The enigmatic Palaeocene-Eocene coralline Distichoplax: Approaching the structural complexities, ecological affinities and extinction hypotheses

The alluring but scarcely understood Palaeocene-Eocene coralline alga Distichoplax has been examined in detail from multiple carbonate sections outcropping in Mawmluh near Cherrapunji (East Khasi Hills) in Meghalaya, N-E India. Despite having a wide geographical distribution Distichoplax still remains a mysterious taxon with faint knowledge pertaining to its evolution, ecology and causes of ultimate extinction. The commonly known feather-shaped and zipper-shaped cell arrangements need detailed reassessment considering the morphological intraspecific variability. The threefold objective of this paper is to analyse the complex cell/anatomical variations in Distichoplax, provide some insights into its ecological affinity, and most importantly propose hypotheses on the possible causes of its untimely demise in sharp contrast to other coralline algae distinguished by a long evolutionary record continuing to the present-day. The abundance of Distichoplax varies from monodominant assemblages to their co-dominance with geniculate coralline algae, non-geniculate Sporolithon and larger benthic foraminifera (orthophragminids) before facing sharp decline and ultimate extinction. Rare fairly well-preserved uniporate conceptacles and absence of cell fusions link Distichoplax to the subfamily Lithophylloideae of the family Corallinaceae. Very poor incidence of reproductive features, significant blooms of the species during the Thanetian (Late Palaeocene) and its eventual disappearance in the Eocene indicate Distichoplax may have been excessively dependent on vegetative propagation, and perished due to the climate adversities like the Palaeocene-Eocene Thermal Maximum (PETM) and biotic pressures like the Larger Foraminiferal Turnover (LFT).