Lapispira: A Double Helicoidal Burrow from Jurassic Marine Nearshore Environments

The hitherto rarely recorded ichnogenus Lapispira Lange is a concentrically arranged double helicoidal U-burrow, subperpendicular to bedding, without branching. It is redescribed on the basis of abundant material occurring in Sinemurian to Pliensbachian deposits at the Atuel valley area of the Neuquén basin (Mendoza, Argentina). The regular coiling remains remarkably constant, with greatest diameters in arenites and smallest in silty sandstones. The burrows are generally filled with massive, fine to very fine sandstone, mainly finer and better size-sorted than the host sediments. The unlined burrow walls may show scratch traces and are usually mined by small Chondrites isp., though less densely than in the surrounding beds. Architectural, taphonomical, constructional, and functional aspects of Lapispira are discussed. Though its trace maker is unknown, some structural details hint at decapod crustaceans as the likely producers. This deliberate complex trace can be regarded as a mixed-purpose one, reflecting somewhat different activities, thus defying straightforward ethologic allocation. Lapispira bispiralis Lange is a presumably elite, facies-dependent burrow: in the studied area it occurs in storm- and fair-weather sandstones deposited in the shoreface and offshore-shoreface transition zone (where it attains the largest sizes); only exceptionally does it appear also in very fine silty sandstones (offshore tempestites). The overall distribution of the ichnogenus shows a wider stratigraphic and geographic range than previously acknowledged, though the facies fidelity is maintained.