Ammonites reward that curiosity. Their fossils preserve clues to ancient animals, vanished oceans and the passage of geological time.
Here are ten questions to start your exploration, with a few favourite Fossil Huntress stories along the way.
WHAT WAS AN AMMONITE?
Imagine a sea animal with a soft body, arms around its mouth and a shell it carried wherever it went. That was an ammonite!
Many ammonites had a shell curled into a spiral, a little like a tightly rolled garden hose. Some shells had ridges or spines, while others were much smoother.
Ammonites were relatives of today’s squid, octopuses and cuttlefish. These animals belong to a group called cephalopods. Ammonites are extinct, which means none are alive today. We learn about them by studying their fossils.
Inside the shell was a row of little spaces called chambers. Think of rooms separated by thin walls. The ammonite’s soft body lived in the largest room, near the shell’s opening. As it grew, it added a bigger room and closed off the old one behind it.
The older chambers held gas and some liquid. Together, they helped the ammonite stay afloat in the sea instead of sinking to the bottom. Scientists call this buoyancy. Its shell was both a protective home and part of its floating equipment!
For a look inside, visit Ammonites: Chambered Beauty, now illustrated with examples of chambered shells and different coiling patterns.
CAN I MEET A LIVING AMMONITE?
Only in your imagination. Ammonites are extinct, with most disappearing during the mass extinction about 66 million years ago. The fossil shell in your hand is evidence of an animal whose world has gone.
IS A NAUTILUS AN AMMONITE THAT SURVIVED?
A tempting thought, but no. A living nautilus also has a chambered shell, yet it belongs to a different cephalopod lineage. Similar shells do not make the animals interchangeable.
Meet these living shelled cephalopods in Nautilus: Nautilidae.
HOW CAN I TELL WHETHER A FOSSIL IS AN AMMONITE?
Begin by observing rather than naming. Look for the overall shell shape, ribs or other ornament, visible chambers and suture lines. Examine both sides and the edge when possible. Record its size and what is known about the rocks it came from.
A spiral is a clue, not a verdict. Other shells and natural forms can resemble ammonites, and incomplete fossils may need a specialist’s eye. Compare the examples in Chambered Beauty before settling on an identification.
WHY DO SOME AMMONITES LOOK LIKE HOOKS OR KNOTS?
The familiar flat spiral was only one possibility. Heteromorph ammonites developed open coils, hooks, helical forms and extraordinary bends. These were characteristic growth patterns, rather than shells that accidentally went wrong.
Explore Heteromorph Ammonites, then meet the wonderfully tangled Nipponites mirabilis. Research into its buoyancy and balance offers a glimpse of how such an unlikely-looking shell might have worked in the water.
WHAT ARE THOSE LACY PATTERNS?
Some fossils reveal lines that resemble branching rivers or finely cut leaves. These are sutures, marking where the internal walls between chambers met the outer shell wall. They are different from the raised ribs on the outside of the shell.
Those delicate-looking lines trace a three-dimensional structure inside the shell. The chamber walls, called septa, were folded where they joined the outer wall. One long-standing idea is that these folds acted like internal bracing, helping the shell resist the surrounding water pressure, which increases with depth.
But more elaborate lace did not necessarily mean a stronger shell or a deeper-diving ammonite. Mechanical studies have produced mixed results: extra folding can reduce stress in the outer shell wall while increasing stress in the septa themselves. The relationship between sutures, shell strength and living depth remains debated—a beautiful pattern with a scientific mystery still tucked inside. For a closer look at the evidence, see research on ammonite septa and water pressure.
Their patterns vary among ammonites and help palaeontologists distinguish groups. Explore the detail in Sutures, Ridges and Lobes.
HOW CAN AN AMMONITE HELP DATE A ROCK?
Some ammonite species were widespread but existed for comparatively short intervals of geological time. Recognising them can help scientists correlate rock layers in different places and establish their relative ages. This use of fossils is called biostratigraphy.
The fossil does not carry an exact numerical date. Its identity is one piece of evidence considered alongside the surrounding geology. Read more in Ammonite Time Keepers: Index Fossils.
HOW BIG COULD AN AMMONITE GROW?
Ammonites came in an amazing range of sizes. Some were smaller than a coin. Others grew shells wider than a bicycle wheel! A famous giant called Parapuzosia seppenradensis, found in Germany, preserves a shell about 1.74 metres across—roughly the height of an adult. Even that huge fossil is incomplete.
Size alone does not tell us an ammonite’s age. Different species grew to different sizes, and a small fossil might be a youngster or a fully grown member of a small species.
Meet a Canadian giant in The Giant Fossil Ammonite of Fernie. Can you imagine a spiral that big?
WHAT DID AMMONITES EAT—AND WHAT ATE THEM?
Not every ammonite had the same menu. Some ate tiny animals drifting in the water, called plankton. Scientists have found the remains of tiny crustaceans and a baby sea snail near the mouthparts of a fossil Baculites, a straight-shelled ammonite. That is a wonderful clue to an ancient meal!
Other ammonites may have eaten larger prey or scavenged food. Their diets probably varied, and scientists study their jaws and rare food remains to work out what they ate.
Ammonites could become someone else’s dinner, too. Marine reptiles, including some mosasaurs and ichthyosaurs, ate them. Their shells offered protection, but could not keep every hungry hunter away.
Meet one of those ocean hunters in Mosasaur of Vancouver Island, or explore the research into an ammonite’s last meal.
HOW DOES AN AMMONITE BECOME A FOSSIL?
Imagine an empty ammonite shell settling onto the sea floor. Mud or sand buries it. Burial can help protect it from being broken apart, although most shells never become fossils.
Over a very long time, the sediment around the shell can harden into rock. Sometimes parts of the original shell survive. Sometimes minerals replace the shell. In other cases, the shell dissolves but leaves its shape behind. Sediment or minerals filling the inside can form a cast—like a model of the space inside the shell.
Later, erosion can wear away the surrounding rock and reveal the fossil. Some ammonites are preserved inside rounded lumps of cemented sediment called concretions. Discover these natural time capsules in Ammonites in Concretion.
KEEP THE QUESTIONS COMING
Ready for a little fossil detective work? Pick an ammonite photograph, trace its spiral with your finger and sketch what catches your eye. Ribs? Spines? Lacy lines? Gather your clues before giving your fossil a name. Every good mystery begins with a closer look—and one more question!
Have a question you'd like answered? Drop me a message over on Facebook or in the contact form at www.fossilhuntress.com.
For the permanent FAQ, photographs and a collection of related stories, bookmark Ammonites: A Guide to Ancient Oceans.
