There are different types of fossils, some of which apply to soft tissue:

• Impression: A shallow imprint of a fossil organism that does not retain any organic material.

• Compression: A fossil that has been crushed or flattened but retains some organic material, although it has been chemically altered.

• Carbonization: A process that occurs during fossilization in which complex organic molecules are converted into a more stable carbon compound that generally has a dark brown color.

This appears to be an impression fossil.