The soft tissue comes in a variety of forms. not just remnants of collagen, or chiton as per @Bill_II’s rational wiki link. Including but not limited to
Fibres and cellular structures preserved in 75-million–year-old dinosaur specimens
Microstructure and Biogeochemistry of the Organically Preserved Ediacaran Metazoan Sabellidites
Soft tissue and cellular preservation in vertebrate skeletal elements from the Cretaceous to the present “flexible and fibrous bone matrix; transparent, hollow and pliable blood vessels; intravascular material, including in some cases, structures morphologically reminiscent of vertebrate red blood cells; and osteocytes with intracellular contents and flexible filipodia.”