Vertebrate fossils have been collected for hundreds years and are stored in museum collections around the world. These remains provide a readily available resource to search for preserved proteins; however, the vast majority of paleoproteomic studies have focused on relatively recently collected bones with a well-known handling history. Here, we characterize proteins from the nasal turbinates of the first Castoroides ohioensis skull ever discovered. Collected in 1845, this is the oldest museum curated specimen characterized using paleoproteomic tools. Our mass spectrometry analysis detected many collagen I peptides, a peptide from hemoglobin beta, and in vivo and diagenetic post-translational modifications. Additionally, the identified collagen I sequences provide enough resolution to place C. ohioensis within Rodentia. This study illustrates the utility of archived museum specimens for both the recovery of preserved proteins and phylogenetic analyses.