Crinoid

You make lightweight armour from the calcium carbonate dissolved in sea water, but concentrate your energy on reproducing as fast as possible.

You float on the end of your 'stalk' or 'holdfast', a cup shaped flower, with both mouth and bum on top.  Many of you are eaten by creatures who have evolved jaws, but you grow fast enough to spread far and wide.  You spread like weeds across the seas of the prehistoric world, shedding so many thin calcium carbonate armour plates that entire chalk deposits, tens of metres thick, are entirely made of the broken up bodies of Crinoids.

Around 252 million years ago: the planet either gets hit by a huge space rock, or super-volcanoes explode, and the climate changes rapidly.  The seas change, becoming a little more acidic, and you can't make the plates of calcium-carbonate armour any more. Luckily, that's not crucial for your survival, and many of your species live on, adapting to become more flexible again.  Some of you become free-swimming as adults, with many flexible arms which float in the currents like grass in the wind, and inhabit both deep waters and reefs, where you are admired for your beautiful colours.