Palaeonitella trifurcata n. sp., a cortoid building charophyte from the Lower Cretaceous of Catalonia

Abstract

Palaeonitella trifurcata n. sp. is a minute ecorticate thallus of a charophyte from a non-marine upper Barremian-Lower Aptian section of the Garraf Massif, near Barcelona (Catalonia, Spain). Whorls of trifurcated branchlets built glomerules about 1 mm across. Small radial branchlets were trifurcated once in three terminal acuminate dactyls. Longer and more robust branches, two per whorl emerge from a basal trifurcation, bore opposite secondary branchlets and ended in three terminal branchlets bearing one oospore. The whole set of characters suggests affinity with Nitella, making of this fossil one of the oldest and unique possible records of thalli of this extant genus. The thallus of P. trifurcata n. sp. was encrusted by a thin micrite film, and additionally, the whorls were coated by a thicker crust while the plant was still alive. It is suggested that this crust was formed upon the periphyton which grew perhaps upon a mucilage covering the whorls and apical heads, as it happens in extant Nitella. After encrustation, a small (ca. 5 mm across) constructive micrite envelope, called a cortoid, was formed attached to the plant, preserving in the inside the thallus articulated and slightly encrusted. Once the plant died, such cortoids accumulated on the lake bottom and were buried and lithified, forming a 'cortolite.' This is the first report of constructive micrite envelopes protecting delicate and poorly calcified charophyte thalli from being destroyed.