What Are the Lines on an Ammonite? Walls Meeting Shell in a Complicated Curve

An ammonite shell is divided internally into chambers by walls called septa, and the animal lived only in the last and largest one, with the earlier chambers holding gas and fluid for buoyancy. Each septum meets the inner surface of the outer shell along a line, and where the outer shell is worn away or the specimen is sectioned that line becomes visible. What makes it striking is that the septa are not simple discs but are folded and refolded, so the line of contact is a complex curve rather than a circle, with branching patterns that in advanced forms resemble foliage or frost on a window. The pattern is consistent within a species and varies between them, which makes it the single most useful character for identification and one of the main tools for dating rocks.

Three broad grades of complexity are recognised and they follow each other in time:

• •Goniatitic, with simple angular zigzags, characteristic of the Palaeozoic
• •Ceratitic, with rounded forward lobes and finely divided backward ones, characteristic of the Triassic
• •Ammonitic, elaborately subdivided in both directions, characteristic of the Jurassic and Cretaceous
• •The sequence is a broad trend rather than a strict rule, with reversals in particular lineages
• •Nautiloid relatives kept simple curved sutures throughout and never developed the complexity
• •The pattern in any individual also becomes more complex as the animal grows

Several explanations have been offered and none commands agreement. The strengthening hypothesis holds that a folded wall braces the shell against external pressure, allowing the animal to live deeper, which is intuitive and has been tested with engineering models that give mixed and sometimes contradictory results, with some finding the complex forms weaker rather than stronger. A surface area explanation holds that more contact between septum and shell attaches the wall more securely, or that increased surface assists in moving fluid in and out of the chambers to adjust buoyancy, which connects the shape to a function the animal certainly performed. A developmental explanation holds that the pattern arises from the mechanics of how the tissue secreting it behaved, so the complexity is a by-product rather than an adaptation. The disagreement has persisted for over a century.

The chambered part of the shell was a buoyancy device and understanding it explains why the walls existed at all. A tube of tissue ran back through every chamber, and by moving salts into and out of the chamber fluid the animal could draw water in or push it out osmotically, adjusting its overall density slowly. That allowed it to hang in the water column without swimming, which is an enormous saving for an animal that propelled itself by jetting. The living relative with the same arrangement adjusts buoyancy over hours rather than minutes and cannot use the system to change depth rapidly, which probably applied to ammonites too and constrains how they are imagined to have lived. Chamber construction proceeded as the animal grew forward, sealing off the space behind it, so the number of chambers records age in a rough way.

Ammonites are the standard against which much of the Mesozoic timescale is calibrated and the sutures are central to that. The animals were abundant, widely distributed in the open ocean, evolved rapidly and left durable shells, which is the exact combination that makes a good index fossil, and the suture supplies a character that changes recognisably between closely related forms. Sequences of ammonite species define zones that are frequently shorter than a million years, which is fine resolution by geological standards, and the same zones can be recognised across continents because the animals travelled. That framework was established largely in the nineteenth century by patient collection and comparison, and it underpins the correlation of rock sequences worldwide, including much of the work that dated the events at the end of the Cretaceous when the group itself disappeared.

The lines are where internal dividing walls met the outer shell, visible when the shell is worn or sectioned, and the walls are folded so the contact is a complex curve. Simple zigzags, rounded forms and elaborately subdivided patterns follow each other through geological time. Whether the folding strengthens the shell, serves buoyancy or is a by-product is still argued.