FOSSIL PLANTS/Calcareous Algae 429 Figure Section of the coralline alga Lithothamnion The plant body consists of closely packed cell filaments with calcitic walls Reproductive structures (conceptacles, multiporate in this example) remain empty after release of the spores Dark and lighter areas represent seasonal growth zones Upper Miocene, south eastern Spain Scale bar 500 microns except those at the surface (epithallial cells), are calcified Consequently, the entire plant has a rigid structure with high preservation potential Non-geniculate corallines can grow on one another, and on firm substrates, including other plants, or live unattached on mobile sediments They can form thick crusts and develop long protuberances and branches Individual plants can reach several centimetres in size In geniculate (articulated) species, calcified segments of the thallus are separated from one another by noncalcified joints (genicula) These articulated corallines form erect branching thalli usually attached to the substrate by an encrusting holdfast They have a bushy appearance and grow up to 10 cm in height After death, articulated corallines disintegrate and calcified segments are shed as sand and gravel particles Sexual and asexual reproduction both occur in the life cycle of most extant species Gametes (male and female) and spores are produced in separate plants that otherwise exhibit similar vegetative morphology Gametes and spores are produced in conceptacles within the thallus The shape and size of conceptacles producing male gametes (and sometimes those producing female gametes too) differ from those in which spores develop These differences can be recognised in fossil plants (Figure 2) The nature and number of openings (pores) of spore-producing conceptacles are key characters in corallinalean taxonomy Coralline algae constitute a cosmopolitan marine group with tropic to polar distribution and from the intertidal zone to depths of 270 m In addition to this wide environmental range, they can tolerate very variable levels of nutrient supply and turbulence, as well as hypersaline to brackish coastal settings Corallines are significant components of tropical shallow-water reefs (see Sedimentary Environments: Reefs (‘Build-Ups’)) but are also common on seafloors with low terrigenous supply at depths to 100–120 m in all climatic belts In both shallow and deeper settings, they build rigid frameworks or lie unattached on loose substrates where they form nodules called rhodoliths Despite the wide geographic and environmental distribution of corallines as a whole, individual families and sub-families exhibit substantially differing depth-temperature distributions Sporolithaceans are almost entirely restricted to low latitudes where they mainly occupy deep-water or cryptic habitats Melobesioids dominate deep-water coralline assemblages in low–mid latitudes and occupy shallow water in high latitudes Lithophylloids and mastophoroids live mainly in shallow water in low–mid latitudes, mastophoroids predominate in the tropics, and lithophylloids are more common in sub-tropical and warm temperate conditions These habitat preferences of different taxa, together with variations in growth morphology according to levels of turbulence and light intensity, make fossil corallines valuable palaeoenvironmental indicators Coralline-like algae are known in the Ordovician (e.g., Arenigiphyllum) and Silurian (e.g., Graticula) The evolutionary relationships of these fossils, and of Late Palaeozoic fossils that have been called ancestral corallines (e.g., Archaeolithophyllum, see Phylloid Algae), to younger examples are still being elucidated The continuous well-documented subsequent history of corallines commenced in the Early Cretaceous, and the group diversified throughout the Late Cretaceous and Cenozoic (Figure 1) About 30 extant genera are recognized ‘Solenoporaceae’ Solenoporaceans are traditionally regarded as an extinct family of red algae, morphologically similar to but simpler than corallines, ranging from the Cambrian to the Palaeogene The superficial similarities that unite the ‘Solenoporaceae’ are simple, nodular form and an internal structure composed of narrow, juxtaposed branching tubes with diameters up to 100 microns (Figure 3) Details that indicate systematic differences include the cross-sectional shape of the tubes and the presence or absence of cross-partitions However, the group is heterogeneous and contains some sponges as well as red algae, together with fossils resembling cyanobacteria The type species of Solenopora is a chaetetid sponge Some Ordovician and Silurian fossils formerly attributed to Solenopora, such as Graticula, closely resemble the extant coralline Sporolithon (see Corallinales above)