THE CONODONTA AND THE EVOLUTION OF VERTEBRATE TISSUE

Since their earliest discovery, conodonts have been an enigmatic group attributed to a variety of animal phyla from Arthropoda to Annelida, and have even been classified within the Kingdom Plantae. This ambiguity can largely be attributed to the lack of body fossils of conodonts, as they typically leave only mineralized elements or teeth behind for classification. With the discovery of the conodont body fossil in 1983, much of this ambiguity dissolved. The carbonized body clearly indicated an affinity within the Chordata, bearing such distinguishing features as a notochord, ray supported fins and myomeres. However, their exact affinity within the Chordata remains open to debate. Some workers have made a compelling case for the classification of the Conodonta within the Vertebrata. While compelling, this classification has not been accepted by all workers, largely on the basis of a lack of clear evidence for specific tissues that are unique to vertebrates.

For the purposes of this study, a wide array of Scanning Electron Microscopy (SEM) techniques were applied in an attempt to distinguish different tissues with the conodont elements. Combining backscatter electron imaging, cathodoluminescence and x-ray spectrometry, histological images of conodont elements were acquired in a detail that has not been previously obtained. These images have led to the identification of several of the most basic vertebrate hard tissues, including some previously unidentified structures such as dentin tubules and pulp cavity. In addition, residual traces of collagen can be found within some of these tissues further supporting the interpretation of dentin and dental pulp tissues. These interpretations strongly support the designation of conodonts as vertebrates and also point toward the origins of primary vertebrate tissues. In addition, this study has laid the groundwork for future research using this array of SEM techniques.