Classical and new bioerosion trace fossils in Cretaceous belemnite guards characterised via micro-CT

Max Wisshak,Jürgen Titschack,Wolf-Achim Kahl,Peter Girod

doi:10.5194/fr-20-173-2017

Abstract

Abstract. The ongoing technical revolution in non-destructive 3-D visualisation via micro-computed tomography (micro-CT) finds a valuable application in the studies of bioerosion trace fossils, since their three-dimensional architecture is hidden within hard substrates. This technique, in concert with advanced segmentation algorithms, allows a detailed visualisation and targeted morphometric analyses even of those bioerosion traces that are otherwise inaccessible to the widely applied cast-embedding technique, because they either are filled with lithified sediment or cement or are preserved in inherently insoluble or silicified host substrates, or because they are established type material and should not be altered. In the present contribution selected examples of such cases are illustrated by reference to bioerosion trace fossils preserved in Late Cretaceous belemnite guards from the European Chalk Province. These case studies comprise an analysis of a diverse ichno-assemblage found associated with the lectotype of the microboring Dendrina dendrina (Morris, 1851) in a belemnite from the upper Campanian to lower Maastrichtian chalk of Norfolk, England, and the description of two new bioerosion trace fossils with type specimens found in belemnite guards from the lower Campanian limestones of Höver, Germany. The latter are Lapispecus hastatus isp. n., a tubular and occasionally branched macroboring for which a sipunculan or a phoronid trace maker are discussed, and Entobia colaria isp. n., a camerate network formed by an excavating sponge that eroded diagnostic grated apertures at the locations of the presumed inhalant papillae or exhaling pores, adding to or replacing filtering devices that are otherwise made of tissue and spicules. As an added value to the non-destructive visualisation procedure, the processed X-ray micro-CT scans of the studied type material provide 3-D models that may now serve as digitypes that can be studied as digital facsimile without the necessity of consulting the actual type specimens.

Highlights

X-ray computed tomography (CT) and especially microcomputed tomography have been firmly established in many fields of earth sciences, including palaeontology, where they have demonstrated their fidelity as nondestructive methods that are about to revolutionise 3-D visualisation of fossils
While Morris limited himself to illustrating three different types of bioerosion trace fossils only, namely Dendrina dendrina, Talpina ramosa, and Trypanites solitarius, the present analysis shows that the belemnite bears traces of at least 10 different ichnospecies (Figs. 2–6)
Micro-computed tomography in conjunction with algorithm-based segmentation procedures and morphometric assessments is a prime tool for non-destructive visualisation and analysis of bioerosion trace fossils

Summary

Introduction

X-ray computed tomography (CT) and especially microcomputed tomography (micro-CT) have been firmly established in many fields of earth sciences, including palaeontology, where they have demonstrated their fidelity as nondestructive methods that are about to revolutionise 3-D visualisation of fossils (for a review, see Cunningham et al, 2014). This includes trace fossils, and bioerosion trace fossils (Tapanila, 2008; Beuck et al, 2007, 2008; Amon et al, 2015; Färber et al, 2016).

Methods

Results

Conclusion