New insights on the red alga Archaeolithophyllum and its preservation from the Pennsylvanian of the Cantabrian Zone (NW Spain)

Abstract

The new species Archaeolithophyllum asymmetricum nov. sp., from the Bachende Formation (Pennsylvanian, Cantabrian Zone, NW Spain), is described herein using cathodoluminescence microscopy. Under plane-polarized light, A. asymmetricum occurs as elongate and arcuate sheets preserved as calcitic mosaics of tiny anhedral to subhedral crystals. Cathodoluminescence has revealed that skeletal walls are composed of dull-bright (locally bright) luminescent calcite that contrasts sharply with the nonluminescent cements filling the intraskeletal pores. Skeletal walls are currently composed of low-Mg calcite (0.5–2 mol % MgCO3) with low Sr content (average 415 ppm). A. asymmetricum shows a strong asymmetry of the thallus organization. The internal tissue is well differentiated into a thick medullar hypothallus and a thin upper cortical perithallus, the latter being composed of nearly rectangular cells arranged in rows perpendicular to the external surface. Cell fusions commonly occur in the perithallial tissue whereas conceptacles exhibit a highly arched geometry lacking any preserved aperture. A. asymmetricum accumulations display a growth pattern similar to that reported from Late Paleozoic “phylloid algae”, and also resemble Miocene frameworks of the corallinacean Mesophyllum. These accumulations of A. asymmetricum formed micrite-rich bioherms with abundant shelter porosities, which are filled up with radiaxial-fibrous calcite (originally high-Mg calcite) and subsequent blocky spar. They constructed a rigid framework that was basically a combination of the foliaceous growth form, crust fusion and division, and synsedimentary marine cementation. Paleontological and sedimentological evidence suggests that A. asymmetricum thrived in an outer platform environment with relative quiet conditions. The exceptional preservation of these algae was favored by a rapid cementation of the intraskeletal pores under oxidizing conditions in a marine phreatic environment, protecting skeletons from early dissolution and recrystallization. Although the resulting neomorphic microsparite fabric suggests an aragonite precursor, the morphological similarities (especially reproductive organs) between Archaeolithophyllum and Recent calcitic corallinaceans, and the similar trace element composition of the algal thalli and the surrounding high-Mg radiaxial-fibrous cements, suggest that originally, Archaeolithophyllum was probably composed of high-Mg calcite. Based on the morphologic features, framework strategies (crust fusion and division) and growth modes, it is suggested that Archaeolithophyllum might be phylogenetically related to the modern coralline algae.