Charophytes or stoneworts are one of the largest and most structurally complex of the green algae, and are thought to be part of the evolutionary lineage that lead to vascular plants (Kenrick 1994). As green algae, charophytes are sometimes classed within the Chlorophyta but are often classed as a distinct group, the Charophyta. These plants are aquatic, and modern forms are found in many freshwater to brackish habitats such as ponds, lakes, lagoons and streams. The main axes of mature plants comprise a series of multicellular nodes interspersed by relatively long single cells or internodes. Branches and branchlets grow from individual nodal cells in a whorled arrangement (see inset below). The gametangia, or fertile elements of the plant are quite characteristic in charophytes. The female gametangium or oogonium is formed from a number of divided cells that grow out from one of the nodes; the outer most cells are elongate and spiral around the oogonium forming a protective sheath (see inset below). The male gametangium or antheridium also grows from a nodal cell and is a complex spherical structure usually comprising eight shield-cells, each one enclosing a stalked cell bearing tiny rounded head-cells which in turn bear filaments comprising the cells which produce the spermatozoids. The position of the gametangia on the plants is variable with different charophyte species and can be diagnostic.
As fossils, charophytes can be quite in sediments deposited in suitable environments. It should be noted that the oogonia may be allochthonous or easily transported, but in instances where the axes of the plant are also preserved it is likely these are more-or-less in situ or autochthonous.
Charophytes are quite well represented in the fossil record from the Tertiary, Cretaceous and Jurassic where they are locally abundant, particularly in limestones and marls deposited in brackish or freshwater settings. In these rocks it is usually the fossilised oogonia that are found (commonly called gyrogonites). This is because the oogonia are often calcified and are easily preserved as a mineral of calcium carbonate, typically calcite. In some instances, for example in the Upper Jurassic/Lower Cretaceous Purbeck Limestones of southern England, fossil charophyte oogonia are so common, diverse and widespread they can be used as zone fossils for biostratigraphy (e.g. Feist et al. 1995). The preservation of whole plants is not common, but they can be quite exquisite in their detail (e.g. Martín-Closas & Diéguez 1998).
The earliest fossil charophytes have been recorded from the Upper Silurian, though whether or not these are true charophytes is still debated. Early Devonian charophytes have been found exquisitely preserved in the Rhynie chert and were first recorded and described by Kidston & Lang (1921b). The Rhynie charophytes show many similarities with the extant charophyte group, the Nitelleae, and have been assigned to the species Palaeonitella cranii. The morphology of Palaeonitella is outlined below.
The morphology of Palaeonitella is relatively simple, its structure being closely comparable to that of modern Nitelleae (see inset above). An entire reconstruction of the alga has not been published, though much of the plant is known, primarily the branch whorls, rhizoid nodes and possible bulbils. The status of Palaeonitella has been questioned by Tappan (1980), but recent discoveries of fertile elements in the chert indicate unequivocally that the plant is a charophyte alga (Kelman et al. in press). The fertile elements will be figured here once published.
The overall size of the plant is not known, but it comprises upright primary axes that bear regularly spaced clusters of nodal cells with whorls of lateral branches. The spacing of nodal cells or rather the length of the internodes does, however, tend to decrease towards the distal end of the axes (see inset below left). These branches in turn may bear secondary branches. The number of branches or rays in a whorl can be variable. Whorls commonly bearing up to ten secondary rays have been observed on the lateral branches. The primary axes are slender generally being up to 200µm in diameter, lateral branches are conspicuously narrower being up to 100µm in diameter with very fine secondary branches being up to 50µm in diameter (see inset below right). Occasionally the nodal and internodal cells appear hypertrophic; mutated and swollen where they have been infested by aquatic fungi (Taylor et al. 1992).
Rhizoids and Bulbils
Palaeonitella was undoubtedly an aquatic plant. It is typically found in chert associated with other aquatic biota, particularly crustaceans, chlorophytic algae and chytrids (tiny aquatic fungi). The enclosing matrix commonly contains coprolites and exhibits a clotted or 'mulm-like' texture. It is likely that the alga was an early coloniser, primarily living in still, relatively shallow freshwater temporary ponds, apparently with rather soft silty and organic-rich substrates. If Palaeonitella had a similar ecology to that of most modern Nitelleae it is likely the waters it inhabited were generally quite alkaline. Most extant charophytes tolerate waters with a range of between pH 6 and pH 9. In a few charophyte-bearing chert horizons, many of the specimens preserved in situ exhibit an orientation with axes appearing to be swept or bent over in a particular direction. This probably represents current alignment (Fayers and Trewin in press); maybe Palaeonitella could also tolerate areas of slow moving water, perhaps colonising rather sheltered areas in the cool distal reaches of run-off channels from hot springs.