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Chapter 34 Deuterostomes
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)
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