432 FOSSIL PLANTS/Calcareous Algae currents and other oceanographic factors, they can be used as proxies for palaeo-oceanographic conditions in ancient pelagic sediments Chlorophyta (Green Algae) Halimedales This order (formerly included in the Codiaceae or Udoteaceae) comprises marine algae of siphonous organisation, i.e., the plant is a single multinucleate cell forming a branching tube or siphon (the parts of the tube resulting from branching are also called siphons) In Halimeda the thallus is a bundle of interwoven, subparallel siphons that branch into swollen vesicles (utricles) to form the outer surface The thallus is composed of segments encrusted with aragonite needles that form in the spaces between siphons and vesicles (Figure 6) Thalli, typically a few decimetres in size, can reach m or more in length They are articulated by having uncalcified connections between calcified segments Halimeda attaches to hard substrates or uses a large bulbous holdfast (aggregate of rhizoidal siphons) to anchor itself in mobile sandgravel substrates Plants can be erect or, especially when larger, sprawling with several points of attachment Male and female gametes are produced in separate plants in branched stalks at the upper margins of segments In sexual reproduction the cell contents of the entire plant are transformed into gametes (mass spawning), leaving the thallus empty and dead Halimeda occurs in tropical to warm temperate seas In temperate waters such as the Mediterranean Sea the thallus is weakly calcified In tropical carbonate environments Halimeda is common in coral reefs and lagoons from depths of less than m to 150 m Its Figure Halimeda segments Aragonitic crystals (now calcite) form a cast of the complex, interwoven cell tubes of the alga, which remain as voids later filled with cement Upper Miocene, south eastern Spain Scale bar mm fast growth produces copious quantities of gravel-size segments that are shed both during life and after death Halimeda sediment production is particularly important in relatively deep water, at depths of 20–50 m Halimeda mounds at this depth cover areas of hundreds of square kilometres on the outer Queensland shelf of the Great Barrier Reef In the Florida Keys, Halimeda is the single most important component of carbonate sand and gravel, rivalling coral in abundance Halimeda-like fossils are present in the Palaeozoic, e.g., the Ordovician Dimorphosiphon and the Devonian Litanaia The group diversified in the Mesozoic with Boueina (Late Triassic–Late Cretaceous) and Arabicodium (Mid-Jurassic–Early Tertiary) Halimeda ranges from Early Cretaceous to present Other extant halimedaleans, such as Udotea, Penicillus, Rhipocephalus, and Tydemania, are also important carbonate producers in tropical shallow water environments Udotea has a fan-shaped thallus with a simple stalk Penicillus and Rhipocephalus have brush-like thalli anchored by rhizoidal siphons Tydemania is globular Greater post-mortem skeletal disaggregation than in Halimeda limits the preservation potential of these genera as recognizable fossils Dasycladales As in halimedaleans, the thallus is siphonous, consisting of a single multinucleate cell with a large central vacuole surrounded by cytoplasm Plants range from a few millimetres to 20 cm in height, and are anchored by a rhizoidal holdfast They have a single erect axis (dichotomously divided in a few species) with lateral branches Gametes (zooids, no sexual differentiation occurs) are produced in fertile laterals Dasycladalean taxonomy is complex Six major families can be recognized, four of which (Seletonellaceae, Beresellaceae, Diploporaceae, Triploporellaceae) are extinct Dasycladaceae and Polyphysaceae (formerly Acetabulariaceae) are extant Present-day Dasycladaceae (six genera) occupy shallow marine environments often less than m in depth, in sheltered areas in tropical and subtropical waters Spaces between the main axis and lateral branches are filled to varying degrees by aragonite The resulting calcareous skeleton forms a cast of the plant that preserves the shapes of the main axis and laterals as voids (Figure 7) Degree and site of calcification vary with the species This strongly influences its potential to preserve details of thallus anatomy Calcification generally decreases with illumination and temperature A few poorly calcified species grow in warm-temperate seas Batophora, which does not calcify, can also be found in brackish and fresh water