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Sphenodiscus lenticularis, Fox Hills Formation, South Dakota. Late Cretaceous (~ 70 mya). Both sides of this handsome specimen display the extraordinarily intricate suture patterns that are characteristic of this species. Also evident is part of the beautifully iridescent nacreous layer. A laterally flattened ammonite, this animal is thought to have been a relatively fast swimmer.

Ammonite,Sphenodiscus lenticularis, from the Fox Hills Formation, South Dakota.Both sides display intricate suture patterns and an iridescent nacreous layer. This laterally flattened Ammonite is thought to be a fast swimmer.