3D morphology of post mortem acrothoracican borings in Famennian heterocorals – a morphological diversification or a continuum ichnospecies?

Abstract

Barnacle borings were found in six heterocoral skeletons. They are present as small (up to 2.9 mm long), slender, pouch-shaped borings with tapering, slit-like openings. The investigated borings were made by acrothoracican cirripedes, which mostly do not have a shell and bore into hard substrates to protect their “naked” bodies. The first occurrence of borings in skeletons of the heterocoral Oligophylloides Różkowska, 1969 from the Upper Devonian Tafilalt Platform was reported recently by Weyer in 2016. Here, the authors present the results of a detailed study of heterocoral remains with numerous acrothoracican borings from Jebel Bou Ifarherioun (Famennian, Anti-Atlas, Morocco). The borings were found on the basal part but also on broken branches and stems of the heterocoral corallum and occurred post mortem. There is no indication of a syn vivo coral-barnacle interaction with borings in tissue-covered areas. The authors used micro-CT scans to visualize the 3D morphology of the pits, their orientation, and distribution. Additionally, the 3D morphology of an assemblage of 75 pits was used to carry out ordination and cluster analyses, which showed that previously proposed ichnospecies may be a continuum of morphological variability. In the basis of measurements by the present authors, the studied borings do not fit any known ichnotaxa. The absence of bourrelets excludes the possibility that the borings studied belong to the ichnogenus Rogerella Saint-Seine, 1951. Hence, the results seem to contradict a synonymization that was proposed by Bromley and D’Alessandro (1987) and subsequent authors and leave room for further research and discussion on this topic. Although the inferred boring organism is a filter feeder and, thus, depends on currents, the authors did not find a preferential orientation of the borings. The samples considered here are the best-preserved Devonian barnacle borings to date.