Earth Sciences 2266F/G Lecture Notes - Lecture 4: Agnatha, Cambrian Explosion, Chondrichthyes

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Lec 4
Lamprey (jawless fish, with sucking disc with teeth lining the round disc) is a parasite, sucking
on a fish for its blood
Petromyzontida represent the lamprey
This is the earliest known living jawless fish
There are also hagfish
Part of the Myxinoidea group
Some people think the Conodonta aren’t true vertebrae, so from D and onwards those are true
Only 2 survivors related to jawless fish (lamprey + hagfish)
This is the early shark fossil
Teeth are multi-cusped (multiple prongs); there are rows of teeth
We do have fossils of early cartilaginous fish
Earliest fossils have cartilage (Cambrian, Silurian), no mineralized anatomic skeleton; they may
have armours but vertebrae and fins are not mineralized
Then how do anatomic bones develop?
Buffer is important b/c for mammals, when the mother is nursing a baby, most of the Ca came
from the mother (she gives Ca from her own bones)
How will a piece of cartilage mineralize into hard bone?
Function of mineralized bone cannot be for movement b/c beginning, only part of it is
And then some of the bone started to store calcium, but why?
Even for movement: fish live in the sea so they don’t need much movement; only on land we
need hard bones
Best swimmers are sharks because they are more flexible (don’t have hard bones), lighter
(density is same as sea water), and don’t have air sacs so it naturally floats
Original verts lived in marine environment, which is sea water full of calcium
As a result, origin of bones is a problem – maybe calcium came out as a metabolic byproduct and
animals found a use for it
In the Cambrian explosion, many animals came out; nearly all of them started to secrete
mineralized skeletons except vertebrates
Even bacteria secreted calcified sheaths
Trilobites used chitin-based
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