The evolution of vision in
vertebrates is an important theme in the history of
animal life, however, aside from the calcified lenses of fossilised arthropods, other parts of the visual system are not usually preserved in the fossil record because the soft
tissue of the eye and brain decays rapidly days after death.
Finally, «soup bones» include collagen, a protein found in connective
tissue of
vertebrate animals, which is abundant in bone, marrow, cartilage, tendons, and ligaments.