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OneShotHelpful t1_jdwo8iz wrote

Reply to comment by ebinWaitee in Why are nonhuman erect bipedal animals so rare? by violetmammal4694

Archosaurs are a common ancestor of everything we consider a dinosaur and a lot of things we don't, like crocodiles. If you only want to go back as far as the ancestor of all dinosaurs, then Ornithodera is what you're looking for and it's also a biped.

Birds are theropods, which are one of the three big groups of dinosaurs. They are the ones that survived to today and kept the bipedal bodyplan all the way through.

Ornithicians are things like triceratops. Many became quadripetal, but things like the duck billed dinosaur didn't.

Sauropods are things like brontosaurus. They probably became quadripetal early after the split because I don't know of any bipedal examples.

If you look at quadripetal dinosaurs you might notice a tendency for the front limbs to be short and underdeveloped or have odd rounded shoulders. That could be called a remnant of the bipedal ancestry.

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ebinWaitee t1_jdwshcu wrote

Thanks! That is fascinating, never knew they're all believed to descent from a bipedal ancestor.

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jqbr t1_jdxkzf4 wrote

That's not believed--OneShotHelpful is wrong. Read their Wikipedia link carefully--there's no support for the claim. See also https://en.wikipedia.org/wiki/Ornithischia

>Ornithischia (/ˌɔːrnəˈθɪski.ə/) is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds.

The key word here being "superficially".

>However, birds are only distantly related to this group as birds are theropod dinosaurs.[3] Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, and the ornithopods.

Only the last of those was bipedal. And of course all of these bipeds had quadrupedal ancestors--we're all tetrapods.

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