Ethology and tracemaker. Undoubtedly, the best example of equilibrium structure is represented by the ichnogenus Rosselia. Nara (1995) undertook a morpho-ethologic analysis of Rosselia socialis, concluding that the most likely tracemakers are terebellid polychaetes. He suggested that the burrow lining was produced by the outward expansion resulting from accretion of muddy material
on the inner surface of the lining. Nara (1995) concluded that this model supports a detritus-feeding strategy. In addition, he noted that similar structures are made in modern shallow-marine settings by terebellid polychaetes (Aller and Yingst, 1978; Sch€afer, 1972). Subsequently, modern Rosselia-like burrows were also attributed to cirratulids (Gingras et al., 1999, 2008), with the genus Cirriformia recognized as a producer (Zorn et al., 2007). Based on modern and Holocene Rosselia-like examples, spionid polychaetes have been proposed as producers as well (Olivero et al., 2012).
Diagnosis. Rosselia displaying bulb-like swelling(s) in the proximal part of burrow (emended by Uchman and Krenmayr, 1995).
Description. Conical structure, sub-perpendicular to bedding, with a maximum diameter (2.3 cm) at the top and a minimum diameter (0.8 cm) at the base. The approximate length is 10 cm. Cone-in-cone laminae are observed internally. It is preserved as endichnia.
Diagnosis Small central burrows surrounded by concentric, funnellike laminae, nested convex downward, terminating in a subcylindrical, concentrically layeredstem.