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Up until two years ago the clade of Dinosauria was defined as all the descendants of the most recent common ancestor of Triceratops and birds. From Wikipedia:

Under phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of the most recent common ancestor (MRCA) of Triceratops and Neornithes, and all its descendants. 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.

However, after the resurrection of the clade Ornithoscelida by Baron et al in 2017 (see here), as the clade consisting Ornithschia and Theropoda, the MRCA of Triceratops and birds is defining Ornithoscelida and not the whole Dinosauria, assuming we still define Sauropodomorphs as dinosaurs.

So if dinosaurs include Sauropodomorpha, Herrerasaurs, Ornithoschia and Theropoda, what is the most updated phylogenetic definition of the clade Dinosauria?

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As of right now it is still the same, the evidence for sauropodomorpha being a outgroup is not statistically more reliable, this may change, however this will still not have much impact. There are several ways in which dinosauria is defined.

The most recent common ancestor of Megalosaurus and Iguanodon is also sometimes used since they were the original animals used to define the group.

The last common ancestor of Triceratops horridus, Passer domesticus, Diplodocus carnegii, and all of its descendants is also used just becasue of Baron's work

Finally their are several unambiguous synapomorphies used to define the group, including such things as ankle structure. a full list can be found here.

In a group as well known as dinosaurs, a discovery like Barons is if it holds up is just going to lead to a change in the definition.

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