BLG 144 Lecture Notes - Lecture 9: Gnathostomata, Amniote, Yolk Sac

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5 May 2015
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Chapter 34 Deuterostomes
Vertebrate evolution
recall: early vertebrates with cartilaginous "armed" fishes
notochord for structural support (still in some living lineages)
no bony endoskeleton but early fishes with bony exoskeleton
teeth but no jaws
Why selection for jaws? How?
You can’t bite without jaws!
early vertebrates limited to filter/deposit-feeding
selection for bulk feeding – predation
So where did jaws come from?
strong support for gill-arch hypothesisy
modification of support structures into jaw
cartilaginous support structure later taken over by bony endoskeleton
more powerful muscles
bone also allows greater support in appendages (fins) but mainly rays
Very elaborate in one fish lineage…
bony elements enlarged & more complex in “lobe-finned” fishes – fleshy fins
support more body weight
Why would this be advantageous?
easier to exploit shallow water habitats
extra oxygen availability (lungs as well)
excellent fossil record detailing lobe fin elaboration to become true terrestrial limb
key transitional fossil from Canada! (Tiktaaklik)
also other skeletal changes
result in 1st tetrapods
So now you want to be a terrestrial vertebrate…
same challenges as other groups: 1) avoiding drying out, 2) structural support, and 3)
reproduction
aquatic reproduction usually easier
external fertilization, embryos do
not dry out
early tetrapods with jelly-coated eggs (still in amphibians today
But what if you truly want to free yourself from aquatic habitats?
evolution of amniotic egg major evolutionary event
seen in Reptilia & Mammalia – creates Amniote lineage
shell provides protection against drying out, also source of minerals
greater structural support – larger embryos
nutrition via yolk sac
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