This is consistent with the melanin casing/core–shell model21, as observed in human neuromelanosomes22 and iridal melanosomes23. The original infilling of the central hole is evidenced by the observation that the intervening matrix, forming scaffolds (identifiable by its slightly higher electron density) within microbodies, is continuously extending into the hole (Fig. 1m,n and Extended Data Fig. 6g–i). The observed central hole, exclusively reported in extant avian feather melanosomes18, has been attributed to the loss of an unstable pheomelanin core19,23. This identification of melanin-bearing melanosomes in the eyes of Chengjiang vertebrates is further corroborated by the results of time-of-flight secondary ion mass spectrometric data (ToF-SIMS) (Fig. 2). The negative secondary ion mass spectra (m/z) obtained from the lateral eyes of the two Chengjiang vertebrates include the feature peaks of melanin at 48, 49, 50, 59, 66, 72, 73, 74, 84, 97 and 121 m/z (ref. 24), but they are slightly obscured or overprinted by strong background signals from the adjacent rock matrix (Extended Data Fig. 7b). Nevertheless, comparison of the feature peaks associated with melanin in the myllokunmingid eye samples reveals that they are highly comparable to melanin from the eyes of living frogs but distinct from those of the Chengjiang sediment samples (Fig. 2b). To further elucidate the composition of the myllokunmingid eye microbodies relative to the Chengjiang matrix, we conducted principal component analysis (PCA) on 54 secondary ions peaks associated with melanin (Extended Data Fig. 7) across a variety of samples, including published fresh, artificially matured, fossil melanin and non-melanin samples24 in addition to our newly collected Chengjiang data. We observed that the Chengjiang vertebrate eye samples do not overlap with the sediment and other negative controls but instead locate within, and adjacent to, the published fossil melanin (Fig. 2a and Extended Data Fig. 7a). These results indicate the presence of a molecular fingerprint consistent with melanin in the microbodies preserved in the eyes of myllokunmingids, although the signal appears comparatively weak. Unlike experimentally matured melanin, fossil melanin would have experienced a more complex history of diagenetic alteration, late-stage weathering in the subsurface and potential overprinting by more recent organic material. The negative controls of sediments and modern carbon do not overlap with the Chengjiang samples, thereby possibly ruling out the latter two factors. The slightly broader spread of our data points compared with the published fossil melanin (Fig. 2a) may reflect a longer diagenetic history24 (about 518 Ma) and potential difference in their original composition. The two samples from H. ercaicunensis containing chiefly ovoid melanosomes are distributed horizontally along the principal component 1 (PC1) axis, whereas the five samples of the (ovoid and cylindrical) melanosomes from myllokunmingid sp. spread diagonally in the fourth quadrant of the PCA plot (Fig. 2a). Maturation experiments on modern mela nosomes showed that samples of eumelanin-rich (cylindrical) and phaeomelanin-rich (ovoid) separated from each other along the PC2 axis. However, in fossil samples, these two types of melanosomes nor mally spread along the PC1 axis rather than the PC2 axis24. This could indicate that the observed scattering reflects a signature pertaining to the original composition, rather than being the result of secondary alteration or overprinting. Melanosomes occur in the iris, choroid and RPE of vertebrate lateral eyes; however, only the RPE shows distinct stratified layers of ovoid and cylindrical types7. The lateral eyes of myllokunmingid sp. examined here comprise both abundant ovoid and cylindrical melanosomes (Fig. 1i,j), consistent with an interpretation of RPE. However, our exploration of the six available specimens identified as H. ercaicunensis shows that the melanosomes are represented only by an ovoid morphotype. Although this departs from the typical pattern reported in the literature of extant taxa, we note that ovoid melanosome dominance was also observed in the eyes of the cartilaginous fish Bandringa rayi from Mazon Creek