Amniotes

Amniotes

1. Basal Amniotes

   • Amniotic Egg
     • Tough outer shell - allowing for colonisation of land
       • Chorion - overall enclosure
         • Amnion - prevents embryo from drying out
           • Allontois - stores waste away from embryo
             • Yolk - holds and digests nutrients
   • Lay eggs on land or fertilised and remain within mother
     • Existed since Mid-Carboniferous
       • Oldest eggs from Triassic

2. Temporal Fenstra

   • Anapsids - no opening (most basal amniotes)
     • Synapsids - 1 opening
       • Diapsids - 2 openings
         • Euryapsids - 1 opening
   • Functions
     • Strengthen jaw muscle by providing attachment and bulging space
       • Reduces skull weight but increases strength

3. Synapsids

   • Therapsids
     • Biarmosuchia
       • Most basal group, carnivorous and pectoral/pelvic girdles indicate advanced posture
     • Dinocephalia
       • Large therapsids, carnivores that evolved to prey on large herbivores e.g. moschops
     • Dichynodont
       • Beaked jaw with no teeth expect two tusks in upper jaw
     • Gorgonops
       • Late Permian carnivore, two large canine teeth
     • Thereocephalia
       • Groups most closely related to cynodonts with large skulls
     • High cheek bones provide more efficient attachment sites for jaw muscles
       • Enlarged temporal fenstra increase jaw muscle efficiency
         • A more forward position of jaw joint - more efficient and effective jaw muscles enabled diversification into a wide range of feeding strategies
           • Advanced therapsids achieved a nearly erect stance and gait - better at running
             • Most diverse in temperate to cool-temperate - warm blooded
   • 'Mammal like reptiles'
     • Oldest known is Archaeothyris
       • Ophiacodon - diet expanded from insects to fish and tetrapods
         • Back sail developed in both herbivores and carnivores - temperature regulation
           • E.g. Dimetrodon

4. Sauropsids

   • Anapsids
     • Herbivory
       • Minimal selection pressure - didn't need to be fast as there were only small carnivores
         • More habitats so they could search for new vegetation
           • Need to grow large to hold digestive system need to digest plant material
             • Evolution probably triggered co-evolution of large carnivores
     • Insectivory
       • Major driving force for early amniote evolution on land
       • Diapsids evolved a light weight skull with improved jaw muscles for efficient catching and eating of insects
     • 5 finger bones
       • E.g. Mesosaurus (plate tectonics)
         • Hind limbs girdle solidifying
           • Extinct groups were only terrestrial
             • Scutosaurs - sister group to turtles, covered in thick armoured plates
   • Diapsids
     • Euryapsids
       • All members are extinct
       • Includes ichthyosaurs
     • E.g. coelurosaurus
     • Protosaurs - earliest basal form of Archosauria
       • Dinosaurs and pterosaurs are two of the many crown groups that radiated during the Mesozoic