CONULARIID TESTS: MICROSTRUCTURE, ORGANIC MATRIX, AND MINERALOGY

Conulariids are generally accepted as the mineralized tests of an extinct group of scyphozoan cnidarians, perhaps with affinities to coronates. As an obscure group, they have received only sporadic attention, with most investigations on their tests taking place decades ago. Those results indicated that the tests were primarily phosphatic in composition, with suggestions of carbonate-rich apatite, chitinophosphatic, or simple apatites. Organic components have been suggested, but evidence for their presence is very limited.

Well-preserved specimens of four species of two genera (Conularia and Paraconularia) of conulariids were collected from sites of Ordovician age in Iowa, and of Mississippian age in central Tennessee. Optical microscopy of thin sections of each of these confirmed that preservation was excellent, including optical extinction patterns characteristic of original microstructure. Extension of this investigation into scanning electron microscopy (SEM) confirmed that fine details of the microstructure were present, even at levels beyond the limits of optical microscopy.

Earlier investigations have suggested that the conulariid test is formed by the addition of layers to the inside of the structure, extending the skeleton in much the same way as one might stack ice cream cones within one another. In this investigation, both optical and electron microscopy were used to confirm and refine our understanding of the process. One significant observation was that the newly added layers were consistent in thickness in the interspaces but increased in thickness when adding to the ribs and carinae, thus thickening and strengthening those structures.

X-ray fluorescence demonstrated significant amounts of phosphorous, calcium and carbon, and a preliminary x-ray diffraction analysis showed peaks for carbonate-rich apatite. This work will be continued.

Some specimens were sectioned, polished, etched, and dried by a critical-point process that preserved the three-dimensional structure of organic material. As a result, SEM analysis showed that some layers of the test were extraordinarily rich in organic matrix, while other layers contained little or none. This seemed to correspond to the alternating pattern of light and dark observed under the optical microscope.