a rift

Cephalopod evolution 101: the Beak

I received this question today, I have to say this puzzled me the whole day. Let’s delve into the subject I never thought I would delve in: cephalopod jaw evolution!
Behold, the beak of a very large species of squid. Some people use it as the perfect example of convergent evolution: both birds as well as cephalopods developed a sturdy beak to crack hard materials (e.g. nuts, shells or crabs). But to be quite honest, the evolution and possible precursor of the cephalopod jaws have puzzled scientists for ages.
Beak of a freshly caught squid.
Royal society of Chemistry; Photographer: Mark Conlin
Let’s first address the elephant in the room, the squid jaws are not homologous with the radula we know from snails. Even more interesting, squid, octopi and cuttlefish even have a tongue-like radula behind the two jaws to scrape the flesh of their prey. But if the jaws are not a derived form of the radula, what are they derived from?
Let’s say you’re a cephalopod in the late Jurassic period, the sea is full of predators and you need to protect yourself. You have your hard shell, but the predators can just get in there through the front door. Thank your ancestors, because you have what you call an operculum, a hard plate you can use to close your shell.
Dzik describes the evolution of the cephalopod operculum in detail as part of his thesis in 1981. Here he explains, based on fossils and previous findings by other authors, that the operculum in the “Hypothetical ancestor of all shelled mollusks (Coniconchia)” can also be found in the most basal groups of cephalopods (Endoceratida):
Evolutionary relationships between main groups of early Cephalopods, with medial sections and apertural views of all groups.
Dzik, 1981
In more derived groups, something interesting happened: the operculum splits in two parts, in structures we call the Aptychi. During evolution, the aptychi migrated deeper inside the body, but could still be pushed to the outside to act as an operculum. While the aptychi are retracted, the pointy ends emerged a little and could be used as some way to destroy sturdy preys, like shelled invertebrates, thus functioning like mandibles or real “jaws”.
Some examples of aptychi (top right: Oppelia from Late Jurassic of Solnhofen, Germany; bottom left: aptychi (recto and versus) from Late Jurassic of Lombardy, Italy), and conceptual scheme of their function if indeed they were used to close the shell aperture, as opposed to being jaws.
Wikipedia commons; Antonov
Aptychi are often found inside the shell of ammonoids, together with a single plate, what we call an Anaptychus. The function of the Anaptychus, closing the upper part of the shell-opening, can still be found in extant nautili (Nautilus sp.), where a leathery flap closes the shell. It is believed that modern day cephalopods simply removed the problem of protecting the shell entrance by having new structures take care of that (like in the case of the nautilus), or by just loosing the shell partially (e.g. cuttlefish, squid) or even entirely (octopi).
The diet of extinct cephalopods cannot easily be studied, so we cannot be totally sure on where the closing-hatches were used for. For now, this sounds like the most plausible explanation, but there’s still a lot to be discovered.
-Werner

[http://www.paleo.pan.pl/people/Dzik/Publications/Cephalopoda.pdf Dzik, 1981]