Biotic interaction between spionid polychaetes and bouchardiid brachiopods: Paleoecological, taphonomic and evolutionary implications

Shells of Bouchardia rosea (Brachiopoda, Rhynchonelliformea) are abundant in Late Holocene death assemblages of the Ubatuba Bight, Brazil, SW Atlantic. This genus is also known from multiple localities in the Cenozoic fossil record of South America. A total of 1211 valves of B. rosea, 2086 shells of sympatric bivalve mollusks (14 nearshore localities ranging in depth from 0 to 30 m), 80 shells of Bouchardia zitteli, San Julián Formation, Paleogene, Argentina, and 135 shells of Bouchardia transplatina, Camacho Formation, Neogene, Uruguay were examined for bioerosion traces. All examined bouchardiid shells represent shallow-water, subtropical marine settings. Out of 1211 brachiopod shells of B. rosea, 1201 represent dead individuals. A total of 149 dead specimens displayed polychaete traces (Caulostrepsis). Live polychaetes were found inside Caulostrepsis borings in 10 life-collected brachiopods, indicating a syn-vivo interaction (Caulostrepsis traces in dead shells of B. rosea were always empty). The long and coiled peristomial palps, large chaetae on both sides of the 5th segment, and flanged pygidium found in the polychaetes are characteristic of the polychaete genus Polydora (Spionidae). The fact that 100% of the Caulostrepsis found in living brachiopods were still inhabited by the trace-making spionids, whereas none was found in dead hosts, implies active biotic interaction between the two living organisms rather than colonization of dead brachiopod shells. The absence of blisters, the lack of valve/site stereotypy, and the fact that tubes open only externally are all suggestive of a commensal relationship. These data document a new host group (bouchardiid rhynchonelliform brachiopods) with which spionids can interact (interestingly, spionid-infested sympatric bivalves have not been found in the study area despite extensive sampling). The syn-vivo interaction indicates that substantial bioerosion may occur when the host is alive. Thus, the presence of such bioerosion traces on fossil shells need not imply a prolonged post-mortem exposure of shells on the sea floor. Also, none of the Paleogene and Neogene Bouchardia species included any ichnological evidence for spionid infestation. This indicates that the Spionidae/ Bouchardia association may be geologically young, although the lack of older records may also reflect limited sampling and/or taphonomic biases.

Life mode of in situ Conularia in a Middle Devonian epibole

Exceptionally abundant specimens of Conularia aff. desiderata Hall occur in multiple marine obrution deposits, in a single sixth-order parasequence composed of argillaceous and silty very fine sandstone, in the Otsego Member of the Mount Marion Formation (Middle Devonian, Givetian) in eastern New York State, USA. Associated fossils consist mostly of rhynchonelliform brachiopods but also include bivalve molluscs, orthoconic nautiloids, linguliform brachiopods and gastropods. Many of the brachiopods, bivalve molluscs and conulariids have been buried in situ. Conulariids buried in situ are oriented with their aperture facing obliquely upward and with their long axis inclined at up to 87degree to bedding. Most specimens are solitary, but some occur in V-like pairs or in radial clusters consisting of three specimens, with the component specimens being about equally long or (less frequently) substantially different in length. The compacted apical end of Conularia buried in situ generally rests upon argillaceous sandstone. With one possible exception, none of the examined specimens terminates in a schott (apical wall), and internal schotts appear to be absent. The apical ends of specimens in V-like pairs and radial clusters show no direct evidence of interconnection of their periderms. The apical, middle or apertural region of some inclined specimens abuts or is in close lateral proximity to a recumbent conulariid or to one or more spiriferid brachiopods, some of which have been buried in their original life orientation. The azimuthal bearings of Conularia and nautiloid long axes and the directions in which conulariids open are nonrandom, with conulariids being preferentially aligned between 350 and 50degree and with their apertural end facing north-east, and nautiloids being preferentially aligned between 30 and 70degree. Otsego Member Conularia were erect or semi-erect, epifaunal or partially infaunal animals, the apical end of which rested upon very fine bottom sediment. The origin of V-like pairs and radial clusters remains enigmatic, but it is probable that production of schotts was not a regular feature of this animal's life history. Finally, conulariids and associated fauna were occasionally smothered by distal storm deposits, under the influence of relatively weak bottom currents. © The Palaeontological Association.