When one thinks of a shelled cephalopod, odds are high that the first thing to come to mind are the ammonites. They were one of evolution's success stories, thriving from 400 to 66 million years ago. Why they became extinct is still a subject of much debate, but some theories have been considered:
- Ammonites started life as planktonic larvae, so if they died out before reaching adulthood, then the species would be doomed.
- Ammonites themselves were planktivores, so when a crucial element of the marine food web collapsed, they simply starved into extinction.
- Their shells were made of calcium carbonate, which were susceptible to ocean acidification, which may be what exactly happened when the Chicxulub impactor slammed on the sulfur-and-carbon-rich Gulf of Mexico 66 million years ago.
The last one may be the least likely, as it doesn't explain why the nautilus is the only shelled cephalopod left on Earth, but ocean acidification IS an issue that many of today's bivalves face, as their shells are made of calcium carbonate. As the oceans become more and more acidic, their shells get eaten and thinned out.
There is another aspect to the equation of this question. So far, bone is an organic material unique to the majority of only one phylum, the chordates. But what is bone made of?
Bone is not uniformly solid, but consists of a flexible matrix (about 30%) and bound minerals (about 70%) which are intricately woven and endlessly remodeled by a group of specialized bone cells. Their unique composition and design allows bones to be relatively hard and strong, while remaining lightweight.
Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein, and the remainder is ground substance. The elasticity of collagen improves fracture resistance. The matrix is hardened by the binding of inorganic mineral salt calcium phosphate in a chemical arrangement known as calcium hydroxylapatite. It is the bone mineralization that give bones rigidity.
Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts. Within any single bone, the tissue is woven into two main patterns, known cortical and cancellous bone, and each with different appearance and characteristics.
Note that calcium carbonate is not mentioned in this blockquote, so the overall question is: In an alternate Earth where a mass extinction involves ocean acidification, could a mollusk--be it a cephalopod or bivalve--develop a bony shell independently from chordates?