Enigmatic traces in infaunal bivalves from the late Quaternary of Argentina, Southwestern Atlantic. Bioerosion, bioclaustration or nothing?

RICHIANO, SEBASTIÁN; AGUIRRE, MARINA; FARINATI, ESTER; DAVIES, KAREN; CASTELLANOS, IGNACIO; GÓMEZ-PERAL, LUCIA E.

GEOBIOS

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

Año: 2018 vol. 51 p. 161 - 172

0016-6995

Ichnological investigations were carried out on late Quaternary shells of the intertidal deep infaunalbivalve Tagelus plebeius (Lightfoot, 1786) found along the southwestern Atlantic, between Uruguay andthe southernmost Buenos Aires Province in Argentina. Analyses reveal distinctive marks that are spreadon the outer shell surface only. The marks are regular-unbranched-elongate, perpendicular to the outershell growth lines, with deflections on the margins, never interconnected, without bifurcations,conforming bottom-up constructions. They occur in hundreds of specimens from many samples takenfrom sediments ranging in age from the late Pleistocene to the Recent. These marks have never beenreported or described for this species and their origin and formation remain elusive. We describe thesetraces thoroughly and we propose an explanation for their preservation on about half the shellsexamined. Potential destructive boring structures (excavated from outside-in) or bioerosion activities byother macro- or micro-organisms are dismissed. These antimarginal asymmetric traces point instead to aprocess of constructive bioclaustrations (grown from the bottom-up) produced in situ during the life ofthe bivalve by unknown symbiont organisms. Additionally, the regular pattern observed for the marksexclude host growth as a consequence of abiotic/extrinsic causes. From a palaeoecological perspective,these structures suggest a biotic interaction that was hitherto undescribed neither for bivalves nor forthe late Quaternary of the southwestern Atlantic.