South Dakota

Details

A RARE HORNED HETEROMORPH AMMONITE
South Dakota
From the Upper Campanian, Cretaceous (circa 72 million years ago) the specimen of Didymoceras cheyennense supported with stand on block of its original matrix.
11in. (29cm.) long

Special notice

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Lot Essay

This very rare and uncoiled ammonite is "among the most striking and remarkable natural objects known from the Cretaceous rocks of the U.S. Western Interior". The species was first described by 1856, and has been subsequently revised to Nostoceras monotuberculatum in 2000. The helical shape with looped tapering finial has provided much debate for palaeontologists as it would not have made an efficient swimmer, and the three different orientations of coiling would present different challenges to the mollusc living inside the shell as they changed the direction of growth.

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