Blastozoa

You make thicker and thicker armour from the calcium carbonate dissolved in sea water.  It keeps you safe - for now.

You float on the end of your 'stalk' or 'holdfast', a small, five-sided ball, with a mouth on the top, and a bum on one side as far down as possible to take advantage of gravity.  On each of your sides, you grow an armour plate, and you filter feed in the shallower waters of the prehistoric world.  Many of you are eaten by creatures who have evolved jaws, but you grow fast enough to spread far and wide.  It's a good balance between the cost of armour and speed of reproduction.

We know of you now as one of the most common fossils of the Cambrian era, showing that there were uncountable billions of you.

Sadly, we can't know of you as a living species, because something quite bad happens 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.

You all die.  All the uncounted billions of you.

On the other hand, you're not alone, because around half of all life forms on the planet also die.  This is the Permian-Triassic extinction event, commonly known as The Great Dying.  It will take millions of years for the planet to recover.