Editing Ammonoidea (section)

=== Aptychus ===
{{Main|Aptychus}}

[[File:Trigonellites latus.jpg|thumb|A drawing of an aptychus which was mistakenly described as a [[bivalve]] and given the name "''[[Trigonellites|Trigonellites latus]]''", from the [[Kimmeridge Clay]] Formation in England|left]]

Some ammonites have been found in association with a single horny plate or a pair of calcitic plates. In the past, these plates were assumed to serve in closing the opening of the shell in much the same way as an [[operculum (gastropod)|operculum]], but more recently they are postulated to have been a jaw apparatus.<ref>{{cite journal | last1=Morton | first1=N | year=1981 | title=Aptychi: the myth of the ammonite operculum | journal=Lethaia | volume=14 | issue=1| pages=57–61 | doi=10.1111/j.1502-3931.1981.tb01074.x | bibcode=1981Letha..14...57M }}</ref><ref>{{cite journal | last1=Morton | first1=N. | last2=Nixon | first2=M. | year=1987 | title=Size and function of ammonite aptychi in comparison with buccal masses of modem cephalopods | journal=Lethaia | volume=20 | issue=3| pages=231–238 | doi=10.1111/j.1502-3931.1987.tb02043.x | bibcode=1987Letha..20..231M }}</ref><ref>{{cite journal | last1=Lehmann | first1=U. | last2=Kulicki | first2=C. | year=1990 | title=Double function of aptychi (Ammonoidea) as jaw elements and opercula | journal=Lethaia | volume=23 | issue=4| pages=325–331 | doi=10.1111/j.1502-3931.1990.tb01365.x | bibcode=1990Letha..23..325L }}</ref><ref>{{cite journal | last1=Seilacher | first1=A | year=1993 | title=Ammonite aptychi; how to transform a jaw into an operculum? | journal=American Journal of Science | volume=293 | pages=20–32 | doi=10.2475/ajs.293.A.20 | bibcode=1993AmJS..293...20S }}</ref>

The plates are collectively termed the [[aptychus]] or aptychi in the case of a pair of plates, and anaptychus in the case of a single plate. The paired aptychi were symmetric to one another and equal in size and appearance.

Anaptychi are relatively rare as fossils. They are found representing ammonites from the Devonian period through those of the Cretaceous period.

Calcified aptychi only occur in ammonites from the [[Mesozoic]] era. They are almost always found detached from the shell, and are only very rarely preserved in place. Still, sufficient numbers have been found closing the apertures of fossil ammonite shells as to leave no doubt as to their identity as part of the anatomy of an ammonite.

Large numbers of detached aptychi occur in certain beds of rock (such as those from the Mesozoic in the [[Alps]]). These rocks are usually accumulated at great depths. The modern ''Nautilus'' lacks any calcitic plate for closing its shell, and only one extinct [[nautiloid]] genus is known to have borne anything similar. ''Nautilus'' does, however, have a leathery head shield (the hood) which it uses to cover the opening when it retreats inside.

There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the shell of the ammonite it is often unclear to which species of ammonite one kind of aptychus belongs. A number of aptychi have been given their own genus and even species names independent of their unknown owners' genus and species, pending future discovery of verified occurrences within ammonite shells.