A Completely Different Kind of Reef
In this scenario, corals, sponges and bryozoans have been extinct for 65 million years. In their place as reefbuilders are echinoderms, bivalves, barnacles and worms of the infraclass canalipalpata.
As reefbuilders, what advantage or edge would any of them have over coral?
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I don't think that any of them would have an advantage over corals when it comes to building reefs. Perhaps they could manage to build some, but it would be hard.
Coral create coral reefs by secreting calcium carbonate[1]. Echinoderms have partial skeletons composed of calcite[2], which is, for our purposes, the same thing. Echinoderms can lose limbs ad regenerate them so it is possible that you could create structures with some of the skeletal material from disused limbs. A stretch? Yes. But not impossible.
The thing is, echinoderms like to live on the seabed. This is not conducive to reef-building. The raw materials are there, but the ability to assemble them may not be.
Bivalves are also composed in part of calcium carbonate[3],1, like echinoderms and corals. They are, however - to quote Wikipedia - "sedentary", preferring to stay where they are on the seafloor. Once again, it appears like it would be difficult to create complex three-dimensional reef-like structures out of them.
Barnacles, too, do not move[4], meaning that their placement for reef-building must be crucial. That said, they grow en masse in large groups, making it especially easy for them to take over large areas. They can quickly cover an area in themselves. They are made of - yes, you guessed it - calcite, the same building material as corals. They seem to be the best bet so far to replace corals.
Worms have soft bodies, and are invertebrates. They would not be good reef-builders.
1 I'm sensing a pattern here. I assume you already knew about composition.
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