What Are Those Paired Plates Found With Ammonites? A Two-Hundred-Year Argument
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Flat paired calcite plates turn up in the same rocks as ammonites and sometimes inside their shells. Whether they closed the shell or lay in the mouth was argued for two centuries and is now largely settled.
What they are
An aptychus is a pair of roughly triangular calcite plates, found either separately or fitted together along a straight edge, occurring in marine rocks of the same age as ammonites and frequently within ammonite body chambers. A related single structure of different composition occurs in other groups. They were named and classified as separate fossils long before their association with ammonites was established, because they were found in isolation and in enormous numbers, and in some rock units they are far commoner than the ammonites themselves, which is explained by their different composition surviving conditions that dissolved the shells.
The two interpretations
Both readings had serious support and each explained different evidence:
- •An operculum, a lid closing the shell aperture when the animal withdrew, which their shape and position when found together suggest
- •A lower jaw, part of the feeding apparatus, which comparison with living cephalopods supports
- •Specimens found in the correct position for a lid are common
- •Specimens found alongside an upper jaw element of different composition are also known
- •The plates are too large to close some apertures and the wrong shape for others
- •The two functions are not mutually exclusive, which is the modern resolution
How the argument was resolved
The weight of evidence shifted decisively towards the feeding interpretation. Specimens preserving both the paired plates and a separate upper element in articulation, in the position a beak would occupy, established that the structure is part of a jaw apparatus comparable to that of living cephalopods, and several such specimens are now known from exceptional deposits. Detailed comparison of the surfaces shows wear and structure consistent with feeding rather than with sealing an aperture. The position within the body chamber in which they are usually found is where a collapsed animal's jaw apparatus would settle. The current view is that they functioned as a lower jaw and may also have served to close the aperture in some species, which accounts for the evidence on both sides.
What ammonites ate
Establishing the jaw interpretation opened a question that is still being answered. Ammonite jaws are varied in form, with some slender and delicate and others heavy and equipped with hardened tips, which suggests different diets across the group rather than one feeding habit. Exceptional specimens preserving gut contents have been found containing small crustaceans and planktonic organisms, indicating that at least some species fed on very small prey rather than hunting anything substantial. Analysis of one group's jaw structure supported a filter-feeding or plankton-collecting habit, which was unexpected and which fits an animal drifting in the water column adjusting its buoyancy slowly. Others plainly captured prey. The picture emerging is of a group occupying a range of feeding roles comparable to modern cephalopods rather than sharing one habit.
Why the plates outlast the shells
The mineralogy explains why these objects are so abundant and so often isolated. Ammonite shells are made of aragonite, a form of calcium carbonate that is unstable over geological time and dissolves or recrystallises readily, while aptychi are made of calcite, a more stable form of the same compound that survives conditions destroying aragonite. In deep water deposits where aragonite dissolves before burial is complete, the plates are preserved and the shells are not, producing rock units full of them with no ammonites at all, which is why they were classified independently for so long. That difference makes them useful for dating and correlating such deposits, where they are effectively the only ammonite evidence available.
The takeaway
Paired calcite plates found with ammonites were read either as a lid closing the shell or as part of a jaw, and the argument ran for two centuries. Specimens preserving them articulated with a separate upper element settled it towards the feeding interpretation. They are made of calcite while the shells are aragonite, which is why they survive in deposits containing no ammonites.