Middle to Late Ordovician (444 to 470 million years ago)
Endoceras, an extinct genus of Ordovician cephalopods, was a distant relative of our modern Nautilus and an even more distant cousin of modern squids, octopi, and cuttlefish. Fragments of its long, straight shells are common fossils in Minnesota’s Ordovician strata. Its presence in Minnesota rocks (including the bluffs below the Washington Avenue Bridge) reflects the breakup of an ancient super continent that raised sea level to cover much of the state. Endoceras had a wide range, extending across North America and eastern Eurasia, with some specimens even found in Australian rocks.
Endoceras was among the largest animals of its time with a straight shell up to six meters long with unconfirmed reports of nine-meter-long shells. Even at six meters, Endoceras was the largest known fossil cephalopod in terms of shell length, as later forms evolved coiled shells. Endoceras may have been the apex predator of Minnesota’s Ordovician seas for most of its time, hunting close to the seafloor. Adult specimens were wider than they were tall and most likely lived as ambush predators that lay in wait on the seafloor for prey to pass by. Younger forms had narrower shells and may have been more mobile, actively stalking or chasing down prey.
While all chambered cephalopods had siphuncles, the siphuncle in Endoceras was particularly large relative to its shell size. The purpose of the siphuncle was to regulate the animal’s buoyancy. Endoceras, like all chambered cephalopods, only lived in the shell’s last, largest chamber. As it grew, the animal would extend the length and size of the living chamber, shift its body forward, and then secrete a new wall behind its body to close off part of the previous living chamber. To offset the mass of this elongated shell, the siphuncle extended back through the empty chambers to allow the animal to change the proportions of water and gas present to change the shell’s buoyancy. It did that by increasing the saltiness of the fluids in the siphuncle, which caused the more dilute water in the chambers to flow into the siphuncle. At the same time, gases from the animal’s tissues passively diffused into the emptying chamber. This process reduced the density of the chambered shell so it could be used as a floatation device to keep the animal’s density close to that of seawater. This allowed the cephalopod to maintain a stable buoyancy and move with minimal effort, which in turn allowed it to grow to a much larger size.
However, as other large predators arose, its long straight shell may have made Endoceras vulnerable to predation from forms that could attack its unprotected shell. Endoceras became extinct at the end of the Ordovician while other lines of chambered cephalopods that coiled their shells continued to flourish.
This specimen was collected in Fillmore County, Minnesota by Timothy ‘Tim’ G. Stenerson, a talented fossil collector, and donated by Peter Giangrande of ‘The Enchanted Rock Garden’ in Richfield, MN.