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Synapsid evolution, characteristics and time frames

Plecosauria (more primitive) Early Permian (320mya) varied diet/activity level large dorsal sail ~3m long Ancestral characteristics: weakly heterodont, angular/articular in mandible, single occipital condyle, no secondary palate
Early therapsida middle Permian (~265mya) active and diverse some dominant terrestrial herbivores for 60my most extinct after Permian-Triassic extinction event Derived characteristics: enlarged temporal opening, partial secondary palate, major jaw and skull changes
Cynodonts (advanced therapsids) Triassic-middle Jurassic diverse herbivores and carnivores lot of transitional fossils Transitional characteristics: double occipital condyle, complete secondary palate, expansion temporal opening and brain case, strong heterodont dentition
Early mammals 250 mya overlap with cynodonts relatively rare and insignificant for first 100my: most small, probably nocturnal (smallest dinosaurs many times larger than largest mammals)
Early Prototherians (early mammals) Morganucodontids: small (~10cm), good hearing, probably nocturnal (eye sockets, olfactory, auditory capabilities), expanded dentary bone with dentary-squamosal articulation, mammalian posture
Early Prototherians Triconodonts: until recently only had teeth and skeletal fragments, 3 cusps in a row (teeth), small carnivores Jeholodens discovered in China few years ago: derived pectoral girdle (like mammals), ancestral pelvic girdle, sprawling posture
Early Prototherians Docodonta: late Jurassic, complex (advanced) teeth- probably omnivores, primitive jaw articulation
Early Prototherians Multituberculata: diverse and persitent (120my beginning late Jurassic), mouse to marmot sized, coexisted with modern mammals, unique dentition- chisel like incisors, complex grinding molars (up to 8 cusps), allowed for exploitation of range niches
Early Therians Symmetrodonta Eupontotheria Tribosphenic molars: basic pattern for later mammals- both shearing and grinding
Created by: jebeard