Engines do work when chemical energy is changed from:

In a combustion engine, chemical energy is first converted to heat to cause the expansion of a fluid which then produces motion. In contrast, motor proteins are molecules that can convert the chemical energy released from adenosine triphosphate (ATP) hydrolysis directly into useful mechanical work.

Organisms are more likely to be more preserved in mud than sand because mud is moist and wet, while sand is very dry.