THE LINK BETWEEN MELANOSOMES AND METABOLISM: NEW EVIDENCE FROM MIDDLE-LATE JURASSIC AGNATHANS AND AMPHIBIANS FROM NORTHEASTERN CHINA

A recent study (Li et al., 2014) showed that shape of melanin-containing organelles, melanosomes, is more diverse in amniotes with higher metabolic rates. However, whether this pattern holds true in non-amniote vertebrates has not been tested. Samples of fossilized melanosomes in integument from Middle-Late Jurassic lampreys and amphibians from northeastern China were measured using standard techniques. Melanosome morphology (i.e. length of the long axis, short axis width, and aspect ratio) was compared to extant non-amniote skin samples and a database of previously published measurements for extant and fossil amniotes. Melanosome morphology was not significantly different between fossil and extant amphibians and agnathans. Melanosomes in agnathan and amphibian skin are small in size, subround in shape, and, like lepidosaurs, testudines, and non-maniraptoran archosaurs, exhibit low morphological diversity. In contrast, melanosomes in mammal hairs and maniraptoran dinosaur feathers exhibit higher ranges of lengths, diameters, and aspect ratios. Low morphological diversity of melanosomes in fossil and extant agnathans and amphibians supports the hypothesis that convergent changes in melanin-based coloration are pleiotropically linked to increases in metabolism between ectothermic and endothermic vertebrates.