Septal implosion in Late Carboniferous coiled nautiloids from Ohio

Abstract

Mapes, R.H. & McComas, G.A. 2010: Septal implosion in Late Carboniferous coiled nautiloids from Ohio. Lethaia, 10.1111/j.1502–3931.2009.00213.x More than 200 relatively mature coiled nautiloid specimens, assigned to Metacoceras mcchesneyi, were recovered from an Upper Carboniferous shale in northeastern Ohio. Twenty-seven undistorted specimens reveal that the septa in every specimen were collapsed and/or telescoped. This septal collapse without external shell distortion could only have been accomplished by limited implosion due to excessive pressure. Analysis of the fossils, sediment and the depositional environment indicate that after burial, the nautiloid cameral spaces were probably filled with both liquid and gas, and the body chamber was filled with semi-solid thixotropic mud. To prevent conch collapse at the time of septal implosion, the thixotropic mud filling the nautiloid body chamber acted as a liquid at the time of stress release during septal failure. The stress was produced by combined lithostatic and hydrostatic pressures, which fluidized the unlithified thixotropic mud that flowed from the body chamber into the phragmocone during septal collapse. After the septal implosion and when flowage ceased, the thixotropic mud quickly resolidified into a solid state providing internal conch support that prevented the collapse of the conch. □Carboniferous, nautiloids, septal implosion, taphonomy, thixotropic mud.