New applications of spectroscopy techniques reveal phylogenetically significant soft tissue residue in Paleozoic conodonts

Abstract

Mineralized tissues such as bones and teeth form the vast majority of what is known of the vertebrate fossil record, whereas non-mineralized tissues are primarily known only from exceptional localities. New chemical techniques have been developed or applied over the last two decades to analyze potential biomarkers for evidence of soft tissues such as keratin, including immunohistochemistry and synchrotron-based chemical analyses among others. These techniques have led to the identification of keratin in fossil feathers and claws by the presence of biologic sulfur residues. Histological sections of Permian and Ordovician aged conodont dental elements are examined for the presence and distribution of soft tissue biomarkers utilizing a suite of spectrometry techniques. Data obtained using energy dispersive X-ray spectrometry consistently show elemental sulfur distributed within the earliest growth stages of the conodont crown as well as in the connected basal body, while X-ray photoelectron spectroscopy analysis supports an organically endogenous origin for at least some of this sulfur. These data suggest that conodont elements, at least in early growth stages, were partly composed of soft tissue, possibly keratin, and were not purely phosphatic. Soft tissue data such as these can have a dramatic effect on our understanding of the early vertebrate fossil record. Incorporating these new data into a phylogenetic analysis suggests conodonts are stem cyclostomes, which is contrary to their current identification as stem gnathostomes.