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.

Triceratops is an ornithician, which has the Latin word for bird right there in its name. Their ancestors were bipedal.

> In the type where two sperms fertilize one egg, a fetus can form in addition to the abnormal cystic placenta, but it's typically nonviable.

Typically??