Final answer:
Smooth and compact bone fragments, particularly cranial bones with a thin profile, suggest they belong to a human fetus. Fetal bones haven't undergone the same ossification process as those of an adult, child, or animal, aligning with the provided description.
Step-by-step explanation:
The description of the bone fragments as being smooth in appearance and consisting of thin, compact cranial bones may indicate that these belong to a human fetus. In human development, fetal bones are typically smoother and more compact, reflecting the absence of complete ossification and the presence of more cartilaginous material. Adult human and animal bones tend to be denser with more pronounced features developed over time, and a child's bones would show some degree of growth and development not present in a fetus.
In the context of fossil remains, the state of preservation and appearance can provide clues to the developmental stage of the individual. The absence of a fully developed bony structure, coupled with the described thinness and compactness, is more consistent with fetal remains than with those of an adult human, a human juvenile, or an animal.
Given the thin, smooth, and compact nature of the cranial bone fragments, it is concluded that they likely belong to a human fetus. These characteristics are consistent with the underdeveloped, cartilage-rich skeletal framework seen in fetal stages.