Triassic
Chalk Mountain
Nevada, USA

Good 2.3 inch black limestone ammonite.

Ammonite. Cephalopod.

Eutomoceras is an ammonite known from the Early Triassic of Nevada, Russia, and China.  The Early Triassic was a time when life was recovering from the most devastating mass extinction in the past 550 million years.  Ammonites and other groups were diversifying as surface conditions slowly improved.  It was also at this time that all the continents had drifted into each other forming the supercontinent, Pangaea.