Serpulids on living Eocene larger foraminifer Discocyclina

Abstract

I report the first observation of the symbiotic relationship between a serpulid polychaete and a larger foraminifer, Discocyclina, from the Eocene of the Ebro Basin (NE Spain). Discocyclina tests are the larger and most abundant element of the fossil assemblage in a limestone bed within a fan delta series, and seems to be a preferred substrate by the serpulid larvae to settle. The bioimmuration of the serpulid tube within the foraminiferal test shows that the polychaetes attached and grew on living foraminifers, which continued growing after the serpulid’s death. The foraminifer could benefit from the filtering activity of the serpulid, although in some cases the serpulid attachment caused an anomalous growth of the foraminiferal test. Some foraminifer individuals survived to two, possibly three successive episodes of serpulid attachment, which suggests a life span of 1–3 years for this Discocyclina species, similar to that of recent hyaline larger foraminifers such as Amphistegina or Paleonummulites. This sheds new light on the paleobiology of this type of larger foraminifers without recent counterparts that thrieved during Late Cretaceous and Paleogene times.