Pseudoreticulate-Subreticulate-Reticulate Nummulites and their possible evolution in the middle Eocene-lower Oligocene succession of Surat-Bharuch, Gujarat India

A biostratigraphically well constrained middle Eocene to lower Oligocene shelf succession (ca. 33 m) of the Amravati Formation in the Vagalkhod-Dinod area of Surat-Bharuch Districts, Gujarat, India contains Nummulites having varied septal filaments, among which complicated filaments show progressive changes in the succession. Complications by the intersection of filaments with spiral laminae/ridge, bifurcation of filaments at low angle and joining by transverse trabeculae, and bifurcation and re-joining of filaments at high and varied angles, and consecutive bifurcation and trifurcation of filaments allowed their categorization in different species. Successive appearances of pseudoreticulate filaments (Nummulites acutus, Nummulites broachensis and Nummulites longilocula), subreticulate filaments (Nummulites hormoensis and Nummulites ptukhiani) and reticulate filaments (Nummulites fabianii, Nummulites retiatus and Nummulites fichteli) reflect gradual complication with time, producing a bioseries that is also manifested by changes in canals and equatorial chambers. The eight species of Nummulites are differentiated and described within the pseudoreticulate subreticulate- reticulate group and based on their stratigraphic distribution, seven zones in ascending order namely, the Nummulites acutus Assemblage Zone, Nummulites broachensis Range Zone, SRG Barren zone, Nummulites hormoensis-Nummulites fabianii Interval Zone, Nummulites fabianii Range Zone, Nummulites fabianii-Nummulites retiatus Interval Zone and Nummulites fichteli Range Zone are recognized and described. The zones are tentatively correlated with the SB zones and their ages are assigned by planktonic foraminiferal zones. Complexity of septal filaments within the Nummulites of the succession began with the development of pseudoreticulation (by the intersection of radial filaments with spiral laminae) during the late Lutetian; afterward during the early Bartonian Stage bifurcation develops in a septal filament along with occasional joining by transverse trabeculae; subreticulation remained restricted to the late Bartonian,while reticulation appeared at the beginning of the Priabonian to flourish during the Priabonian to mid Rupelian.The bioseries by septal filaments shows three evolutionary trends in these species: 1) evolution of N. acutus to N. fichteli through N. broachensis-N. hormoensis-N. ptukhiani-N. fabianii; besides acquiring complication of septal filaments this trend shows a general increase in the size of the proloculus, an increase of the size of the B-form, increasing complication in canals, and lowering the height of equatorial chambers concomitant with the increase of length; 2) evolution of N. acutus to N. broachensis to N. longilocula; this trend is restricted to the pseudoreticulate group; discerning changes involve connecting adjacent septal filaments by transverse trabeculae and spiral laminae, and development of rudimentary mesh of septal filaments, complication in canals and slit-like long equatorial chambers; 3). evolution of N. hormoensis to N. retiatus; changes involve in the development of reticulation from subreticulate septal filaments, decrease in the height of chambers, complication in canals, and development of laminae and mesh as grooves. Nummulites acutus is ancestral to all the discussed species. The evolution of Nummulites leading to complicated septal filaments is known during the corresponding span in the Mediterranean region, East African region, and a few locations in western European and southeast Asian regions, but the trends of evolution as studied in the Amravati Formation differs in the distinct developments of pseudoreticulate lineage, N. ptukhiani as an intermediate evolutionary form between N. hormoensis and N. fabianii, and in the development of complex subreticulation from a simple one. The studied succession is the only one of its kind in the Indian region to have successive development of pseudoreticulate-subreticulate-reticulate Nummulites across the Lutetian/Bartonian, Bartonian/Priabonian and Priabonian/Rupelian boundaries.