Sabrina Coelho Rodrigues

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.

New data on the anatomy of Conularia milwaukeensis Cleland, 1911 (Middle Devonian, Iowa and Wisconsin)

Reexamination of previously described conulariids has shown that the morphology of steeply pyramidal, generally four-sided conulariid exoskeletons is more complex than was realized by earlier generations of conulariid specialists (e.g., Van Iten, 1992a, 1992b; Jerre, 1994; Van Iten et al., 1996). Much of this complexity is expressed as variation in the anatomy of the corners and midlines (Van Iten, 1992a). In nearly all conulariids, the corners are sulcate, and in some genera (e.g., Archaeoconularia Boucek, 1939 and Pseudoconularia Boucek, 1939) the longitudinal center line, or midline, of the faces also is sulcate or is marked by a straight or zigzagged ridge. In the oldest known conulariid, Baccaconularia Hughes, Gunderson, and Weedon, 2000 (Cambrian: Furongian Series), the midlines are marked by a series of elongate invaginations. Corners and/or midlines of many other conulariids exhibit subtle or pronounced internal thickening (e.g., Van Iten, 1992a; Jerre, 1994). Most such conulariids exhibit one or two sets of longitudinal ridges, or carinae, with a single carina at each corner and/or a single carina or a pair of carinae at each midline. The height of the carinae varies considerably between species, ranging from less than one-fiftieth to over one-half the distance to the center of the exoskeletal cavity. Both corner and midline carinae may be continuous or (less frequently) seriated. Also, the abaxial edge of single midline carinae may have a single crest or (rarely) it may be weakly or strongly bifurcate. Conulariids having internal carinae figure prominently in many previous discussions of the functional morphology and phylogenetic affinities of conulariids (e.g., Kiderlen, 1937; Moore and Harrington, 1956; Werner, 1966, 1967; Bischoff, 1978; Van Iten, 1991, 1992a, 1992b; Jerre, 1994; Bergstrom, 1995; Van Iten et al., 1996; Hughes et al., 2000 …