Storm-Induced Community Dynamics in the Fezouata Biota (Lower Ordovician, Morocco)

Abstract

In the Central Anti-Atlas (Morocco), the lower part of the Fezouata Shale has yielded locally abundant remains of soft-bodied to lightly-sclerotized taxa, occurring in low diversity assemblages characterized by strong spatial and taxonomic heterogeneities, and frequently, by the occurrence of small-sized individuals. Size frequency analyses of Celdobolus sp., Wosekella sp. (both linguliformean brachiopods) and Babinka prima (babinkid bivalve) collected in deposits of the Fezouata Shale and associated with distinct paleoenvironmental conditions show that short-lived communities of epifaunal and shallow infaunal taxa were regularly smothered and killed by distal storm deposits. Smallsized individuals more likely represent juveniles, rather than 'dwarfed' adults (Lilliput Effect). Consequently, unstable environmental conditions (regular storms, and possibly low oxygenation of the water column) probably explain the unusual community dynamics of late Tremadocian assemblages of the Fezouata Biota (high density of individuals, low α-diversity, and high γ-diversity), interpreted as short-lived, opportunistic populations. This process has wider implications for the understanding of occurrences of small individuals elsewhere in the fossil record.

Taphonomic pathway of exceptionally preserved fossils in the Lower Ordovician of Morocco

“…This hypothesis is confirmed by the absence of benthic fauna (both hard and soft parts) in this interval, showing that environmental conditions on surface sediments were probably not favorable for the colonization of this environment (Saleh et al, 2018). Statistical analyses show that an alternation of clusters, which are reconstructed based on the proportion of different sedimentary facies, can predict with a 95% fidelity the presence and location of intervals with exceptional preservation discovered in the field (Fig.…”

“…The other type of preservation is associated with shale (Martin et al, 2016). In these levels, fossils occur exclusively at bed junctions and not within beds (Vaucher et al, 2016(Vaucher et al, , 2017, strongly supporting the view that organisms were smothered on the seafloor under a new blanket of distal storm deposits, rather than having been carried in sediment flows (Saleh et al, 2018).…”

“…10(C)). This is because Fc2 combines rather calm environmental conditions with lower energy events compared to other facies, allowing living organisms to colonize the sea floor (Saleh et al, 2018), in addition to burial during event deposition, a prerequisite condition for exceptional preservation (Vaucher et al, 2017).…”

Ordovician stratigraphy and benthic community replacements in the eastern Anti-Atlas, Morocco

“…Although a direct correlation of these six sequences with other Ordovician sequence frameworks is not available (for comparison, see e.g., Ross & Ross 1992 Fig. 7) in a wave-dominant tide-modulated platform with predominance of offshore clayey substrates (Álvaro et al 2014c, d;Vaucher et al 2017;Saleh et al 2018). About 200 taxa have been reported from these two formations, mainly controlled by the exceptional preservation conditions of the Fezouata Biota (Gaines et al 2012;Lefebvre et al 2016aLefebvre et al , b, 2018; see list in Annex 1), the taphonomic conditions of which were virtually controlled by external climatic forcing (Saleh et al 2019).…”

Biogenic Iron Preserves Structures during Fossilization: A Hypothesis

“…[ 46 ] Increased accumulations of ferritin‐ferrihydrites were evidenced in marine invertebrates after their exposure to dysoxic/anoxic conditions [ 47 ] comparable to the environments in which animals from the Chengjiang Biota and the Fezouata Shale were preserved. [ 48,49 ] In experimental studies, it was shown that under bacterial sulfate reducing (BSR) conditions and when sulfates are present, ferrihydrites release high quantities (≈87%) of reactive Fe [ 50 ] (i.e., 0.22 m ). This iron delivery is 40% higher than the yield from the same quantity of hematite.…”