X-ray nanotomography and electron backscatter diffraction demonstrate the crystalline, heterogeneous and impermeable nature of conodont white matter.

Ayse Atakul-Özdemir,Peter G Martin,Carlos Martínez-Pérez,Manuel Guizar-Sicairos,Philip C J Donoghue,Xander Warren,Federica Marone,Mirko Holler

doi:10.1098/rsos.202013

Abstract

Conodont elements, microfossil remains of extinct primitive vertebrates, are commonly exploited as mineral archives of ocean chemistry, yielding fundamental insights into the palaeotemperature and chemical composition of past oceans. Geochemical assays have been traditionally focused on the so-called lamellar and white matter crown tissues; however, the porosity and crystallographic nature of the white matter and its inferred permeability are disputed, raising concerns over its suitability as a geochemical archive. Here, we constrain the characteristics of this tissue and address conflicting interpretations using ptychographic X-ray-computed tomography (PXCT), pore network analysis, synchrotron radiation X-ray tomographic microscopy (srXTM) and electron back-scatter diffraction (EBSD). PXCT and pore network analyses based on these data reveal that while white matter is extremely porous, the pores are unconnected, rendering this tissue closed to postmortem fluid percolation. EBSD analyses demonstrate that white matter is crystalline and comprised of a single crystal typically tens of micrometres in dimensions. Combined with evidence that conodont elements grow episodically, these data suggest that white matter, which comprises the denticles of conodont elements, grows syntactically, indicating that individual crystals are time heterogeneous. Together these data provide support for the interpretation of conodont white matter as a closed geochemical system and, therefore, its utility of the conodont fossil record as a historical archive of Palaeozoic and Early Mesozoic ocean chemistry.

Highlights

Conodonts are an extinct lineage of primitive vertebrates that have been widely accepted as the earliest members of our evolutionary lineage to possess a mineralized skeleton, manifest as a complex array of ‘elements’ that comprise a feeding apparatus
Conodont elements are bicomponent, composed of a basal body and crown that grew in synchrony as evidenced by the growth lines that pass between these structures [3,4]
This study aims to resolve a physical characterization of conodont white matter structure using state-ofthe-art equipment and methods and to inform their utility as geochemical archives of past ocean chemistry, corroborating previous conclusions [6,7]

Summary

Introduction

Conodonts are an extinct lineage of primitive vertebrates that have been widely accepted as the earliest members of our evolutionary lineage to possess a mineralized skeleton, manifest as a complex array of ‘elements’ that comprise a feeding apparatus. The conodont skeleton harbours additional and more general insights as an archive of seawater chemistry and temperature, preserved in the calcium phosphate mineral from which the elements are composed, providing fundamental insights into past climates throughout their 300 Myr evolutionary history, including episodes of past climate change associated with mass extinctions, analogous to present [1,2]. White matter has been suggested as the principal tissue to target for geochemical analyses because of its assumed low permeability [6]; there is little agreement on the physical nature of the crown tissues (see discussion in [7] and references ) and still less agreement on their inferred development, which is crucial for elucidating their significance in understanding the early evolution of vertebrate skeletons [8]

Objectives

Methods

Findings

Discussion

Conclusion