(Yes, taking a break from saving the world stuff)
For those who don't know, Wikipedia has a great primer.
So, you'll notice a lot of weird issues with how many legs they have, right? Now look at the fossils. . . closely!
Those aren't 'stubby legs' on the one end. Those are discs with spines only sticking out on one side!
Imagine a creature that isn't a Sea Urchin yet. It has pentaradial symmetry and flexible, starfish like skin (which is what we see fossilized) It has evolved semi-hard spines that stick out from seven rings of support structures around their body.
There's some variance in this, because they actually walk primarily on five rows of tiny tube feet, though some also have more flexible, less spiny projections they use for support. They also may use these tube feet to feed, collecting detritus from the ocean floor or plucking them out of the water and carrying them to it's mouth. We're not seeing the tube feet, because they don't fossilize
This creature specialized over time. Some began to connect the seven discs into a more solid support structure and specialized in sediment feeding, they tipped over so that their mouth was now right by their food.
From there some evolved an internal shell to support fiercer spines, again starting with those seven pentaradial rings, but eventually with a 'more the merrier' philosophy, they became the urchins.
Some of them opted to maintain some flexibility and speed and moved their rows of tube feet closer to the ground with a thornier protective layer on top, they began to evolve chemical defenses as well (or hallucigenia had started us down that path long ago) and adopted a more predatory lifestyle. The sea stars.
Some of the urchins decided to dig and became sand dollars.
And some of the rest mixed it up and became brittle stars, and so on.
Meanwhile, another branch went for flexibility from the get-go, they maintained that pentaradial symmetry and seven rows of support structures and became the sea cucumbers while evolving other. . . more creative defenses, They kept the seven rows of support structures, you can still see it today in the sea pig and many others, though after all this time rings have been lost by some and others have adopted other modes of locomotion
Meanwhile, we have some odd mixes that start to make sense, like Xyoplax
Maybe this is what hallucigenia larvae looked like, too they sure look like baby starfish, and now we know they're related!
Then we have our suspension feeding tall skinny urchins, a logical step now, right?
As is the old Sphaeraster
See? I bet we got all that awesomeness from lil' old hallucigenia.
And those are totally discs!
HUGE credit to Joseph Jameson-Gould over at Realmonstrosities.com by inspiring the thought and enhancing it!
(Edit: To add. . )
And that also makes these guys start to make more sense. .
And these delightful critters
As do these guys!
And yet another fun hybrid
And a lot of the weirder old fossils, which suddenly make sense as variants on a theme!