Biomineralization, taphonomy, and diagenesis of Paleozoic lingulide brachiopod shells preserved in silicified mudstone concretions

Exceptionally preserved Devonian lingulides, found in spherical-to-subspherical mudstone concretions (Paranل Basin, Brazil), were analyzed using an electron microscopy-based approach (scanning electron microscopy, secondary and backscattered electron imaging) augmented with semi-quantitative elemental analyses (energy dispersive X-ray spectroscopy). Elemental composition, spatial biomineralization gradients, microstructural details, and growth banding patterns are similar to those observed in live-collected lingulid brachiopods (Glottidia palmeri and Lingula sp.). These parallels suggest a faithful preservation of primary biomineralization and microstructure in the Devonian lingulides. While our results suggest that physicochemical aspects of lingulide shells have been conserved throughout their evolutionary history, comparisons of carbon and phosphorus elemental maps and transects indicate that the Devonian lingulides may have been more extensively biomineralized than their present-day relatives, a pattern suggestive of their greater intrinsic fossilization potential. These Devonian shells, some of which are preserved in situ and in interpreted life position (with both valves intact and oriented vertically relative to bedding), tend to be associated with pyrite and rarer sphalerite grains at much higher concentrations than the surrounding matrix. This localized sulfide-mineral enrichment suggests that organic-rich lingulide shells may have acted as focal zones for bacterial sulfate reduction, resulting in degradation of the shell-associated organics and enhanced sulfide mineral precipitation. In addition to sulfide minerals, barite is also observed in direct proximity to the surfaces of some lingulide shells. Their in-situ preservation indicates rapid burial and a lack of reworking, and the early-diagenetic precipitation of sulfide and sulfate minerals suggests microbially-mediated authigenic processes in an anoxic microenvironment. In addition, the presence of three-dimensionally preserved acanthomorphic acritarchs indicates that the concretions were likely lithified rapidly prior to the degradation and compaction of these comparatively labile organisms. This combination of sedimentological, microbial, and geochemical conditions may have enhanced preservation of the lingulide shells, conserving original microstructural and biomineral details, as well as the acritarch fossils. The data presented here indicate that concretion-hosted organo-phosphatic shells may offer an exceptional taphonomic view into lingulide biomineralization and taphonomic processes.