Say Hello to Ornithoscelida
So, this happened. It’s a thing now. As always, proceed with caution, of course, but look what happened to turtles, and in fact has happened to many other extant groups. Reshuffling of taxonomic relationships happens a lot.
Also, this is about names and taxonomic relationships; the fundamental animals being discussed remain unchanged; what has changed is how we see them.
What hasn’t happened (and isn’t actually a thing): some folk are interpreting this to mean Diplodocus is no longer a dinosaur, and while taxonomy is plenty confusing, that’s not what’s happening. Diplodocus is still a dinosaur (cf. the diagram above; everything inside the large black circle is a dinosaur).
What has happened: Diplodocus still belongs to Saurischia, but Theropods (including the birds) do not. Not any more. Instead, a new taxon encloses the Theropods and the Ornithischian dinosaurs like Triceratops: this is the Ornithoscelida. This is to indicate that sparrows and Triceratops are more closely related to each other than either are to sauropods like Diplodocus.
Which blows my mind a bit, because since looooong before I was born, the sauropods were widely agreed to be sisters to theropods, closer to each other than either were to Ornithischian dinosaurs.
This is going to take some getting used to, but it makes sense to proceed with caution. This is just one paper, and this particular discussion of the early relationships of Triassic animals justifying this shake-up has just begun.