The official story says like Cambrian Ordovician transition was caused/accompanied by animals learned how to build the Ca-based skeletons. These skeletons and shells started to fall upon the sea bed causing the rich and durable limestone formations on top of earth crust. I wonder didn't limestone form itself in pre-Cambrian epochs? You already had the CaO in the primordial crust (I wonder why do you give the leverage to he people who chronically cannot understand a question even after it was answered). You do not need animals for the CaO to react with CO2. Actually, there was much more CO2 even before Cambrian, billions of years ago. But we do not see any limestone formations at that time. So, why do we see the limestone on the top layer sediments 460-60 mln years ago rather than at the very bottom of the crust?
No. CaO will not exist in most environments for any significant time in the presence of CO2, it will react to form CaCO3. I doubt CaO is the primary source of Ca in our world. Silicates are likely the original source of Ca in our oceans https://en.wikipedia.org/wiki/Silicate_minerals. CaCO3 will partially dissolve or corrode in water to form an aqueous solution of Ca +2 + 2 HCO3 -. The dissolution of the CaCO3 in water is an equilibrium reaction based on the acidity and temperature of the water, this is why the acidification of the oceans is a concern for our marine life. I am not a chemist or a biologist but organisms will extract the aqueous Ca +2 ions found in the water to form shells etc. Because these organisms are not evenly distributed in the ocean their shells and skeletons will and have accumulated the CaCO3 in distinct areas.