Dinosorts

Like most kids, I loved dinosaurs, but much of what we learned
Turned out to be inaccurate. You might just be concerned
With which of them had feathers or who scavenged for a meal
Or when and how they died, but I would ask which ones are real—
Not whether they existed but what scientists agree
Are properly called dinosaurs, not just familiarly.
I knew that they excluded the dimetrodon somehow.
I didn’t know a fuller definition up to now:
Among the large reptilians that all have gone extinct,
The dinos are distinguished by the way their legs were linked
To hipbones at the sides and how their back legs stood erect,
Not splayed but perpendicular, a detail to reflect
Their evolution’s history; they’re not like crocs or gators.
The pterosaurs don’t qualify (unlike the birds of later!),
And neither do the mosasaurs and others in the sea.
There may be further differences, but that’s enough for me.