COMPARATIVE HISTOLOGY OF IDIOGNATHODUS PA ELEMENTS: DOES GERONTIC MORPHOLOGY HAVE A MICROSTRUCTURAL SIGNATURE?

Conodont elements were the first mineralized feeding structures in vertebrate history. Elements record the growth history and dynamics of the individual across the successive deposition of lamellae, including episodes of accretion, remodeling, and damage repair. Growth shapes the appearance of the element, including the development of characteristic morphology, or sometimes “gerontic” or pathological features. Gerontic elements are relatively common in collections, but their genesis is not well understood, although they are thought to belong to relatively old individuals due to their typically larger size. This research investigates whether gerontic elements develop microstructural differences over their lifetimes, such as changes in the amount or spatial arrangement of growth centers, spatiotemporal changes in relative rates of growth, or relationships between damage, remodeling, and “gerontic” features.

The specimens used in this study are both normal and gerontic (large, thickened, unusually sculptured) Pa elements of Idiognathodus from the Hushpuckney Shale of the Swope cyclothem (Late Pennsylvanian, Missourian/ Kasimovian; I. cancellosus Zone). Choosing one stratigraphic level protects against possible interference from signals of biotic or seawater geochemical evolution.

Elements were embedded in epoxy, ground to the plane of interest, and imaged with backscattered electrons to visualize the growth layers. Initial results demonstrate possible roles for addition of growth centers and changes in rates of growth, but extensive remodeling concentrated near both the oral and aboral surface of the specimens obscures microstructural details, and may represent the dominant factor in the morphology of gerontic individuals.