Schrödinger's Lingulid: Alive or Dead?

Lingulids, infaunal brachiopods with records from the Cambrian to the present day, preferentially inhabit sandy substrates, remaining in their terrier's in a perpendicular position to the lodging plane (PV). Through the description of the taphonomic signatures of lingulids preserved on eight floors, collected in Siluro-Devonian rocks of the Amazon Basin, it was possible to detail the series of events that represent this record. The studied pavements are composed of silty sandstone with ESE-WNW oriented symmetrical wave structures. The pavements contain an abundant and paucispecific occurrence of lingulids, fragments of ex situ tubular ichnofossils and in situ ichnofossils. Approach to quantification consisted of the use of the FIJI software aligned with traditional taphonomy. Allied methodologies were microscopic petrography and SEM. Of the 480 three-dimensional valves parallel to the bed (PA), 70% are articulated (complete or fragmented) and have a filling similar to the matrix (with shell fragments inside). The valves have an exfoliation feature and different degrees of fragmentation. Its occurrence is associated with the ripple troughs and preferentially oriented to the south (indicated by the orientation of the posterior region of the shell), as well as the more elongated tubes that are also reoriented in the troughs of the wave marks. Whole valve sizes have normal distribution. Next, we will list the sequence of events that generated this record: populations are buried in a living position in their preferred habitat, preserving valve articulation. Pervasive fragments and matrix infilling are life position burial indicaors. Eodiagenesis enhances shell endurance, maintaining articulated valves preserved. Exhumation and burial sequenced episodes confers varied degree of fragmentation to those shells. Reorientation at ripple trough by proximal bidiretional wave interference corroborates with lingulid life habitat. Ex-situ ichnofossil fragments relates to the same interpretation. In life burial taphonomic signatures (articulation, filled and well preserved shells) occurs associted with death assemblage signatures (reorientation, fragmentation and exfoliation). Those unique condition is enhanced by eodiagenetic endurance confered to fossil shells and tubes. The fossil assemblage is autochtonous to parautochtonous. At section scale, it can be associated with an aggradation sequence.