A VAUCHERIA-LIKE FOSSIL FROM THE NEOPROTEROZOIC OF SPITSBERGEN

Shallow-water shales from the ca. 750 Ma-old Svanbergfjellet Fm, Spitsbergen, preserve large populations of tomaculate (sausage-shaped) microfossils, typically around 50 mm wide and 200 mm long. Combined with a range of more idiosyncratic characteristics, it is possible to reconstruct significant aspects of their biology. Such characteristics include: 1) linear internal structures, reliably interpreted as condensed cytoplasm, and indicating a coenocytic grade of organisation; 2) complete and rare incomplete division of filamentous coenocytes, typically resulting in unevenly partitioned uniseriate filaments; 3) rare equidimensional branching, usually at 90º; 4) tomaculate cells with external thin-walled vase-shaped to spheroidal structures, reliably interpreted as differentiated reproductive structures; 5) tomaculate cells with external (in some instances possibly internal) thick-walled spheroidal structures, sometimes with medial splits, sometimes in series with thinner-walled cells, reliably interpreted as cysts; 6) tomaculate cells with double walls; 7) tomaculate cells with an external filamentous envelope; 8) tomaculate cells with unobstructed terminal filamentous extentions ca. half the diameter of the "parental" cell; and 9) clavate coenocytic filaments approximately half the diameter of tomaculate cells.

Among extant organisms, these Neoproterozoic fossils are most comparable to the Xanthophyte alga Vaucheria, with specific similarities to its so-called Gongrosira stage where the cytoplasm is partitioned into a series of mostly, but not entirely, disconnected, desiccation-resistant segments. The comparison with the fossils is striking. Most, but not all, of the remaining fossil characteristics can be accommodated within the extant vaucherian bauplan.

Among other fossils, these various components are readily compared to a number of form-taxa, including Arctacellularia, Brevitrichoides, Digitus, Archaeoellipsoides, Torgia, Eosynechococcus, Caudosphaera, Leiosphaerida, certain VSMs and, possibly, Palaeovaucheria. Most of these form-taxa co-occur in the late Mesoproterozoic Lakhanda Formation, and it is worth considering whether the recorded diversity of that biota might largely derive from the ontogenetic complexity of a Vaucheria-like alga.