2.7" Iridescent Fossil Cephalopod (Baculites) - South Dakota

Baculites ("walking stick rock") is an extinct cephalopod genus with a nearly straight shell, included in the heteromorph ammonites. The genus lived worldwide throughout most of the Late Cretaceous.

As with other ammonites, the shell consisted of a series of camerae, or chambers, connected to the animal by a narrow tube called a siphuncle by which gas content and thereby buoyancy could be regulated, the same way as Nautilus does today.

From shell isotope studies, it is thought that Baculites inhabited the middle part of the water column, not too close to either the bottom or surface of the ocean.

Baculites fossils are very brittle and almost always break. They are most commonly found broken in half or several pieces, usually along suture lines. Individual chambers found this way are sometimes referred to as "stone buffaloes" (due to their shapes), though the Indigenous attribution typically given as part of the story behind the name is likely apocryphal.