Abstract

The scaphitid ammonite genus PonteixitesWarren, 1934, has remained poorly known and poorly represented in collections up to the present. All previously described specimens of Ponteixites, including the original type specimens, are small and appear to be juveniles. A larger, clearly adult specimen was recently discovered in the Pierre Shale of eastern Montana. If not for the larger size, it would be identified as P. robustusWarren, 1934. A comparison of the new, larger specimen with hundreds of typical, smaller specimens of P. robustus provides evidence for replacing the name Ponteixites with RhaebocerasMeek, 1876. An alternative interpretation of the large size of the new specimen would be gigantism. However, it is within the size range of other known Rhaeboceras species and has normal scaphite adult characteristics, thus disqualifying it as a candidate for gigantism. The species P. robustus should be renamed R. robustus. Another species, R. coloradoenseCobban, 1987, strongly resembles R. robustus and might be the latter species. The species formerly described as P. gracilisWarren, 1934, is believed either to represent the juvenile stage of another Rhaeboceras species or to be within the range of morphologic variation for R. robustus.

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