Pisanosaurus

MIT again. The first dinos.

The discussion is over the divergence between "lizard-hipped" versus "bird-hipped" dinosaurs. This distinction confuses me to this day: actual freakin' birds are lizard-hipped, like all 'raptors and the apatosaurus. Obviously at the timescales they're talking, they'll confuse the researchers too because this is, again, when they diverged in the first place. Up to 2017 the question was reopened: it was mooted that lizard-hipped dinosaurs came first. In 2017 the sauropods and therapods were mooted paraphyles: the therapods broke off from the birdhips, and the true lizard-hips – the sauropods – are basal.

(The article erroneously cites the infamous chimera "brontosaurus". Ignore that, pedants.)

Earlier palaeontologists had found Pisanosaurus (bird-hipped) in the southern parts of Triassic-era South America - "Ischigualasto". Apparently they hadn't constrained the date; a dark-age breaks up the postCarnian Triassic record from 225-209 Mya. The new result is a redate of the context: these sediments span 230-221 Mya. Thus it rolls back this dark age - although, the fossil here is (postextinction) Carnian at 229 Mya. South America and Africa were as one then. I think it was all still Pangaea in fact.

Ischigualasto is, here, matched to Chinle here in North America. All the dinos evolved in the south. The north had... nothing. (Close to home, that's the Colorado Plateau; hence the Coring Project.) Partly that's because the Triassic from 252-229 Mya wasn't exactly nice for life at all; but they're saying the north should have had something.

Thus is constrained the sauropods - they broke off during the Triassic. The therapods came later, after Pisanosaurus.