Pahasapasaurus

Pahasapasaurus is an extinct genus of polycotylid plesiosaurs from Late Cretaceous rocks of the United States. Distinctive features of the taxon include elongate epipodial bones (radius/ulna - tibia/fibula) and the nature of the palate bones (roof of the mouth). The genus contains two species; type species, Pahasapasaurus haasi, was named in 2007 based on remains from the late Cenomanian-aged Greenhorn Limestone of South Dakota, In 2025, a second species was named, Pahasapasaurus gillettei, based on a complete skull and partial skeleton from the early Turonian-aged Tropic Shale of Utah.

In August 1934, entrepreneur Charles Christian Haas and his son Arthur while searching for shark teeth discovered a plesiosaur skeleton near the Maloney Creek, south of the Belle Fourche River in the northern Black Hills. They collected and partially prepared the specimen, studying the relevant scientific literature to better understand the bones and documenting the dig via photographs and matrix samples. A report was written, published by the Science Service Bureau. Haas hoped to sell the specimen to some museum but when no buyer showed interest, he donated it to the Pioneer Museum at Deadwood, the present Adams Museum & House. By 1937 it had been identified as Trinacromerum bentonianum by Stephen Chapman Simms, the director of the Field Museum.