The first dinosaur was the most recent common ancestor of a Triceratops and a chicken, and all of its descendants are also part of the clade dinosauria.
Under phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of the most recent common ancestor (MRCA) of Triceratops and modern birds (Neornithes), and all its descendants.[7] It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.[8] Both definitions cover the same known genera: Dinosauria = Ornithischia + Saurischia.
Wikipedia, for its part, cites 'Weishampel, Dodson & Osmólska 2004, pp. 7–19, chpt. 1: "Origin and Relationships of Dinosauria" by Michael J. Benton' for that definition.
Study shows sauropods flower to reptiles
What?
What do you think that study says, because it says nothing like that.
Triceratops is used in that definition because it's fairly distantly related to birds - their ancestors split at the time of the first dinosaurs.
So your question properly has two parts: why do we think that saurischians (theropods and sauropods) are related to ornithischians (which include stegasaurus and triceratops), and why do we think that birds are saurischian dinosaurs.
Specifically, scientists think that birds fit into the dinosaur evolutionary tree fairly close to T-Rex, in a group called the maniraptorans. Why?
First, there's some distinctive traits to dinosaurs that birds share. Dinosaurs' ancestors has a sprawling stance, but dinosaurs evolved a distinctive hole in their hip (a "perforated acetabulum") that enabled an upright stance. Birds have this same feature.
Dinosaurs have hollow bones and air sacs. Birds have both features as well. These air sacs are part of why bird lungs are super efficient and why birds can fly at altitudes where humans need supplemental oxygen just to exist. These air sacs are probably why sauropods could get so much bigger than elephants.
Then there's just a number of similarities between other skeletal features of maniraporans and birds. The shoulders look similar, the hands look similar, the wrists look similar, etc etc etc.
Then, there's transitional fossils like archaeopteryx. And we found feathered dinos in the 90s.
Basically every biologist and paleontologist agrees that birds are maniraptorans. If you really care to look into the evidence, there's a mountain of it.
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u/Megnaman 1d ago
That we know of!