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Atractosteus simplex and A. atrox occur mainly in Fossil Lake deposits where they are relatively rare (less than 0.05% of the fish fauna). The long snouted gars are much rarer in Lake Gosiute and Lake Uinta deposits. Taxonomy from Fossilworks.org. Emended species diagnosis from Grande 2010, p. 471: “†Atractosteus simplex differs from other species in the genus by the following characters A-E. (A) Having a lower number of lateral line scales than any other species except for †A. messelensis sp. nov. (Table 184). (B) Having a lower number of abdominal vertebrae than any other species except for †A. messelensis sp. nov. (Table 182). (C) Differs from †A. messelensis sp. nov. in having a shorter premaxilla relative to head length (Table 180). (D) Differs from †A. messelensis sp. nov. in having a shorter head relative to standard length (Table 180). (E) differs from A. spatula and A. tristoechus in that the dermosphenotic forms part of the orbital margin as in A. tropicus." Line drawing from Grande 2010, p. 475 : Identified by oilshale. References: Grande, L. (2001) An Updated Review of the Fish Faunas From the Green River Formation, the World’s Most Productive Freshwater Lagerstätten. Eocene Biodiversity, 1–38. doi:10.1007/978-1-4615-1271-4_1 . Grande, L., Kammerer, Ch. & Westneat, M. (2006) Comparative and Developmental Functional Morphology of the Jaws of Living and Fossil Gars. // Journal of Morphology, Vol 267, Issue 9, 1017-1031. Grande, L. (2010) An Empirical Synthetic Pattern Study of Gars (Lepisosteiformes) and closely related Species, based mostly on Skeletal Anatomy. The Resurrection of Holostei.// Copeia, 2010, No 2A, 1-863.