Description and phylogenetic reassessment of the monotypic Ostariostomidae (Teleostei)

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Abstract
The monotypic family †Ostariostomidae Schaeffer, 1949 is represented by a single specimen †Ostariostoma wilseyi Schaeffer, 1949. This unique and enigmatic fossil is the impression of the left side of a complete, articulated skeleton from Upper Cretaceous or lower Paleocene freshwater deposits of the Livingston Group in Montana. It was originally described based on the incompletely prepared type specimen and identified as a clupeoid. Based on a more complete preparation of the holotype, we redescribe †O. wilseyi in previously unknown detail and reexamine its phylogenetic relationships. It clearly is not a clupeoid because it lacks all known derived characters of Clupeomorpha. Unfortunately, even based on the additional morphological information available to us, we were unable to clearly identify †Ostariostoma even to superorder. Our questionable assignment of †Ostariostoma to Osteoglossomorpha (possibly a hiodontoid) is not strongly supported and needs to be further tested with additional material, should it be discovered. Although less than five centimeters in total length, the specimen of †Ostariostoma appears to be an adult based on certain features of its skeletal structure, making it one of the smallest species known from Late Cretaceous or Early Tertiary freshwater deposits. †Ostariostoma is one of only a few teleost genera known from Cretaceous through middle Paleocene freshwater deposits of North America. It gives us additional insight into the little known transitional teleostean fauna that existed between Jurassic and Eocene time in North America.