Final answer:
The skull found near Merrimack River in West Newbury, MA, based on its size, is likely from a large fish or a marine mammal, according to the information provided which includes references to a shark-like fish called Helicoprion ferri.
Step-by-step explanation:
Based on the information provided and the context of the question, which asks about the potential source of a skull found near Merrimack River in West Newbury, MA, and considering its size, we can infer a few possibilities. The mention of Helicoprion ferri, an extinct shark-like fish known for its distinctive saw-toothed whorl of teeth, suggests that if the skull has similar features, it might have belonged to a large fish or a marine mammal.
Since the fossilized jaw from Helicoprion provided is estimated at 35 cm belonging to a roughly 4m long creature, a similarly sized skull could feasibly be from a creature of comparable size. Since insects and most birds would not have skulls of that size, options (b) and (d) would be highly unlikely. In the context of the given examples like shark fins and dolphin fins, or the mention of a marine biologist discovering a fish with a cartilaginous skeleton, we can deduce that if the skull has significant size and marine features, it most likely belongs to category (c) Mammal or (a) Fish.