Scalamagnus is an extinct genus of polycotylid plesiosaur from the Late Cretaceous Tropic Shale Formation of the United States.

D. tropicensis extended the known stratigraphic range for Dolichorhynchops back by approximately 7 million years.

[2] Previously three additional polycotylid taxa, Eopolycotylus, Palmulasaurus and Trinacromerum, have been named from the same formation, two of which are currently endemic to the Tropic Shale.

The generic name is a combination of the Latin scala, "staircase", and magnus, "large", referring to the Grand Staircase-Escalante National Monument.

The results of their phylogenetic analyses are shown in the cladogram below:[1] Scalamagnus Dolichorhynchops herschelensis Martinectes ROM 29010 (Niobrara polycotyline)[4] Unktaheela The holotype is associated with 289 gastroliths, which is unusual in comparison to most polycotylid skeletons that generally lack gastroliths.