Stem Ornithodirans
- Members of the Ornithodira that don't belong to either Dinosauria or Pterosauria
- Quadrupedal, with limbs held beneath body
- Relatively long, narrow skull
- Long neck
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Basal Dinosaurs: Ornithischia
- "Bird-hipped Dinosaurs" with pubis that points backwards
- Predominantly herbivorous
- Quadrupedality evolved multiple times independently from bipedal ancestors
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Silesauridae
- Formerly thought to be outside the Dinosauria (i.e. split off before the Ornithischia-Saurischia split)
- Recent studies suggest they are the most basal Ornithischian dinosaurs
- Small to medium sized
- Quadrupedal, with long front and hind legs
- Relatively long neck, small head
- Mostly herbivorous (earliest forms were omnivores)
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Asilisaurus kongwe skeleton, Middle Triassic, Tanzania (Chicago Field Museum)
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Chilesaurus
- Bipedal herbivore
- Has been considered part of Theropoda, Sauropodomorpha, or Ornithischia
- Most recent study suggests it is a basal Ornithischian (despite its late appearance)
- Fairly long neck; small head
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Basal Dinosaurs: Saurischia
- Saurischia are the "lizard-hipped dinosaurs" that includes the Sauropods and Theropods (and birds)
- Included here are primitive dinosaurs that belong to the clade Saurischia but may fall outside either the Sauropodamorpha or Theropoda
- Sometimes considered dinosauromorphs instead of true dinosaurs
- Small to medium size with slender build
- Bipedal, with longer hind legs than front legs
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Herrerasauridae
- Bipedal carnivores with fairly long hind legs
- 3 functional fingers (4th vestigial and not visible externally)
- Skull squarish
Tawa
- Bipedal carnivore with fairly long hind legs
- May be a basal Theropod or a Dinosauromorph
- 3 functional fingers (4th vestigial and not visible externally)
- Snout narrow, pointed
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Herrerasaurus skeleton, Middle Triassic, Argentina (Chicago Field Museum)
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