Articulated and dislocated infaunal echinoids as unique markers of hypoxic environments from the Miocene of Central Paratethys

Abstract

Exceptional preservation of macrobenthic invertebrates with articulated remains is typically explained by episodic rapid burial events or by onset of anoxia, both aborting mixing and disintegration processes. However, these scenarios do not explain the preservation of articulated remains of infaunal organisms in the Lower Miocene diatom-rich mudstones (Schlier-type deposits) in the Central Paratethys epicontinental sea. We show that bathyal macrobenthic assemblages and dominated by the shallow-burrowing echinoid Lovenia are best preserved in background mudstones with burrow-disrupted diatomaceous lamination, conforming to intermediate ichnofabric typical of hypoxic environments. Lovenia occurs in three types of assemblages that differ in preservation, size structure, and species diversity: (1) dispersed or clustered, frequently complete echinoid tests with spines occur in homogeneous or partially-laminated silty claystones; (2) sandy pavements with densely-packed, almost monospecific echinoid concentrations exhibit intermediate frequency of intact tests with spines; and (3) well-sorted echinoid fragments co-occur with plant remains in species-rich sandy lags. Alternation of laminae formed by Thalassionema or Coscinodiscus-dominated diatom assemblages with terrigenous laminae indicates that: (1) postmortem burial of echinoids below the taphonomically-active zone was induced by rapid export of ungrazed diatoms to the seafloor and by seasonal fallout of terrigenous muds from hypopycnal plumes or low-density hyperpycnal flows (rather than by sudden burial by thicker event beds); and that, (2) sediment mixing and irrigation rates were slow and patchy because the diatomaceous mats were not eliminated by echinoid and crustacean burrows and the laminae-forming diatom frustules remained intact. Although winnowing and test exhumation to sediment-water interface contributed to the formation of pavements with echinoid concentrations, their dense packing and low evenness can rather reflect population outbreaks of echinoids exploiting seasonal diatom fluxes to the sediment surface. The echinoid fragments with spines, dislocated remains with cross-plate fractures, and molluscs with sharp-edged margins in silty claystones suggest that some mortality events were induced by predation rather than by anoxia. Seasonal hypoxia was a key factor that limited test disintegration and displacement and thus preserved not only unique ichnofabric but also intact or dislocated, weakly time-averaged remains of benthic fauna adapted to hypoxic conditions.