SPECIMEN GUIDE
Ammonite identification guide
Ammonites are extinct marine animals known for their distinctive coiled shells. They lived for millions of years and are among the most recognizable fossils in sedimentary rock.
What does an ammonite look like?
Most ammonites show a flat, spiral coil divided into chambers. The outer surface may be smooth or marked with ribs, bumps, or spines. A broken or weathered specimen may only reveal part of the spiral or a line of internal chambers.
Look for these key features
Start with the planispiral coil—one that winds in a flat plane. Then examine the shell for repeating ribs and the intricate lines where internal chamber walls meet the outer shell, called sutures. Size varies dramatically, so shape and structure matter more than measurement alone.
Ammonite or nautilus?
Both have chambered, coiled shells. Ammonite sutures are often more complex and frilled, while nautilus sutures are typically simpler. The distinction can be difficult from a worn photo, so treat app results as a starting point for further comparison.
Where are ammonites found?
They occur in marine sedimentary rocks around the world. Your region’s geological age is an important clue: different ammonite groups lived during different periods of Earth’s history.