The first identification of fossil Mesophyllum in accordance to the modern taxonomic concepts in coralline algae

Despite their common occurrence, the potential of coralline algae is not yet fully exploited in paleoecological reconstructions. The reasons are mainly grounded in the taxonomic inconsistency caused by poor preservation or insufficient knowledge of the type material of many species, and confusion derived from the difficult recognition of the coralline three-phased life cycle in the fossil record. Specimens of fossil coralline algae from newly collected samples, and historical Schaleková’s collection of middle Miocene Paratethyan limestone were studied under optical and scanning electron microscopes, revealing the occurrence of the asexuate, male gametangial, and carposporangial conceptacles of Mesophyllum crassiusculum here documented for the first time. Based on the recent emendation of Mesophyllum and consequent circumscription of the genera Mesophyllum sensu stricto and Melyvonnea, this is the first and oldest finding of a fossil Mesophyllum sensu stricto. Moreover, we provide further evidence of the preservation potential of important diagnostic characters, such as the shape of epithallial and subepithallial cells, the shape of the conceptacle roofs, the number and shape of pore canals lining cells in the multiporate roof of the asexuate conceptacle chambers. The identification of M. crassiusculum in the middle Miocene of central Paratethys would deserve further biogeographic and paleoclimatic considerations that, however, are prevented by the incomplete exploration of the Paratethyan fossil record and the need of revision of other important type collections of closely related species.