The biological reconstruction of Tetradium Dana, 1846

Abstract

The Ordovician fossil Tetradium Dana, 1846 typically had aragonite tubes with millimetre-size subsquare cross-sections, fourfold symmetry and an inward-projecting septum along the midline of each wall. It increased by axial quadripartite division into four subequal parts when the septa grew together in the centre of the tube. Tetradium is usually classified as a chaetetid sponge or a tabulate coral. However, it is shown that in animals, fourfold symmetry occurs only as a secondary adaptation in medusae, and axial quadripartite division into four subequal parts is unknown. In contrast, both of these characteristics are common in algae. Tetradium is classified here as a rhodophyte, a calcified corticated uniaxial florideophyte. The anatomy of Recent rhodophytes is used to model the living Tetradium, and their branching is used to model growth by axial quadripartite division. The reconstructed microanatomy of the living Tetradium has (1) an uncalcified axial filament of elongated cells, (2) whorls of four lateral axes, rarely calcified and therefore rarely preserved as tabulae, that radiated from each cell of the central filament, and (3) a weakly calcified cortex that formed the subsquare tube and extended into the inward-projecting septa. Aragonite precipitation within the cortex was probably entirely intercellular, and produced a lightly calcified skeleton. Each reconstructed skeletal tube extended distally into a somewhat flexible, photosynthesizing and nutrient-absorbing part of the alga. Holdfasts were weak. This study shows that reconstructing the anatomy of an enigmatic fossil, instead of just comparing hard parts, significantly improves the reliability of its taxonomic identification.