A New Dichotomous Microboring: Abeliella bellafurca isp. nov., Distribution, Variability and Biological Origin

Abstract

Cluster-forming boring traces in carbonate substrates, characterized by regular dichotomous ramifications, are described as a new ichnospecies of Abeliella Mägdefrau 1937: A. bellafurca isp. nov. The new trace differs from A. riccioides Mägdefrau by a three- rather than two-dimensional display of tunnels and branchings and by penetrating carbonatic rather than phosphatic substrates. These boring traces occur in bivalve shells from the Oligocene to the Recent in shallow, illuminated marine settings. Two distinct morphotypes of A. bellafurca were observed, both originating from a single point of entry: one produces prostrate boring system in early developmental stages followed by deeper penetration, and the other, which forms only deep penetrating three-dimensional boring systems. Tunnels with dichotomous ramifications in modern marine environments are produced by the cyanobacterium Hyella stella Al-Thukair et Golubic. The organism penetrates the shells and shell fragments, whereby the apical cell divides alternately parallel and perpendicular to the direction of penetration. Bifurcation follows each longitudinal division of the apical cell. This type of cell division and bifurcations was described in Neoproterozoic endolithic cyanobacterium Eohyella dichotoma Green, Knoll et Swett. Both Proterozoic and modern organisms penetrated carbonatic ooid sand grains in shallow marine settings. Other species of Hyella and Solentia are known to form traces described as Fascichnus (Radtke). Dichotomously branched microborings in phosphatic substrates described as Abeliella riccioides Mägdefrau were attributed to boring fungi. Distinctions between borings produced by phototrophs vs. heterotrophs are discussed because they have bearing on paleobathymetric interpretations.