylic: in modern sharks
Once sharks started to appear in the Devonian, many sharks were discovered
One of them is the Cladoselache, where the entire fossil is preserved; relatively small size, but
shows powerful tail and pectoral fins thus they are already good swimmers
This is the feature that is there in protruding snouts in sharks
All sharks have this organ: Ampullae of Lorenzini
Pores inside the skin with very sensitive nerve cells for sensing electromagnetic signals (even a
few billionth of a volt is hard to detect in modern scientific instruments)
As long as they move, they create the electric field and they can sense anything
Fish are cold-blooded animals, but some sharks are able to maintain a higher body temp to swim
in cool waters quickly still
These are the ampullae of Lorenzini structures (the pores)
Underneath each pore is a hair cell for sensing vibrations
That’s why sharks have strange looking heads because they are enlarging the SA for more
sensory
So they can have very big nostrum (snout) or a hammerhead (also control eyes for stereovision)
b/c they’re cartilaginous, fossils are hard to find
2 possibilities:
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