Amniote skulls

TrirachodonCynognathus Therapsida – advanced cynodonts Probainognathus

Secondary Palate

Therapsid evolution toward Mammalian grade

Feeding 1) expansion of temporal opening, elaboration of jaw muscles 2) reduction in post-dentary jaw bones, “second” jaw articulation 3) tooth specialization Breathing 1) secondary palate 2) reduction in lumbar ribs, formation of ribcage (& diaphragm?) 3) nasal turbinals (temp. & water conservation) Posture & locomotion 1) double occipital condyle, atlas & axis 2) enlargement of limb girdles 3) specialization of lumbar & sacral regions 4) vertical orientation of limbs 5) formation of a heel bone Hearing 1) reduced size & flexible attachment of post-dentary 2) reduction in size of stapes MAMMALIAN “GRADE” TRENDS IN CYNODONTS

Jaw evolution – pelycosaur to mammal

LOBE-FIN AMPHIBIAN PELYCOSAUR FISH MAMMAL AMPHIBIAN CYNODONT EAR EVOLUTION

Sorex (Recent shrew) – ear region TYMPANIC MALLEUS (manubrium)

TRITHELODONTS (ICTIDOSAURS)

Morganucodon (late Triassic)

Morganucodon -- (late Triassic)

Morganucodon -- skull

MORGANUCODONTS vs ADVANCED CYNODONTS

Coelophysis (carnivorous dinosaur) Tritylodon (herbivorous therapsid) Morganucodon (insectivorous “mammaliaform”) 3 meters 1 meter 10 cm Plateosaurus (herbivorous dinosaur) 10 meters LATE TRISASSIC VERTEBRATES

CONTRASTS BETWEEN LATE TRIASSIC VERTEBRATES EARLY DINOSAURS medium-large sized herbivores & carnivores probably diurnal, some endothermic CROCODILES small-medium sized, carnivores LATE THERAPSIDS Small-medium sized herbivores (tritylodonts) insectivores (ictidosaurs) EARLY “MAMMALIFORMS” Very small Insectivores probably nocturnal, endothermic