ABSTRACT
The Dyrosauridae Hyposaurus derbianus Cope was loaned by Orville A. Derby in 1880 to Edward D. Cope to be studied. After Cope's death the specimens were not returned and toured several foreign institutions like the American Museum of Natural History in New York and the National History Museum (NHM) in London. We accessed several documents of the Memory and Archive Sector of the National Museum of Brazil (Museu Nacional/Universidade Federal do Rio de Janeiro) to elucidate the real trajectory of the H. derbianus holotype. We noticed several errors in the literature, going from the assignment of the specimen as a syntype, even though it is a holotype according to the ICZN, to registration number problems. In addition, photographs of specimen PV R 8672, previously taken at the NHM, describing it as a cast of H. derbianus in the NHM's official website, were correlated with the original material studied by Cope and recognised through the labels of the Museu Nacional/UFRJ and the Geological Commission of the Brazilian Empire (Comissão Geológica do Império). Lastly, despite the fire at the Museu Nacional/UFRJ and the limitations imposed by the COVID-19 pandemic, we were able to locate the holotypes at the NHM and elucidate its history.