Tullimonstrum gregarium is an iconic soft-bodied fossil from the Carboniferous Mazon Creek Lagerstätte (Illinois, USA)1. Despite a large number of specimens and distinct anatomy, various analyses over the past five decades have failed to determine the phylogenetic affinities of the ‘Tully monster’, and although it has been allied to such disparate phyla as the Mollusca2, Annelida3,4 or Chordata5, it remains enigmatic1,2,3,4,5. The nature and phylogenetic affinities of Tullimonstrum have defied confident systematic placement because none of its preserved anatomy provides unequivocal evidence of homology, without which comparative analysis fails. Here we show that the eyes of Tullimonstrum possess ultrastructural details indicating homology with vertebrate eyes. Anatomical analysis using scanning electron microscopy reveals that the eyes of Tullimonstrum preserve a retina defined by a thick sheet comprising distinct layers of spheroidal and cylindrical melanosomes. Time-of-flight secondary ion mass spectrometry and multivariate statistics provide further evidence that these microbodies are melanosomes. A range of animals have melanin in their eyes, but the possession of melanosomes of two distinct morphologies arranged in layers, forming retinal pigment epithelium, is a synapomorphy of vertebrates. Our analysis indicates that in addition to evidence of colour patterning6, ecology7 and thermoregulation8, fossil melanosomes can also carry a phylogenetic signal. Identification in Tullimonstrum of spheroidal and cylindrical melanosomes forming the remains of retinal pigment epithelium indicates that it is a vertebrate; considering its body parts in this new light suggests it was an anatomically unusual member of total group Vertebrata.
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W. Simpson, P. Mayer, S. Williams, D. Rudkin and K. Seymour are thanked for specimen access and loans. Funding was through a Natural Environment Research Council studentship P14DF19 (to T.C.) and grant NE/K004557/1 (to M.A.P. and S.E.G.). We also acknowledge the National Science Foundation grant DMR-0923096 used to purchase the TOF–SIMS instrument at Texas Materials Institute, UTA. D. Murdock, C. Nedza, S. Wentges and A. Clements are thanked for proofreading. S. Furzeland is thanked for scanning electron microscope optimization. P. Smith is thanked for Adobe Illustrator tutorials.
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Seminars in Cell & Developmental Biology (2019)