Ammonite

The first thing to look for is the shape: a flat, tightly wound spiral that coils in a single plane, like a ram's horn pressed flat or a coiled rope seen from the side. Most ammonites you find are this planispiral form, and many show ribs, ridges or knobs running outward across the coil. The widening tube of the shell gets larger as it spirals out from the center, ending at the open mouth where the animal lived. That regular, expanding coil — rather than the screw-like, three-dimensional twist of a snail — is the strongest first clue that you are holding a cephalopod and not a gastropod.

The decisive feature, though, is the suture lines. Inside, an ammonite shell was partitioned into many chambers by thin curved walls called septa, and where each septum met the outer shell it traced a line. On a worn or polished specimen these lines show up as intricate, wavy, frilly patterns — like the outline of an oak leaf or a fern — looping back and forth across the shell. Truly complex, lacy sutures are unique to ammonites; their living relative the nautilus has only simple, smooth, gently curved partitions. If you can see frilly suture lines, or the spiral pattern of internal chambers on a cut and polished face, the identification as an ammonite is essentially settled.

Ammonites belong to the subclass Ammonoidea, a branch of shelled cephalopods that first appears in the fossil record during the Devonian Period, roughly 409 million years ago. They survived several earlier extinction events and then radiated spectacularly through the Mesozoic Era — the Triassic, Jurassic and Cretaceous periods — evolving so rapidly into so many distinct forms that geologists use them as classic index fossils to date and correlate marine rock layers around the world. For much of that time they were abundant open-water animals, which is why their shells turn up so commonly in marine sediments.

Their long story ended at the close of the Cretaceous Period, about 66 million years ago, in the same end-Cretaceous mass extinction that wiped out the non-bird dinosaurs. Ammonites disappeared entirely and left no descendants, so every genuine ammonite fossil is at least roughly 66 million years old, and many are far older. Their close cousins the nautiloids squeaked through the same event and survive today as the pearly nautilus, which is the nearest living thing to a real ammonite — though even it is only a distant relative, not the same animal.

Ammonites were enormously diverse, and thousands of species have been described across their long history. Most are the familiar tightly coiled, flat spiral, but the group also produced striking oddities: partly uncoiled and hook-shaped forms, loosely wound open spirals, and even nearly straight or irregularly twisted shells, especially toward the end of the Cretaceous. The ornament varies just as much — some shells are smooth, others carry strong ribs, rows of spines or bold knobs — and these differences in coiling and ornament, together with the exact pattern of the suture lines, are how specialists tell one kind from another.

Size ranges from small coins a centimeter or two across up to giants. While most collected specimens fit comfortably in the hand, the largest known ammonites reached well over a meter in diameter — close to two meters in a few exceptional cases — making them some of the biggest shelled animals that ever lived. Preservation also creates distinct 'looks': shells turned to glittering golden pyrite, specimens recrystallized into honey-colored or white calcite, and most prized of all, ammolite — the rare gem-quality fossil shell that displays a brilliant iridescent play of green, red and gold and is cut and sold as a gemstone in its own right.

Common ammonites are inexpensive and widely sold; many polished halves and small whole specimens are affordable curiosities, because the animals were so abundant. Value climbs with size, completeness, sharp natural detail, attractive preservation (well-formed pyritized or fully iridescent ammolite pieces command the highest prices), and reliable information about the species and where it was found. Cut-and-polished pairs that reveal the internal chambers, and large display-quality specimens, sit well above the common tumbled or rough material.

Outright fakes of ammonites are less common than with some fossils, but a few traps are worth knowing. Resin casts and molded copies do exist: tell-tale signs are a suspiciously light, plasticky feel, visible mold seams, air bubbles, perfectly identical 'specimens' repeated across a seller's table, and color that sits on the surface rather than running through the material. More common than total fakes are composites and heavy restoration — real fragments assembled with sculpted or glued-in filler, or cracks and missing sections quietly repaired. Examine the piece for seams where texture, grain or color changes abruptly, and be cautious of detail that looks too crisp and symmetrical to be natural. Because a genuine fossil shows the small imperfections of a once-living shell, buying from a reputable dealer who states the species, locality and any restoration is the surest protection.