A Split Ammonite

While dinosaurs ruled the land during the Late Cretaceous, the ammonite genus Pachydiscus (aka "thick disc") could be found flourishing in oceans worldwide.

Ammonites were a type of cephalopod—the taxonomic class that includes today's octopus, nautilus, and squid. Much like a submarine, they employed gas and fluid-filled chambers to regulate their position in the water column. The animals themselves lived only in the outermost compartment, employing their tubular siphuncle to connect its chambers along their shell's ventral surface.

Pachydiscus fossils are an important index species in the stratigraphic record of the history of life on Earth, as they are used as the "defining biological marker" for the beginning of the Maastrichtian Stage, starting 72.1 ± 0.2 million years ago. The specimen offered here is even more remarkable, as evidenced by the calcified geodes that have taken up residence within each of the Pachydiscus fossil's chambers.