Morphology and ethology of Rosselia erecta in Eocene lignitic mudstone

Abstract

The Middle Eocene lignite-mudstone deposits of the Kutch (Kachchh) Basin, western India, manifest a highly restricted marginal-marine depositional setting, in which an unusual preservation of Rosselia erecta can be observed within organic-carbonaceous-rich mudstone beds. It is the earliest Cenozoic record of the ichnospecies hitherto reported. Rosselia erecta occurs in a paucispecific R. erecta–Trichichnus isp. ichnofabric within thinly laminated mudstones that are intercalated with lignite seams and laminae. The burrows are primarily vertical, characterized by cone-in-cone structures that are penetrated by a centrally positioned cylindrical shaft with concentric or intermittently eccentric lining. The lining intermittently contains sulfide-mineralization, often with the framboidal pustules that probably formed as a result of anaerobiosis by chemautotrophic symbiotic bacteria. The cone opening at the bedding surface displays radial striae, indicating surface-detrital foraging behaviour. We propose that the burrow serves as a facultative multifunctional structure, with the tracemaker employing two distinct feeding mechanisms: (1) surface-detritus-feeding (fodinichnia) and (2) cultivating microbial colonies (agrichnia) of sulfate-reducing bacteria in a mutualistic symbiotic relationship. The well-preserved morphological features in the newly examined specimens facilitate a reassessment of the constructional mechanisms and functional implications of the ichnospecies R. erecta reminiscent of the recently established ethological category sequestrichnia.