Most fish fossils are:

Coprolites?

Explanation:

coprolites are of sea organisms, particularly fish and reptiles. They consist of indigestible remains of the organism's food, such as pieces of scale, teeth, shell and bone. Coprolites are preserved by petrification or cast and mold.

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.