NEW INSIGHTS FROM RECONSTRUCTED GONIATITE AMMONOID (CEPHALOPODA) RADULAE USING SYNCHROTRON MICROTOMOGRAPHY

Examination of several exceptionally preserved goniatite buccal masses from the Carboniferous (Carboniferous: Mississippian of Arkansas and the Pennsylvanian of Oklahoma) reveals new details about the radula morphology and insights into the evolution of this specialized organ, which is considered to be a soft tissue organ that is rarely preserved. The oldest specimen found in situ in Cravenoceras fayettevillae was previously described by Tanabe & Mapes (1995) and is used as a reference. The second specimen is an isolated buccal mass found in the same locality (Fayetteville Formation, near Leslie, Arkansas) and is attributed to the same species by comparison with the published specimen. Both specimens were studied using PPC-SR µCT on the ID19 at the ESRF (European Synchrotron Radiation Facility) with an isotropic size voxel of 3.49µm. The second specimen presents the radula in its original position in the lower jaw. A total of 23 rows of teeth are preserved as well as the bending plane. Tooth wear is investigated along the radular ribbon in order to obtain insights on feeding habits.

The Mississippian specimens were compared to radulae found in the Pennsylvanian of Oklahoma and, more broadly, to the radulae of younger fossil ammonoids and to Recent cephalopods in terms of the number and shape of the teeth. Although the lower jaw morphology of the Pennsylvanian ammonoid specimens is characteristic of the Goniatitina (broad outer lamella), the morphology of the radulae of the specimens examined is closer to Recent coleoids than to the specialized Ammonitina in the Mesozoic.