Ivanovia tebagaensis was a cyathiform Permian codiacean membranous alga with dimorphic cortices

Abstract

Species of Ivanovia (Codiaceae or Udoteaceae, Chlorophyta) have traditionally been characterized as phylloid or leaf-like. Extraordinarily well preserved specimens of I. tebagaensis in limestone from the Upper Permian of southern Tunisia indicate that the thallus was cyathiform or cup-shaped, similar to the broadly conical codiacean Calcipatera and to living Udotea cyathiformis. Ivanovia also shared with Calcipatera the same general membrane structure consisting of bilateral cortices with palisades of utricles, now filled with micrite, and a medulla of tubular coenocytes, now filled with sparry calcite mosaic. A cyathiform thallus would have had an inner and an outer cortex and those of I. tebagaensis were dimorphic. The utricular structure as usually seen in thin sections using transmitted light is much clearer when viewed on polished surfaces of hand specimens using reflected light. Commonly occurring fused membranes suggest that the thalli reproduced vegetatively by budding. The complexity and regularity of the I. tebagaensis membrane structure strongly suggest that a model which proposes that Ivanovia is simply a diagenetic stage in the fossilization of the red alga, Archeolithophyllum, is invalid.