Fossil microbodies are melanosomes: evaluating and rejecting the ‘fossilised decay-associated microbes’ hypothesis

Melanosomes are membrane-bound organelles of varying geometry, commonly
found within a range of vertebrate tissues, that contain the pigment
melanin. Melanosomes have been identified in the fossil record in many
exceptionally preserved fossils allowing reconstructions of the coloration
of many extinct animals. However, these microstructures have also been
interpreted as “microbial cells” or melanin producing bacteria based on
their geometric similarities to melanosomes. Here we test these two
conflicting hypotheses experimentally. Our results demonstrate multiple
lines of evidence that these fossil microbodies are indeed melanosomes:
the geometry of decay-associated microbes differs significantly from
fossil microbodies; fossil microbodies are very strongly localized to in
vivo melanized tissues and are absent in tissues typically unmelanized in
vivo, in all fossils examined regardless of lithology or age. On the basis
of these results, as well as a thorough review of existing literature on
melanin like pigments, we are able to rule out a bacterial origin for
fossil microbodies and demonstrate that fossil microbodies associated with
exceptional vertebrate fossils are in fact preserved melanosomes.