Telescoping in a Cephalopod Assemblage from the Maquoketa Formation (Ordovician)

The exposures of the Maquoketa Formation near Dubuque, Iowa have attracted interest for both their remarkably concentrated Ordovician cephalopod assemblage and their excellent fossil preservation: the phragmocones exhibit the oldest preserved cephalopod nacre yet discovered. The most common cephalopod phragmocones found here are from Isorthoceras sociale; other, less common species represented include Endoceras sp. and Beloitoceras sp. These phragmocones frequently exhibit a phenomenon known as "telescoping," which often involves smaller phragmocones becoming lodged within larger shells. The mechanisms behind telescoping, however, are poorly understood, especially concerning the sliding of phragmocones into one another. No modern analogue exists that might serve as a model for this phenomenon. We found a partial fossil of a large orthoconic cephalopod without its body chamber or apex from the Graf Road outcrop of the Maquoketa Formation. The species is indeterminate because the siphuncle and internal structure is missing, but it may be an Endoceras sp. due to its large size and whorl section geometry. Multiple Isorthoceras sociale phragmocones were lodged within this larger orthocone throughout its entire length and surrounded by matrix. No sedimentary structures were found surrounding the fossil. These observations suggest that the larger orthocone's septa were damaged or partially dissolved, after which it then filled relatively quickly with sediment before or during the time that the smaller phragmocones entered this larger orthocone. We therefore propose that several mechanical and chemical steps took place for this example of telescoping to occur including burial, partial dissolution (and/or mechanical breakage), and lateral shuffling of sediments.