Lithothamnion crispatum: long-lasting morphospecies of nongeniculate calcareous red algae

Lithothamnion crispatum is a nongeniculate coralline alga of the Hapalidiaceae family, with a cosmopolitan distribution. It occurs in the Mediterranean, in the Atlantic and Indian Ocean and in the Pacific Ocean along the Eastern coast of Australia (Basso et al. 2011). The plants either grow unattached, forming rhodoliths, or attached on hard substrate; they have been found from 2 to 80 meters of water depth (Keats et al. 2000; Nobregas-Farias et al. 2010; Basso et al. 2011). Until now L. crispatum has not been recorded in the fossil. A revision of fossil specimens, ranging in age from Eocene to Pleistocene, proved that this species has a long stratigraphic distribution, starting approximately 40 Ma ago, without significant morphological changes in both reproductive and vegetative structures (Fig. 1). The species has a fruticose to foliose growth form with a thallus organized in a plumose hypothallium (ventral core) and a zoned perithallium (peripheral zone). Perithallial and hypothallial cells are connected by cell fusions. Secondary pit connections and trichocytes have not been observed. Epithallial cells are flattened and flared; subepithallial initial are as long or longer than their derivatives. Tetra/bisporangial conceptacles are multiporate, their roof is slightly protruding above the surrounding thallus surface and is characterized by a depression on the top of pore canals (Fig. 1A-D). This depression is generated by the disintegration of the last cell of the roof, and the resulting pit overlays wedge-shaped cells, which, in surface view, appear as a rosette bordering the pore canal (Fig. 1E-H). Conceptacle roofs pitted with depressions originating from the degeneration of the uppermost cells in filament bordering the pore canal is a unique diagnostic feature which distinguishes L. crispatum from other species of its genus (Wilks and Woelkerling 1995; Basso et al. 2011). This morphological character was used to merge Lithothamnion indicum and Lithothamnion he