A fresh look at Lapìspira: a neglected nearshore double helicoidal burrow

LANÉS, S.; MANCEÑIDO, M.O.; DAMBORENEA, S.E.

Trelew, Chubut, Argentina

Congreso; Ichnia 2004. First International Congress on Ichnology; 2004

Museo Paleontológico Egidio Feruglio

The hitherto seldom recorded ichnogenus Lapispira is a concentrically arranged double helicoidal burrow, subperpendicular to bedding, without branching. The regular coiling remains remarkably constant, with greatest diameters in arenites and smallest in silty sandstones. The usual infilling (no pellets observed) comprises fine to very fine sandstone, mainly finer and better size-sorted than the hosting sediments. Burrow walls, unlined, are usually mined with small Chondrites isp., though less densely than in the surrounding beds. Lapispira isp. is a facies-dependent burrow: in Sinemurian to Early Pliensbachian deposits at the Atuel valley area of the Neuquén basin (Mendoza, Argentina) it occurs only in storm-and fair-weather sandstones deposited in the shoreface and offshore-shoreface transition zone (where it attains the largest sizes), and ocassionally appears in very fine silty sandstones, too. Though the trace-maker is largely unknown, the occurrence of decapod moults (Astacidae) outside the burrows hints at this group as a likely producer. The comparison of Lapispira isp. with the simple helicoidal burrows of living thalassinidean crustaceans suggests a feeding behaviour (subsurface deposit feeder?) where the double helicoidal tube optimizes the burrow inner surface for a given volume. The trace may be regarded as a mixed one reflecting somewhat different activities. The relationship with Chondrites isp. led earlier to allocate the burrow among “agrichnia-modified domichnia”, yet it can also point to possible commensal interactions, like those between ghost-shrimps and lucinids inhabiting within simple corkscrew burrows of certain present-day intertidal to shallow subtidal thalassinidean decapods. The overall distribution of the ichnogenus is reviewed revealing wider stratigraphical and geographical ranges than previously acknowledged.