SOFT TISSUE PRESERVATION IN COILED NAUTILOIDS FROM THE BEAR GULCH LAGERSTÄTTE (MISSISSIPPIAN-CHESTERIAN) OF MONTANA

Thirteen specimens of the coiled nautiloid, ?Stroboceras were collected from the Bear Gulch Limestone lagerstätte in Montana. Ten specimens show preserved carbon and phosphate? films that can be interpreted as the remains of the intestinal tract, specifically the mandibles, crop, stomach and intestine. Three specimens contain debris clusters interpreted as partly digested food particles. Also, two specimens show massive breaks in the body chamber suggesting a fish or shark may have predated the animal.

The carbon concentrations that we interpret as the mandibles, crop, stomach and intestine, together with the positioning of food debris clusters are mostly confined to the dorsum of the body chamber. This positioning is logical because: 1) the positions for those organs are the same in modern Nautilus and 2) the ventral area of the body chamber in the Bear Gulch specimens appears to lack organ concentrations. With the exception of the gills, this region in modern Nautilus is a hollow cavity surrounded by muscular tissue used for locomotion and respiration through water intake and outflow. Thus, it seems reasonable to conclude that the dorsal area of the body chambers in the Bear Gulch specimens contained a similar mantle cavity without major organs except for the gills. However, no evidence of gills, tentacles, hood, hyponome, or mantle cavity tissue is preserved in these fossil specimens.

These fossil coiled nautiloids are the first to be described with soft tissue preservation. In some ways they confirm that the fossil coiled nautiloids were anatomically similar to modern Nautilus. It remains a continuing puzzle as to how many tentacles and gills were present in these animals.