Final answer:
The anterior fontanel is the skull feature absent in the adult skull because it closes as a part of normal development within the first two years of life.
Step-by-step explanation:
The skull feature that is absent in the adult skull is the anterior fontanel. Fontanels are broad areas of fibrous connective tissue that form fibrous joints between the bones of the skull in newborns, often referred to as 'soft spots'. These include the diamond-shaped anterior (frontal) fontanel, located at the top front of the skull, and the smaller triangular-shaped posterior (occipital) fontanel, located at the back of the head. The ossification process, where bone tissue forms, causes these fontanels to close a few months to a couple of years after birth. While the posterior, sphenoidal, and mastoid fontanels close within the first few months of life, the anterior fontanel can take longer but will usually be closed by around the age of 2 years.