- Cymatoceras sp.
- Middle Cretaceous Age (Albian Stage – 150 Million Years Old)
- Republic of Madagascar – an island country in the Indian Ocean, off the coast of Southeast Africa.
- Each half measures 5 3/4″ wide (You get BOTH halves)
Nautiloids are a large and diverse group of marine cephalopods (Mollusca) belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day. Nautiloids are among the group of animals known as cephalopods, an advanced class of mollusks which also includes ammonoids, belemnites and modern coleoids such as octopus and squid. Traditionally, the most common classification of the cephalopods has been a three-fold division (by Bather, 1888), into the nautiloids, ammonoids, and coleoids.
The septa are perforated by the siphuncle, which runs through each of the internal chambers of the shell. Surrounding the fleshy tube of the siphuncle are structures made of aragonite (a polymorph of calcium carbonate – which during fossilisation is often recrystalized to calcite, a more stable form of calcium carbonate [CaCO3]): septal necks and connecting rings. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers (called cameral deposits) or within the siphuncle (endosiphuncular deposits), a process which may have been connected with controlling buoyancy. The nature of the siphuncle and its position within the shell are important in classifying nautiloids and can help distinguish them from ammonoids (the siphuncle is on the shell periphery in most ammonoids whereas it runs through the center of the chambers in most nautiloids)