BIOL 2003 Lecture Notes - Lecture 20: Echidna, Bird Egg, Yolk Sac
Amniotes
November 20,2015
Sauropsida – reptilia
• Implications: birds are not reptiles
• Crocodiles are most closely related to birds
• Mammals did not evolve from reptiles
o Ancestors are less closely related to reptiles than anything else
o Lineages usually due to changes in skull (temporal region)
o Dermal bones cover outside surface while braincase is a completely different structure
o Ancestral form (also in fish) is the synapsid skull (vs. Anapsid or Diapsid)
o Ancestor evolved from the early tetrapods (not modern amphibians)
Turtles are very different *Anapsid Skull
• Very distinct depending on terrestrial and aquatic elements
• Hard to position with the tree – hard to determine what they evolved from
Everything except turtles and mammals have a diapsid skull.
Amniotic Egg: three more extra-embryonic membranes
• Yolk sac – found within almost all vertebrates (viviparious)
• New in amniotes:
o Amnion – surrounds embryo
o Chorion – underneath shell in bird egg (part of embryo)
▪ Ends up covering entire surface of the egg for gas exchange
▪ Specialized membrane created from outpocketing behind the gut stores nitrogenous
waste – less toxic then fish as urea
o *maternal layer holds shell together
• In mammals:
o No shell
o When Amnion breaks – water breaks (pool of fluid around fetus)
o Chorion and Allantois support and exchange the nutrients
▪ Creates placenta to exchange gas, water, nutrients, and waste
▪ Invades maternal tissue – no shell
▪ Forms a joining tissue for exchange
• blood vessels are seen throughout components
• Since some animals still lay eggs, it is obvious that mammal eggs evolved from amniotic eggs in fish
Monotremes: egg laying mammals
• Ex. Enchidna, spiny anteater, platypus
• Hatching from egg
• Late pouch young
• Have bizarre modifications
Waterproof Skin (in some)
• Mostly gas-proof; cannot breathe through the skin
• Overlapping scales: only in snakes and lizards *biggest group of non-mammals
• Flexible hinges hold osteoderms (sometimes; like bone but not derived from scales with bony base),
and melanophores
• Layer of dermis w epidermis on top
Turtles: one of three main clades traditionally called reptiles
• Snapping turtle – no teeth; beak is made from keratin (not made of bone)
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