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.

Methods
This raw dataset has been collected by measuring modern melanosome samples, fossil microbodies in exceptionally preserved fossils and experimental decay microbial consortia from scanning electron micrographs. The variables measured and tabulated in this dataset are length, diameter and aspect ratio (length/diameter). Summary statistics of raw data are included in Sheet 9.

黑素体（melanosomes）是一类膜结合型细胞器，具有多样的几何形态，广泛存在于多种脊椎动物组织中，内含黑色素色素。目前已在众多特异保存的化石记录中发现黑素体，这为诸多已灭绝动物的体色重建提供了依据。然而，这些微结构也曾被解读为“微生物细胞”或产黑色素细菌——因为它们与黑素体存在几何形态上的相似性。本研究通过实验对这两种相互矛盾的假说进行了验证。研究结果从多方面证实，这些化石微体确实为黑素体：与腐烂相关的微生物的几何形态与化石微体存在显著差异；在所有检测的化石中，无论其岩性或形成年代如何，化石微体均强烈定位于活体状态下含黑色素的组织，而在活体状态下通常不含黑色素的组织中则不存在此类微体。基于上述结果，以及对现有类黑色素色素相关文献的全面梳理，我们可以排除化石微体的细菌起源假说，并证实与特异保存的脊椎动物化石相关的化石微体实为保存下来的黑素体。

方法
本原始数据集通过扫描电子显微图像，对现代黑素体样本、特异保存化石中的化石微体以及实验腐烂微生物群落进行测量而构建。本数据集所记录并制表的变量包括长度、直径以及长宽比（长度/直径）。原始数据的汇总统计信息详见工作表9。