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not to beat the thing to death, but I'm about to start a new term
teaching invertebrate paleo, so a bit more on echinoids:
In the Triassic, a group of left-over Paleozoic echinoids (i.e. the
Class of echinoderms with biscuit-shaped bodies and (frequently) spines)
called the Cidaroidea evolved a structure called "Aristotle's Lantern"
a cute 5-part jaw apparatus which enables grazing. These were the apparent
root of a diversification of sea-urchins through the Mesozoic and into
modern times. Thus the concept of "appearance of grazing echinoids" is
pretty much correct for the Triassic, with the possibility that some
Permian Cidaroids or their ancestors were early grazers. There are some
additional subtleties not worth mentioning here.