Final answer:
In an organism with a chitin-based exoskeleton, calcium carbonate is the likely additional molecule found in the skeletal system, as chitin indicates an arthropod which may have an exoskeleton reinforced with calcium carbonate.
Step-by-step explanation:
If you chemically analyzed a carcass and found significant amounts of chitin, it indicates that the organism likely has an exoskeleton, as chitin is the primary component of the exoskeletons in arthropods such as crustaceans and insects. Given this information, one would not typically expect to find hydroxyapatite (which is associated with the bones in vertebrates) or collagen and elastin (which are also associated with vertebrate structures). Instead, you might find calcium carbonate, as it is commonly found in the exoskeletons of marine organisms such as those of crustaceans, and provides additional structural support. Thus, the most likely additional molecule associated with the skeletal system of this chitin-containing organism is a) calcium carbonate.