Argonautidae

The Argonauts, also known as paper Nautilus, belong to the Cefalopod family, but their shell is not properly a "shell", and it's calles a pseudo-shell. The females "secrete" the thin calcareous shell in which they live, and their dorsal arms are modified with large flag-shaped membranes that secrete the limestone to build the shell thal will be used as brood chamber. The swiss paleontologist and zoologist Adolf Naef, dedicated his life to the study of cephalopods, and in 1921 noticed the similarity between its shells and the ammonites. He theorized that the first Argonauts, living in the cretaceous, simply used to occupy the empty shells of the ammonites, and developed their arms and learned to secrete the limestone in order to repair small damages that occurred to the shell. After the mass extinction of the Ammonites they developed the ability to build theid own shell, shaped as an evolution of the ammonite shells. But there is a gap of 40 million years between ammonites extionction and the first argonaut fossil information.

The argonauta is a species with a marked dimorphism, and the female can be even 20 times bigger than the male.

The shell, or rather egg's incubation chamber, is very much sought after by collectors, both for this mistery and  the beauty of the shell.

The Argonauts are pelagic of surface tropical waters, but hardly can be found near the coasts.