This is a gorgeous, and absolutely massive, 15.4" wide polished ammonite fossil. It is Late Cretaceous in age and was collected in the Mahajanga Province of Madagascar. It's of the genus Lytoceras which is one of the more uncommon ammonites from the locality.
The exterior of this ammonite has been highly polished, revealing the "suture" patterns of the shell and areas of gorgeous red/green iridescence. There is an impression of another, smaller ammonite near the "mouth" of the shell.
It makes an amazing display on the included metal display stand. And did we mention it's huge!?!
Ammonites were predatory mollusks that resembled a squid with a shell. These cephalopods had eyes, tentacles, and spiral shells. They are more closely related to a living octopus, though the shells resemble that of a nautilus. Ammonites appeared in the fossil record about 240 million years ago and they barely survived several major extinction events. The last lineages disappeared 65 million years ago at the end of the Cretaceous.