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

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

GEOBIOS

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

Lugar: Paris; Año: 2018 vol. 51 p. 161 - 172

0016-6995

Ichnological investigations were carried out on Late Quaternary shells of the intertidal deep infaunal bivalve Tagelus plebeius (Lightfoot, 1786) found along the southwestern Atlantic (between Uruguay and the southernmost Buenos Aires Province in Argentina). Analyses revealed distinctive marks that are spread on the outer shell surface only. The marks are regular?unbranched?elongate, perpendicular to the outer shell growth lines, with deflections on the margins, never interconnected, without bifurcations, conforming bottom?up constructions. They occur in hundreds of specimens from many samples taken from sediments ranging in age from the late Pleistocene to the Recent). These marks have never been reported or described for this species and their origin and formation remain elusive. Here we describe these traces thoroughly and we propose an explanation for the preservation of about half the shells examined. Potential destructive boring structures (excavated from the outside?in) or bioerosion activities by other macro- or micro-organisms are dismissed. The antimarginal asymmetric traces point instead to a process of constructive bioclaustrations (grown from the bottom?up) produced in situ during the life of the bivalve by unknown symbiont organisms. Additionally, the regular pattern observed for the marks exclude 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 for the late Quaternary of the southwestern Atlantic.