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 literatur...