Taphonomy and taphofacies analysis of lingulid brachiopods from Devonian sequences of the Paraná Basin, Brazil

Abstract

The extreme abundance of lingulid valves in some outcrops of the Devonian sequences of the Paraná Basin, and their absence in others, where other benthic taxa dominate, was the starting point to the present research. Twenty-one outcrops were sampled and characterized taphonomically. These outcrops record an epeiric muddy sea, influenced by episodic storms. In twelve out of the 21 outcrops (the Devonian sequences B, C, D and E), massive occurrences of complete and fragmented valves of lingulids, oriented parallel to the bedding plane, were documented. The occurrence of fragments together with complete, but disarticulated valves, suggests that the post-mortem sorting and transport is unlikely to have been notable. This claim is also supported by the fact that in situ preserved lingulid valves were always recorded, even if in low numbers. However, some time averaging may have occurred. This is because entire valves (presumably removed from their burrows just prior to burial) and fragmented ones (probably reworked or exposed on sediment–water interface for some days or a few weeks) were found together. Rosette and stringer arrangements of the lingulid pavements represent the first record of distal reworking of this kind of bioclastic material. The Devonian “lingulid outcrops” studied here represent fossil accumulations that likely formed with contributions from multiple factors. However, the extrinsic physico-chemical factors are difficult to infer by analogy with modern environments because of the unique setting that typified Palaeozoic epeiric seas. On the other hand, an actualistic approach can be used to assess some of the taphonomic controls exerted by intrinsic characteristics of lingulid brachiopods, including shell thickness and life habit. The lingulid-rich units, with their distinct taphonomical signatures, represent a clearly definable taphofacies (“lingulid taphofacies”) characterized by abundant (often complete) occurrence of lingulid valves parallel to the bedding plane in light grey and dark grey siltstones and rare occurrences of lingulid valves in black shales. The lingulid taphofacies is associated with a range of proximal marine settings, ranging from the lower shoreface (lowermost intertidal) to the distal transitional offshore (shallow subtidal) zones. The pattern of patchy distribution of lingulids, observed in some present-day environments, was rarely seen in the studied sequences.